Tag Archives: bird

MEASURING AN EAGLE: REFERENCES

Baird, S.F., T.M. Brewer, and R. Ridgway 1874.  A History of North American Birds: Volume III, Land Birds.  Little, Brown, and Co.: Boston.

Baldwin, S. P., H. C. Oberholser, and L.G. Worley 1931.  Measurements of Birds (1931).  Scientific Publications of the Cleveland Museum of Natural History, Cleveland, OH.

Bent, A.C. 1937.  Life histories of North American birds of prey: order falconiformes.  Smithsonian Institution, Washington, DC.

Bortolotti, G.R. 1984a.  Criteria for determining age and sex of nestling Bald Eagles.  Journal of Field Ornithology 55: 467-481.

Bortolotti, G.R. 1984c.  Sexual size dimorphism and age-related size variation in Bald Eagles.  The Journal of Wildlife Management 48: 72-81.

Bortolotti, G.R. 1984d.  Physical development of nestling Bald Eagles with emphasis on the timing of growth events.  The Wilson Bulletin 96: 524-542.

Broley, C.L. 1947.  Migration and nesting of Florida Bald Eagles.  The Wilson Bulletin. 59: 3-20.

Chura, N.J., and P.A. Stewart 1967.  Care, food consumption, and behavior of Bald Eagles used in DDT tests.  The Wilson Bulletin. 79: 441-448.

Fitzpatrick, B.M. and J.R. Dunk 1999.  Ecogeographic variation in morphology of Red-Tailed hawks in western North America.  Journal of Raptor Research 33: 305-312.

Friedmann, H. 1950.  The birds of North and Middle America: A descriptive catalog.  Part XI.  Smithsonian Institution, Washington, D.C.

Gerrard, J.M. and G.R. Bortolotti 1988.  The Bald Eagles: Haunts and Habits of a Wilderness Monarch.  Smithsonian Institution Press, Washington and London.

Gerrard, J.M., A.R. Harmata, and P.N. Gerrard 1992.  Home range and activity of a pair of Bald Eagles breeding in northern Saskatchewan.  Journal of Raptor Research 26: 229-234.

Imler, R.H. and E.R. Kalmbach 1955.  The Bald Eagle and its economic status.  U.S. Fish and Wildlife Service Circular 30.

Maestrelli, J.R. and S.N. Wiemeyer 1975.  Breeding Bald Eagles in captivity.  The Wilson Bulletin 87: 45-53.

Meiri, S. and T. Dayan 2003.  On the validity of Bergmann’s rule.  Journal of Biogeography 30: 331-351.

Palmer, R.S., ed. 1988.  Handbook of North American Birds vol. 4: Family Cathartidae, New World condors and vultures; Family Accipitridae (first part), Osprey, kites, bald eagle and allies, accipiters, harrier, buteo allies.  Yale University Press, New Haven and London: 187-237.

Salewski, V. and C. Watt 2016.  Bergmann’s rule: a biophysiological rule examined in birds.  Oikos 126: 161-172.

Southern, W.E. 1964.  Additional observations on winter Bald Eagle populations: including remarks on biotelemetry techniques and immature plumages.  The Wilson Bulletin 64: 121-137.

Stalmaster, M.  The Bald Eagle.  Universe Books, New York.

Temple, S.A. 1972.  Systematics and evolution of the North American merlins.  Auk 89: 325-338.

U.S. Fish and Wildlife Service Forensic Laboratory.  The Feather Atlas: Flight feathers of North American birds.

Whaley, W.H. and C.M. White 1994.  Trends in geographic variation of Cooper’s hawk and Northern goshawk in Northern America: a multivariate analysis.  Proceedings of the Western Foundation of Vertebrate Zoology 5: 161-209.

Wright, B.S. 1953.  The relation of Bald Eagles to breeding ducks in New Brunswick.  The Journal of Wildlife Management 17: 55-62.

MEASURING AN EAGLE

©elfruler 2018

Acquiring accurate measurements of a bird is a tricky business, and it turns out that there are relatively few peer-reviewed publications that report measurements for Bald Eagles.  In the pages that follow I present tabulations of the credible numbers of which I am aware.  I begin with some general information on measurements and measuring.  General References are given at this link.

What are the sources of measurements?

The most reliable measurements are taken from a live, wild, healthy eagle, but capturing one is difficult and rarely attempted.  Some researchers have used captive eagles, recently deceased eagles, and museum specimens for measurements, but these present some issues that must be considered:

  • Permanently disabled eagles in captivity may have different measurements from those of their cousins in the wild because of richer diets, less exercise, irregular feather molt, abnormal beak length and depth (due to inconsistent feaking), etc.
  • A rescued eagle in distress is usually dehydrated and emaciated, so its weight is likely to be lower than normal. After being treated in rehab for days or weeks during recovery, its diet may be richer and more abundant than it would be had the eagle continued living in the wild, which coupled with a reduced level of activity may result in a higher weight than normal at its release.
  • A recently deceased eagle can yield accurate measurements unless the eagle succumbed to injury or starvation, or the body has decomposed or been disfigured by a predator.
  • Measurements of carcasses that have ended up as prepared skin specimens in museums or other collections can be useful for some features, such as talons, bill depth, wing length, and tail length. But a specimen is subject to drying and shrinkage which can affect some measurements such as total length, bill width, tarsus width, wing chord, wingspan, and individual feather length.  And since most bones are absent from prepared skins, weight and most skeletal measurements are impossible.  Specimens also can yield erroneous sex classifications, especially with younger birds.  (Bortolotti 1984c)

How do researchers measure?

  • Scales, calipers, and tape measures are standard tools.
    • Digital scales give more precise readings than analog scales. All scales should be well calibrated.
    • Methods for positioning the bird on scales and for a linear measurement can be surprisingly challenging and require study of best practices as well as training and practice.
  • Measurements are best reported in universally used metric units (grams, millimeters, etc.), although some earlier studies and a few recent ones use the “traditional” American system of pounds, ounces, feet, and inches.
    • For ease of comparison, here I use metric units including conversions where necessary.
  • For features that incorporate a curve or arc, such as the folded wing, talons, beak, and feathers, the measurement is of the chord, or a straight line between two end points, thus disregarding the curve. Measuring the curve itself requires a calculation based on the shape of the curve and may lead to only an approximate measurement that can make comparisons difficult.  Feathers and wings are not rigid and can be flattened to eliminate the curve, but this can introduce inconsistencies or inaccuracies and is generally avoided by researchers.
  • Reliable statistical analysis requires a sampling of a large enough number of birds to provide minimum, maximum, and average measurements. The notation “N=x,” where x is the number of birds examined, conveys the size of the sampling.  In general the average is the mean.  Some researchers calculate a standard deviation (SD).

What features do researchers measure?

P. Baldwin, H. C. Oberholser, and L.G. Worley, Measurements of Birds (1931) established a comprehensive and standardized system of obtaining linear measurements of the various parts of birds, which continues to guide researchers. It includes detailed discussion of procedures and challenges and the tools needed to obtain accurate and consistent measurements, along with drawings showing what to measure, how to position the bird, and where to place the tools.

  • Subsequent studies of raptors have offered nuances and modifications necessary for measuring these species.
  • The authors of some studies do not always make clear what exactly what they measured or what procedures they followed, or their procedures may not conform to standard practices. For example, some authors do not indicate whether they used live birds or specimens, they may omit important information on the geographical origins, age, or sex of the birds, or they may use unorthodox techniques without explanation.

The following measurements of Bald Eagles have been reported in published literature.  (Figures from Baldwin et al. 1931 are used under the license agreement with Creative Commons.  Figures from Bortolotti 1984a are used with permission of The Journal of Field Ornithology.  Figures from Bortolotti 1984c are used with permission of The Wildlife Society, Bethesda, MD.)

  • Weight, which would seem to be a straightforward measurement to obtain simply by putting the bird on a scale.  But several factors can skew the reading, and researchers do not always indicate whether they have addressed them:
    • A malnourished or otherwise unhealthy bird.
    • Contents of the stomach and crop, which can vary widely from hour to hour or day to day (according to Gerrard & Bortolotti 1988, a Bald Eagle’s crop can hold about 15% of its body weight). A desirable reading would be a “net” weight with empty stomach and crop.
    • Time of year the bird is weighed, as for instance for females in the lead up to egg-laying when they can gain weight and afterwards when they can lose weight, or around migration or dispersal season which could begin with weight gain and end with weight loss.
    • Recently deceased birds can give an accurate weight, but specimens will not.
  • Length of the bird from the tip of the beak to the tip of the tail.  This measurement is taken with the bird lying flat on its back, its head and tail stretched as flat against the table surface as possible (Figure 1 © Baldwin et al. 1931 as licensed by Creative Commons).
    • This is not the “height” of the bird, which on many internet sites seems to mean a measurement from the crown of the head to either the tip of the tail or the bottom of the feet while the bird is perched or standing. This “height” usually is stated as being about 3 feet for a Bald Eagle, although the method of measuring this is rarely specified.
    • This measurement can be difficult to obtain with a high degree of accuracy because of variability in stretching the neck.  Skin specimens will not give an accurate measurement.
    • There are few reported measurements of a Bald Eagle’s length in the ornithological literature.
  • Depth of the beak, from the maxilla at the front (anterior) edge of the cere to the opposite point on the lower mandible (© Bortolotti 1984c, Fig. 1c, used by permission).
    • See the measurement at C.
    • The exact place at which this measurement is taken can result in significant variation in the measurement and thus can be unreliable for comparison purposes.
      • Specimens can provide accurate readings unless the carcass was dried with the beak open.

 

 

  • Width of the beak, the widest point where the cere meets the feathers (Figure 13 © Baldwin et al. 1931 as licensed by Creative Commons).
    • The exact point at which this measurement is taken can cause significant variation in the reading and thus is not always reliable for comparison purposes.
    • Specimens provide less reliable readings.
  • Length of the exposed culmen of the beak, the chord of the beak from the tip of the culmen (the curve on the top of the upper mandible) to the front (anterior) edge of the cere, thus the length of the culmen without cere (see above, Bortolotti 1984c, Fig. 1a).
    • Specimens can provide reliable readings.
    • Some researchers measure the culmen with cere, i.e. to the back (posterior) edge of the cere where the feathers begin, although they do not always say so; these are not included here.
  • Length of the beak from the gape, a straight line (chord) from the tip of the upper mandible to the gape or corner of the mouth.
    • Specimens provide less reliable readings (see above, Bortolotti 1984c Fig. 1b).
  • Length of the foot, or foot pad, the bottom of the foot from the end of the middle toe to the end of the hallux toe, with both toes fully extended and not including the talons (© Bortolotti 1984a, Fig. 1, used by permission).
    • Taking this measurement on a live bird can be challenging unless the bird is anesthetized and thus relaxed.  Specimens usually will not yield accurate results unless the toes were extended during preparation before the skin dries out.
  • Tarsus or tarsometatarsus, the “foot” or lowest part of the leg, from which two measurements can be made:
    • Length, from the intratarsal joint where the ankle meets the heel to just above the base of the middle toe below the lowest scute (scale) on the leg. This measurement is a long diagonal from the intratarsal joint at the back of the leg to the base of the toe at the front of the foot.  Reports of this measurement can vary, perhaps because of imprecision of identifying the end points.  (Figure 136 (© Baldwin et al. 1931 as licensed by Creative Commons).
    • Width, which Baldwin et al. 1931 prescribe should be taken at the midpoint of the tarsus. The tarsus is wider when measured from front to back and narrower when measured from one side to the other; generally the front-to-back diameter of the tarsus is measured (Fig. 137 © Baldwin et al. 1931 as licensed by Creative Commons).   But the few reported measurements for Bald Eagles are taken just above the toes at the narrowest point of the tarsus, and calculations are of an average of two measurements taken front-to-back and side-to-side, yielding mean width rather than diameter.  Either way, width is a highly variable measurement in reports, and specimens do not always yield reliable readings (Bortolotti 1984a and Bortolotti 1984c).
  • Middle toe (the 3rd and longest digit), measured from above, from its base at the metatarsal joint outward to the end of the toe, not including the talon (Fig. 139 © Baldwin et al. 1931 as licensed by Creative Commons).The toe must be extended as straight as possible.
  • Length of the hallux talon, the chord from the talon’s tip to the point on top where it emerges from the skin of the toe. (see above, Bortolotti 1984c, Fig. 1d).
      • Specimens can provide reliable readings.
  • Length of a feather, the chord of the shaft from where it enters the skin to its tip, without flattening the shaft (Fig. 114 © Baldwin et al. 1931 as licensed by Creative Commons).
      • For Bald Eagles this is usually done only on flight feathers.
      • See these photos of Bald Eagle primaries and secondaries from the U.S.F.W.S. Feather Atlas.
  • Tail, the longest point from the bases of the feathers at the skin to the tips, measured with the tail closed, the ruler placed between the two innermost, or central rectrix, feathers (Fig. 120 © Baldwin et al. 1931 as licensed by Creative Commons).
      • Specimens can give reliable readings of this measurement.
  • Wing chord, the length of the closed, unflattened wing from the wrist joint to the feather tips (Fig. 101 © Baldwin et al. 1931 as licensed by Creative Commons).
    • Some reported Bald Eagle measurements appear to have been taken with flattened wings, hence are longer than measurements of the chord; these are not included here.
    • Specimens may not give reliable measurements.
  • Wingspan, between the tips of the outstretched wings with feathers, measured with the bird lying flat on its back (Fig. 98 © Baldwin et al. 1931 as licensed by Creative Commons).
    • Attempts to measure while the bird is standing or perched are less reliable and consistent.
    • Care must be taken not to injure the wings by over-stretching or flattening them.
    • Specimens will not give accurate readings.
    • There are surprisingly few Bald Eagle wingspan measurements reported in the literature.

MEASURING ADULT, SUBADULT, AND JUVENILE BALD EAGLES

ADULT BALD EAGLE MEASUREMENTS TABLE

SUBADULT AND JUVENILE BALD EAGLE MEASUREMENTS TABLE

REFERENCES

HATCHING

© elfruler 2018, 2021
with thanks to Donna Young

The avian egg is a marvel of nature, a self-enclosed and perfectly effective living environment for the developing bird embryo. The shell  is sturdy but flexible, hard but porous. The egg contains all that is necessary to enable a small and weak organism to develop into a chick with all its parts and enough strength and skill to break through and emerge into the outside world. Here is an account of the many factors involved in a chick’s hatching.

Inside the shell

  • The eggshell is a complex structure of hard calcium carbonate crystals interwoven with collagen fibers and coated by a thin layer of crystalline calcite and smooth protein cuticle. The structure is sturdy to protect the developing embryo, yet permeable with microscopic pores that allow oxygen to pass into the egg and carbon dioxide and water vapors to pass out.
  • Two soft keratin membranes line the inside of the shell, both formed in the isthmus of the oviduct a few hours after fertilization. These membranes facilitate the exchange of oxygen, carbon dioxide, and water through the hard shell. The outer membrane becomes fused to the inside of the shell near the time of hatching, and the thinner membrane lines the inner surface of the outer membrane. A gap between the two membranes forms a small air cell in the large (blunt) end of the egg, which will become very important when hatching is near.
  • A third membrane is adjacent to the inner shell membrane, the chorioallantoic membrane (CAM), which surrounds the embryo and effects the exchange of oxygen and carbon dioxide via a network of blood capillaries connecting it to the embryo. It also collects wastes that cannot be evaporated through the shell from the growing embryo, which it sheds after the egg hatches. The CAM is homologous to the mammalian placenta.
  • The embryo is attached to the yolk sac — which contains fat and protein to feed the growing chick — by a cord, the umbilicus, leading into the abdominal cavity.
  • The yolk sac is greatly reduced in size by hatching time. Now the egg weighs less than when it was laid because it has absorbed and metabolized fats from the yolk and lost evaporated water through the shell. At hatch a Bald Eagle egg might weigh 91-102 g (3.2-3.6 oz.), as opposed to 113-127 g (4-4.5 oz.) when laid.
  • The eggshell itself is much thinner at hatch than when the egg was laid because the chick has absorbed much of the shell’s calcium into its developing bones.
  • Starting about a third of the way through 36-39 days of the embryo’s growth in the egg, an “egg tooth” or “pipping tooth,” a small, hard, sharp protuberance of calcified keratin on the beak’s upper mandible, begins to develop. Here is a closeup of the egg tooth on a hatchling eaglet at the Institute for Wildlife Studies. The egg tooth gradually wears away within a couple of weeks after hatch.
  • A muscle in the back of the chick’s neck (the complexus or hatching muscle) swells in response to the influx of lymphatic fluids. This muscle recedes in size after hatching (although it later plays a role in neck extension in grown eagles).

The hatching process

  • When hatching nears, the air cell in the large (blunt) end of the egg quickly expands and spreads partway down along the upper side of the egg.
  • As the embryo nears full development it takes up most of the space inside the shell – it is crowded in there! The chick has gradually rolled to curl up tightly, lying on its left side with its legs bent in the smaller end of the shell, its back against the air cell. Its head is tucked forward against its breast near the blunt end of the shell and turned to the right under its right wing. This puts the beak and the egg tooth close to the air cell. Here is a drawing of the position of the chick in a chicken egg at 20 days, just before hatching.
  • As it takes hatching position, the embryo absorbs the remainder of the yolk sac into its abdominal cavity.
  • The complexus muscle begins to contract, causing the entire body of the chick to straighten and contract, pushing the egg tooth against the air cell and piercing it. This results in what is called the internal pip. The air cell releases a small supply of oxygen and prompts the chick’s lungs and its 9 air sacs to begin functioning.
  • With its lungs now working, the chick can also begin to vocalize, as can be heard in this video of an egg just as it pips the shell at the Institute for Wildlife Studies incubation facility in 2008.
  • After the internal pipping the chick rests as its lungs learn to directly inhale oxygen and exhale carbon dioxide. At this point the blood circulation and gas exchange via the CAM (chorioallantoic membrane) are winding down and the cord that connects the CAM to the embryo begins to wither.
  • After a few hours the buildup of carbon dioxide inside the shell stimulates the complexus muscle to contract more. The head and beak begin to jerk back against the shell repeatedly and the spine and legs push against the shell, finally piercing it with the egg tooth near the blunt end of the egg. This is seen from the outside as a “pip” or tiny hole or crack in the shell, usually on the side of the shell and near the larger end of the egg (but note that the beginning of a pip is often not in view on a nest cam). This is called the external pip.
  • After the first external pip that allows outside oxygen into the egg, the chick usually rests again for several more hours while its respiratory and circulatory systems continue to adapt.
  • The external pip accelerates fluid loss inside the egg as well as in the chick’s body, which is good because a slightly reduced body mass allows the chick more room to maneuver as it pushes against the shell.
  • The pip may begin as a tiny hole that increases in size over the next few hours. Or it may begin as a cracking of tiny bits of the shell, possibly taking a star-like appearance (“starring”). As the chick pushes outward, small bits of shell may bulge from the hole, often visible in profile as the pip is turned to the side. The chick’s legs flex and contract and the egg tooth pokes and scrapes the shell, creating larger holes and cracks. The chick’s beak, pipping tooth, and head might be visible through some of the cracks. The enlarged complexus muscle at the nape provides cushioning and support during this shell-breaking process.
  • As it pushes against the shell, the chick may begin to rotate, usually counterclockwise, perhaps halfway or more around the inside, until a part of the shell, often a roundish disc at one end, or a “cap,” separates and breaks the shell apart. This has led to the term symmetrical hatching, referring to the more or less symmetrical shape of both the broken-off cap and the rest of the egg. Symmetrical hatching is the norm for most avian species.
  • However, as observers of Bald Eagle cams over the years have noted, not all hatches result in a symmetrical breakup of the shell; in fact, some hatches look downright chaotic and messy. Sometimes the first external pip seems to simply grow in size until the chick breaks through the gap. Sometimes the shell membrane holds the shell together so that it does not break apart cleanly and the chick has to push through both shell and membrane to be free.
  • The hatching process is strenuous and can take up to 72 hours to complete.  The chick rests inbetween efforts to break through the shell.
  • Most biologists and observers consider the egg to be “hatched” when the chick fully emerges free from the shell.
  • The new hatchling is covered with a thin layer of downy feathers – its natal down – which is damp from the fluids inside the shell, matted against its mostly pinkish skin (but dark gray around the eyes). The down will dry out to a soft light gray color within a couple of hours.
  • The hatchling weighs about three-quarters of the weight of the egg when first laid about 37-39 days before – decreasing from about 113-127 g (4-4.5 oz.) to about 85 g (3 oz.). (Sizes vary with latitude – larger in the north than in the south – and also with hatch order – first eggs in a clutch are larger than subsequent eggs.)
  • After hatching the chick will lie in the nest resting for several hours. It will roll about a little, and the wings, legs, neck, and head may jerk spasmodically from time to time. Its breathing can be seen, and it will let out some tiny cheeps, which can be both heard and seen.

Parental behavior during hatching

  • The parents are aware of the hatching when they hear the chick’s vocalizations and possibly also its pecking at the shell. The incubating adult may stand above or to the side of the egg and lean in or cock its head, seeming to listen. Parents may chirp softly to the chick, or champ or click their beaks, perhaps another attempt to communicate.
  • They might continue to gently nudge the hatching egg and even the emerging chick with their beak.
  • They may exhibit restlessness in the egg cup, rising to check the eggs every few minutes, circling the cup, leaning in often to listen. They often pull soft nesting material in toward the nest cup (sometimes building a wall between the cup and the viewers!).
  • Both parents, but especially the male as the female does more of the incubating, may bring food to the nest in anticipation of both the chick’s and the mother’s need for food as brooding begins.
  • The parents do not assist the chick in breaking the shell because they could damage the still fragile blood vessels in the CAM. They may move shell fragments away from the hatching egg.

Post-hatching

  • Bald Eagle hatchlings are “semi-altricial,” which means they are nearly helpless when they hatch, with limited motor skills and strength, entirely dependent on parents for food and warmth, and confined to the nest (“nidicolous” – “nest inhabiting”). All raptors are semi-altricial and must spend several weeks being cared for by their parents in the nest before they fledge and are capable of fending for themselves.
  • Raptors are not considered fully altricial (like songbirds and parrots) because their eyes are open at hatch, they are covered with downy feathers, and they have some mobility.
  • At the other end of the developmental spectrum from altricial are “precocial” chicks, like geese, ducks, swans, chickens, quail, etc., which are capable of walking (and often swimming) and thermoregulating soon after they hatch. They are “nidifugous” (“nest fleeing”), meaning they leave the nest almost immediately after hatching.
  • In the days before it hatched the eagle chick has absorbed the yolk sac into its body, whose nutrients feed it in the few hours before and after hatch. It will not need to be fed by its parents for several hours.

Clearly, hatching is a complex process, and most of the time it ends successfully.  Sometimes, though, things can go wrong.  This page surveys reasons why an egg might fail to hatch.

Here is a compilation video of the hatch of the first eaglet at the West End nest on Catalina Island on 20 March 2018.
Detailed description of the development of a chicken embryo from fertilization through hatch, with great drawings and images.

References

  • Bond, G.M., V.D. Scott, and R.G. Board 1986. Correlation of mechanical properties of avian eggshells with hatching strategies. Proceedings of the Zoological Society of London (A) 209:225-237.
  • Bond, G.M., R.G. Board, and V.D. Scott 1988.  An account of the hatching strategies of birds.  Biological Review 63:395-415.
  • Bortolotti, G.R. 1984.  Physical development of nestling Bald Eagles with emphasis on timing of growth events. Wilson Bulletin 96:524-542.
  • Deeming, D.C. 2002.  Avian Incubation: Behaviour, Environment, and Evolution (Oxford and New York: Oxford University Press).
  • Deeming, D.C. and S.J. Reynolds, eds. 2015.  Nests, Eggs, and Incubation: New Ideas about Avian Reproduction (Oxford: Oxford University Press).
  • Drent, R. 1973.  The natural history of incubation. In Breeding Biology of Birds: Proceedings of a symposium on breeding behavior and reproductive physiology in birds, Denver, Colorado, February 1972, ed. D.S. Farner (Washington, DC: National Academy of Sciences):262-322.
  • Fox, N. 1995.  Understanding the Bird of Prey (Surrey, British Columbia and Blaine, WA: Hancock House Publishers).
  • Gill, F.B. 2007.  Ornithology, 3rd ed.  (New York: W. H. Freeman and Company).
  • Hamburger, V. and R. Oppenheim 1967. Prehatching motility and hatching behavior in the chick. Journal of Exp. Zool. 166:171-204
  • Lovette, I.J. and J.W. Fitzpatrick, eds. 2016.  The Cornell Lab of Ornithology Handbook of Bird Biology, 3rd ed. (Chichester, West Sussex: John Wiley & Sons, Ltd.
  • Oppenheim, Ronald W. 1972. Prehatching and hatching behaviour in birds: a comparative study of altricial and precocial species. Animal Behaviour 20:644-655.
  • Podulka, S., R.W. Rohrbaugh, Jr., & R. Bonney, eds. 2004.  Handbook of Bird Biology, 2nd ed. (Ithaca, NY: The Cornell Lab of Ornithology).
  • Proctor, N.S. and P.J. Lynch 1993.  Manual of Ornithology: Avian Structure & Function (New Haven and London: Yale University Press).
  • Sharpe, P. 1995.   Guide to Bald Eagle Egg Incubation and Chick-Rearing.  Institute for Wildlife Studies.
  • Starck, J. M. and R.E. Ricklefs, eds. 1998.  Avian Growth and Development Evolution within the Altricial-Precocial Spectrum (New York and Oxford: Oxford University Press).

 

CLUTCHES, EGGS, and FLEDGES

These numbers come from all Bald Eagle nests for which I have records, including those observed on camera and from the ground.  See here for a list of these nests.  Excluded from these data are nests in aviaries where non-releasable eagles are provided with food, medical, and other care (Carolina Raptor Center in NC and American Eagle Foundation in TN).

Click on the chart to enlarge.

© elfruler 2018

OVIPOSITION (Egg-laying)

By Donna Young and elfruler
Updated 10/27/18

© elfruler 2018

  • Every female Bald Eagle has her own style of laying an egg that is usually consistent from egg to egg and year to year, although some variations in behavior may occur.  Any departure from a previously observed style may indicate a new female.
  • Rough predictions of when a first egg may be laid at a particular nest can be made on the basis of past years, since a pair tends to lay within a one- or two-week timeframe every year.  A significant departure from timing may point to a change of female, male, or both.  Click here for calendars of egg-laying at Bald Eagle nests observed on live cameras since 2006 (arranged by month).
  • Timing of the first egg is the hardest to predict, but second and third (and in the rarest of cases, fourth) eggs will come at 3-day or 4-day intervals (never fewer than about 69 hours, and only rarely after more than 96 hours).  See this page for statistics showing the time intervals at the eagle cams.

We have compiled a list of behaviors associated with egg-laying (oviposition) that we have observed over ten years of watching Bald Eagle cams online.  We divide the signs into three periods: Prelude, The Main Event, and Postlude.  A few signs are seen in all instances of oviposition, but we emphasize that no two events are alike, even with one specific bird.

 

PRELUDE

  • Nest preparation
    • For a few days before oviposition both male and female usually will spend more time in the nest, bringing in and arranging materials, especially the softer grasses, leaves, fronds, etc. that form the small cup where the egg(s) will be laid. They may dig with their beaks in the cup to help define and deepen it.  Exceptions to this do occur – sometimes there is not much soft material to form a clear cup.
    • Also for several days both parents probably will lie for a time in the nest cup, sometimes scraping backwards with their feet and pulling the soft materials in toward the boundary of the cup, all of which helps form it into a clear rounded indention.
  • Behavior
    • The female may begin to exhibit increasing lethargy for a day or few days before oviposition.  She may stand in the nest or lie in the cup almost motionless for minutes or even hours.  This is a great tease, and it may or may not lead immediately to egg-laying.   Observable lethargy does not always occur.
    • In some instances she may be absent from the nest just before oviposition, flying in (probably from a nearby perch) at the last minute.
    • Just before oviposition she may seem restless, lying down and standing up, or circling the nest cup.  She may rearrange the nest materials, dig in them with her beak or scrape with her feet.
    • Her mate may bring her a gift of food in the hours or minutes preceding oviposition, which she will confiscate and usually mantle, and she may whine to communicate that he is to leave it for her.  He will concede.
Female at MN DNR nest, 1/28/17

THE MAIN EVENT

  • Body position
    • The female must be slightly elevated above the nest cup to allow enough room for the egg to come out.  She assumes a squatting or crouching position, either rising from a resting position or moving to the cup from another part of the nest.  This squatting is usually clear, but sometimes, if the cup is very deep or if she is already incubating an egg or two, it is difficult to detect any lifting of her body.
    • The body will be nearly parallel with the nest or at an angle of about 10-15 degrees with the tail low or flat on the nest.  Wings may be pulled in tight or may bulge out slightly.
    • She may look intently down into the nest cup in front of her.
  • Behavior
    • She may unfold and refold one wing and then the other over her back once or several times, and settle her feet into the cup.
    • She may release a small amount of wastes into the nest, clearing her cloaca for the egg.  The tail usually flips up slightly for this.
    • She may remain very still, seemingly unfocused, or she may look around even during contractions.
    • Her body feathers may fluff outward.
  • Contractions may be marked by any of the following:
    • Her upper back and shoulders may constrict clearly with each contraction.
    • Feathers on her nape, back, sides, and wings may shudder, lift, and/or fluff out with contractions.
    • Her wings may flex outward slightly with each contraction.
    • She usually toggles from foot to foot after each contraction.
    • Her tail may rise slightly with each contraction.
    • Her body may tip backward and forward slightly.
    • She may exhale soft whistles and/or chirps with each contraction.
    • She may lower down further into cup, head hunkered into her shoulders.
    • She may spread her wings at the elbow and appear to prop herself on them during contractions.
  • Visible contractions may last as briefly as 1-2 minutes or as long as 7-8 minutes.
  • Final push is always marked by an end of the contractions. It may also be marked by:
    • Shaking and shuddering of the entire body.
    • A final loud chirp or whistle.
    • A quick, sharp flip of the tail.
    • A dramatic jump and spreading up and/or outward or flapping of the wings.
    • Hardly any detectable movement at all, in which case the cessation of contractions is the only clear indication that the egg has emerged.
  • After the final push any of the following may occur:
    • She may become very still for several minutes.
    • She may rise, look around, or shake her head.
    • She may turn her head and quickly wipe her beak on her wing or scapular feathers.
    • She may look directly down into the cup where the egg lies.

POSTLUDE

  • She begins incubating after laying, but this may occur in one of several ways:
    • She may immediately step up and out of the nest cup and examine the new egg, nudge or roll it, lower herself and shimmy her brood patch onto the egg(s), and settle into incubating.
    • She may remain in a squat above the egg(s) for as long as 20-30 minutes, staying still or perhaps looking around before she steps aside to check the new egg.
    • She may stand aside or above the egg leaving it exposed for many minutes before incubating.
    • She may not step aside at all but immediately or after a few minutes simply lower herself over the cup and begin incubating, in which case the egg may not be seen for many minutes or even hours.

elfruler’s YouTube channel has dozens of videos of oviposition at eagle cams from 2011 to the present.

© elfruler and Donna Young 2018

 

 

LINKS TO STREAMING CAMS

The links here point to live streaming cameras of Bald Eagle nests.  All links open in a new browser tab.  Note: YouTube direct links seem to change almost daily, so here I have provided links to a cam operator’s channel, which should list their current Live Cams. Letters in [brackets] are codes I use to refer to the nests. See key and full list of nest codes here.

The list is current as of 9/17/2024, although some cams are offline until the breeding season begins. Please contact me (here) if you discover any broken links, incorrect information, cams coming back online, or new links not included here.

Alaska, Glacier Gardens, Juneau – Glacier Gardens [AK gla]
Glacier Gardens
YouTube channel

Arizona, Pleasant Lake – Arizona Game and Fish Department [AZ gfd]
GFD

British Columbia, Boundary Bay Central – Hancock Wildlife Foundation [BC bbc]
HWF with map
YouTube channel
HWF all nest links

British Columbia, Delta 2, Vancouver – Hancock Wildlife Foundation [BC dl2]
HWF 2 cams with map
YouTube east channel
YouTube west channel
HWF all nest links

British Columbia, Surrey, Surrey Reserve – Hancock Wildlife Foundation and Dawson & Sawyer [BC sur]
HWF 2 cams with map
YouTube north channel
YouTube south channel
HWF all nest links

British Columbia, White Rock, Boundary Bay – Hancock Wildlife Foundation [BC wht]
HWF 3 cams with map
YouTube overhead channel
YouTube north channel
YouTube wide angle channel
White Rock Eagles 3 cams with chat
HWF all nest links

California, Big Bear Lake, San Bernardino National Forest, Fawnskin – Friends of Big Bear Valley [CA bbl]
YouTube channel 2 cams with chat
Friends of Big Bear Valley

California, Folsom, Lake Natoma – Folsom Lake State Recreation Area, Friends of Lakes Folsom and Natoma (FOLFAN), & American Eagle Foundation [CA fol]
YouTube channel
AEF with chat

California, Redding, Turtle Bay – Friends of the Redding Eagles [CA red]
YouTube channel

California, Two Harbors, Catalina Island – Institute for Wildlife Studies [CA cTH]
IWS all nest links with chat
YouTube channel
Explore

California, West End, Catalina Island – Institute for Wildlife Studies [CA cWE]
IWS all nest links with chat
YouTube channel
Explore nest
Explore overlook

California, Bald Canyon, San Clemente Island – Institute for Wildlife Studies [CA mBC]
IWS all nest links with chat
YouTube channel
Explore (cam not yet linked)

California, Fraser Point, Santa Cruz Island – Institute for Wildlife Studies [CA zFP]
IWS all nest links with chat
YouTube channel
Explore

California, Sauces Canyon, Santa Cruz Island – Institute for Wildlife Studies [CA zSC]
IWS all nest links with chat
YouTube channel
Explore

Colorado, Fort St. Vrain – Xcel Energy Fort St. Vrain Station & Raptor Resource Project [CO fsv]
RRP 2 cams (chat on side cam)
Xcel YouTube channel with links to 2 cams

Delaware, Delaware Botanical Gardens, Dagsboro, DE – Delaware Botanical Gardens  [DE bot] NEW CAM FOR 2024-2025
HDOnTap with chat (2 views)
Delaware Botanical Gardens

Florida, Captiva/Sanibel Island – Lori Covert (ground observer), Window to Wildlife, & Clinic for the Rehabilitation of Wildlife (CROW)  [FL cap]
WtW YouTube channel (2 views)
Window to Wildlife with chat

Florida, central – property owner goes by SuperBeaks [FL cen]
YouTube channel with chat

Florida, Miami-Dade County – Zoo Miami, Wildlife Rescue of Dade County, & Ron Magill Conservation Endowment [FL mdc]
YouTube channel with chat

Florida, Northeast coastal – American Eagle Foundation [FL nef]
AEF 3 cams with chat (toggle “Other views” at bottom right)
HDOnTap 3 cams with chat (toggle “Other Views” at bottom right)
AEF YouTube channel with rewind

Florida, North Fort Myers – Dick Pritchett Real Estate, Inc. [FL swf]
Pritchett  with chat
YouTube channel

Florida, Pine Island – Portfolio Medics [FL pin] NEW CAM FOR 2024-2025
YouTube with chat

Florida, Southwest, Eagle Country ranch – EagleCountry.net [FL ece]
Eagle Country multi-view 4 cams (toggle to individual views at top right)
YouTube channel with chat

Florida, Vero Beach –Mullinax Ford [FL ver] NEW CAM FOR 2024-2025
YouTube with chat

Georgia, Berry – Berry College [GA ber]
Berry College 3 cams with chat (toggle button on menu at left)
Livestream cam 1
Livestream cam 2
Livestream approach cam

Indiana, South Bend, St. Patrick’s County Park – University of Notre Dame Linked Experimental Ecosystem Facility [IN ndl]
ND-LEEF
YouTube channel with chat

Iowa, Davenport – Arconic Davenport Works [IA dav]
Arconic
Livestream

Iowa, Decorah Fish Hatchery – Raptor Resource Project [IA dec]
RRP with chat
Explore
YouTube channel
Links to all RRP cams

Iowa, North Decorah – Raptor Resource Project [IA dnn]
RRP with chat
Explore
YouTube channel
Links to all RRP cams

Iowa, Urbandale, west of Des Moines – Denton Homes Iowa [IA urb]
YouTube channel

Kansas, Flint Hills – Farmer Derek Klingenberg’s Farm [KS der]
YouTube Channel with chat

Louisiana, Benton – Metro Aviation [LA ben]
YouTube

Louisiana, Kisatchie National Forest, Kincaid Lake, southwest of Alexandria, Nest E-1 – USDA Forest Service and Kisatchie National Forest [LA kf1]
YouTube channel

Louisiana, Kisatchie National Forest, Kincaid Lake, southwest of Alexandria, Nest E-3 – USDA Forest Service and Kisatchie National Forest [LA kf3]
YouTube channel

Maine, Piscataquis River – Wildlife of Maine Park [ME pis]
YouTube channel with chat

Maryland, Baltimore, Masonville Cove, Helen Delich Bentley Port of Baltimore – Masonville Cove Urban Wildlife Refuge Partnership [MD mas]
YouTube channel with chat

Maryland, Port Tobacco River Park, Charles County – Wild Streaming, Port Tobacco River Park, & Charles County, MD [MD tob]
Wild Streaming with chat
YouTube channel with chat

Michigan, Ann Arbor – Rob Staples’ neighborhood [Mi arb]
YouTube channel 4 cams split view

Michigan, Traverse City, West Bay, Grand Traverse Bay, Lake Michigan –private property owner, goes by Great Lakes Bald Eagle Cam [MI tvc] NEW CAM FOR 2024-2025
YouTube channel with chat

Minnesota, St. Paul – MN Department of Natural Resources [MN dnr] NEW CAM FOR 2024-2025
DNR
YouTube channel

Minnesota, Albert Lea – St. John’s Lutheran Community [MN alb]
YouTube channel

Montana, Miles City – Miles Community College, Yellowstone Valley Audubon, MT Fish, Wildlife, & Parks [MT mil]
AngelCam

New Jersey, Three Bridges, Hunterdon County – Conserve Wildlife Foundation of NJ, Public Service Electric & Gas, NJ Endangered & Nongame Species Program, & USFWS [NJ 3br]
YouTube Channel

New Jersey, Duke Farms, Hillsborough – Duke Farms Foundation [NJ duk]
Duke Farms
YouTube channel

New York, Long Island, Centerport – Bald Eagles of Centerport [NY ctr]
YouTube channel with chat

New York, Iriquois National Wildlife Refuge – Iriquois National Wildlife Refuge & PixCams [NY irq] NEW CAM FOR 2024-2025
YouTube channel with chat
PixCams with chat

Ohio, Avon Lake – Redwood Elementary School [OH avn]
YouTube channel 2 cams

Ohio, Cincinnati, Bortz Family Nature Preserve – Cardinal Land Conservancy, Duke Energy, Laura Carliss Co., LPA, & Louise Taft Semple Foundation [OH cin]
Cardinal Land Conservancy
HDOnTap with chat
YouTube channel with chat

Ohio, Little Miami Conservancy, Little Miami River between Cincinnati & Dayton – Little Miami Conservancy [OH lmc]
Little Miami Conservancy
YouTube channel with chat

Oklahoma, Bartlesville – City of Bartlesville & George Miksch Sutton Avian Research Center [OK brt]
Sutton
EarthCam

Pennsylvania, rural “Farm Country” – PA Game Commission & HDonTap [PA frm]
HDonTap 2 cams with chat (toggle “Other Views” at bottom right)

Pennsylvania, Hanover, Codorus State Park – Friends of Codorus State Park & PA Game Commission [PA han]
HDonTap 3 cams with chat (toggle “Other Views” at bottom right)

Pennsylvania, Pittsburgh, Hays neighborhood, Monongahela River – Audubon Society of Western Pennsylvania, Duquesne Power & Light, US Steel Irvin Plant, & PixCams  [PA pit]
PixCams with chat
YouTube channel

Pennsylvania, West Mifflin, suburb of Pittsburgh, Monongahela River – U.S. Steel Corp. Irvin Plant, Monongahela Valley Works & PixCams [PA wmf]
PixCams 4 cams
YouTube channel with chat

South Carolina, Dataw Island, on Saint Helena Island – Dataw Island Conservancy [SC dat] NEW CAM FOR 2024-2025
HDOnTap with chat

South Carolina, Hilton Head Island – Hilton Head Land Trust, Russell Patterson Law, & Hargray Communications [SC hil] NEW NEST & CAMS FOR 2024-2025
HDonTap with chat
Hilton Head Island

Tennessee, Bluff City, South Holston River – Dept. of Biological Sciences, East Tennessee State University [TN blf]
YouTube channel with chat
ETSU 2 cams

Tennessee, Johnson City, Boone Lake – Dept. of Biological Sciences, East Tennessee State University [TN jns] (nest tree fell in Hurricane Helene, platform & cameras placed in different tree)
YouTube channel with chat
ETSU 2 cams

Texas, Combine, John Bunker Sands Wetlands – John Bunker Sands Wetland Center [TX jbs]
YouTube Channel with chat

Virginia, Dulles Greenway Wetlands, Leesburg – Dulles Greenway Wetlands & Loudoun Wildlife Conservancy [VA dul]
YouTube channel with chat
Dulles Greenway with chat
HDOnTap with chat

West Virginia, Shepherdstown, National Conservation Training Center –Outdoor Channel, U.S. Fish & Wildlife Service National Conservation Training Center, & Friends of the National Conservation Training Center [WV shp]
Outdoor Channel 2 cams with chat
YouTube Channel

Wisconsin, Chippewa Falls – Heyde Center for the Arts [WI chp]
YouTube channel

Wisconsin, Trempealeau – private property owner & Raptor Resource Project [WI trm] NEW CAM FOR 2023-2024
RRP
YouTube channel

Inactive or offline in 2024-2025

British Columbia, French Creek Estuary, Vancouver – Hancock Wildlife Foundation [BC frn]
HWF cam with map

British Columbia, Harrison Mills – Sandpiper Resort & Hancock Wildlife Foundation [BC har]
HWF 2 cams with map
YouTube north channel
HWF all nest links

District of Columbia, Washington, National Arboretum – U.S. National Arboretum (USDA) & American Eagle Foundation [DC arb] (not streaming)
AEF (click on tabs at top for different views)
YouTube Cam A
YouTube Cam B
HDonTAP with chat (toggle “Other Views” at bottom right)

Tennessee, Dale Hollow Lake – Dale Hollow Lake Marina Operator’s Association & Friends of Dale Hollow Lake [TN dal] (not streaming)
YouTube channel

HATCH TIMINGS

This page presents data about times from egg-laying to hatch and  times between hatches recorded at wild Bald Eagle nest cams from 2006-2016.

The information here comes from JudyB’s charts, the Hancock Wildlife Forum, the Channel Islands EagleCAM Forum, nest cam websites, and my own observations. Scroll down for charts giving the raw data.

Determining the exact time of hatch is difficult for several reasons:
  • A hatch may not be visible on the cam because the camera angle may not give a clear view into the nest cup, or an incubating parent, another egg or nestling, or nesting material may obscure the view.
  • The online video stream may not have an embedded timestamp, or if present it may not be accurate.
  • Most experts agree that a hatch has occurred when a chick is completely free of the shell, but some observers may record a time when the chick is only partially out of the shell.

Thus time of hatch may be approximate, or reported times may vary from one viewer to another. I have attempted to restrict my analysis to data that appear to be as reliable as possible, which includes dates and times for 44 breeding seasons at 31 nests with 2-egg clutches (N=44), and 28 breeding seasons at 20 nests with 3-egg clutches (N=28). In my judgment the overall analysis is plausible given the size of the sampling and consistency among the data.

General observations

Time from egg-laying to hatch among all 72 clutches:

  • The average time was exactly 36.5 days (36 days 12 hours).
  • The shortest time was about 34.5 days (34 days 11 hours 1 minute).
  • The longest time was about 40.5 days (40 days 12 hours 17 minutes).

The interval between egg-laying and hatch almost always decreases for successive hatches in a clutch.

In 2-egg clutches, Egg 1 takes on average about 1.4 days longer to hatch than Egg 2:

In 3-egg clutches, Egg 1 takes on average almost 2 days longer to hatch than Egg 2 and almost 2.5 days longer to hatch than Egg 3:

The data also show that it takes longer for an eagle to lay a complete clutch of eggs (click here for data on egg-laying) than it does for those eggs to hatch:

The reason for these differences can be found in the incubation behavior of the parents.
  • Bald Eagles’ eggs hatch successively in the order in which they were laid, called asynchronous hatching.
  • The first chick to hatch will be larger and more developed than its sibling(s), which gives it an advantage in the competition for food in the nest.
  • To help mitigate this disparity, the parents usually incubate the first egg intermittently until the second egg is laid, which slows the development of the embryos of the earlier eggs.
  • This phenomenon is sometimes referred to as “delayed incubation,” but the parents usually do incubate most of the time, so it is actually the hatching that is delayed. A better description of the behavior might be “intermittent incubation.” Scientists use the term parental attentiveness in reference to the amount of time parents devote to incubating.
  • Parents gradually increase their attentiveness until the clutch is complete.
  • Since later eggs are incubated more consistently than earlier eggs, their development progresses more quickly and they hatch in less time.

Another way to see the effects of parental attentiveness on different eggs in a clutch is to consider the intervals between hatches.  In 3-egg clutches, because Hatch 1 is usually delayed and Hatch 3 is not, the interval between Hatches 1 and 2 is considerably shorter than the interval between Hatches 2 and 3:

  • The shortest hatch-to-hatch time on record was 3 hours 57 minutes between Hatch 1 and Hatch 2 in a 3-egg clutch.
  • The longest hatch-to-hatch time on record was 99 hours 42 minutes between Hatch 2 and Hatch 3 in a 3-egg clutch.
Geography does not appear to play a predictable role in incubation behavior.
  • One might expect that eggs in colder northern climates would be incubated more regularly than those in warmer southern climates. The data do not bear this out.
  • Longer incubation times for first eggs, indicating partial parental attentiveness, occur in both colder climates including Pennsylvania, Maryland, Minnesota, Iowa, and British Columbia, and in warmer climates including Florida, Virginia, and the Channel Islands off the California coast.
  • Conversely, shorter incubation periods for first eggs, indicating close to full parental attentiveness, occur both at nests in colder climates, including Wisconsin, Minnesota, Iowa, New Jersey, and British Columbia, and in warmer climates, including Florida, Virginia, and the Channel Islands.
  • But local climate is not solely a function of latitude. Other conditions such as elevation above sea level, proximity to a large body of water like an ocean or lake, precipitation and humidity, rural or urban habitat, and changeable weather conditions from year to year certainly affect both the dates of egg-laying and the incubation behavior of parents.
The charts below show the information collected.

You can sort on a column by clicking its heading. Nest codes used here are listed in this chart. All times are local nest time and are given in 24-hour format without a colon (0000 = midnight, 1200 = noon).

Abbreviations:
h = hour
m = minute
d = day
< = before or by (not included in calculations)
~ = approximately (not included in calculations)
strikeout = unhatched eggs
italics = nestlings that died before fledge

2-Egg Clutches

NESTEGG 1Time E1-H1HATCH 1EGG 2Time E2-H2HATCH 2Time H1-H2
WI e4k3/2/12 163035d12h49m4/7/12 06193/5/12 185435d10h26m4/10/12 062003d00h01m
BC har3/24/15 204835d19h22m4/29/15 16103/28/15 180935d08h25m5/3/15 023403d10h24m
BC har4/4/13 201035d21h50m5/10/13 18004/8/13 194435d18h55m5/14/13 1439
starv		03d20h39m
BC har4/3/16 202135d22h06m5/9/16 18274/7/16 170735d18h04m5/13/16 111103d16h44m
FL nef11/14/13 134236d02h44m12/20/13 162611/17/13 134835d10h21m12/23/13 000802d7h42m
FL nef11/16/15 134936d03h29m12/22/15 171811/19/15 164235d02h09m12/24/15 185102d1h33m
FL swf11/17/13 163736d07h09m12/23/13 2346
infection?		11/20/13 181834d16h03m12/25/13 102101d10h35m
FL swf11/26/12 134436d09h00m1/1/13 224411/29/12 1838<35d03h16m1/3/13
2149-2154
CA zSC3/2/12 184136d10h51m4/8/12 06323/6/12 0026<4/11/12 am
MN bnd3/1/16 174036d13h40m4/7/16 08203/4/16 182635d11h06m4/9/16 063201d22h12m
CA zPH2/24/08 132936d16h46m4/1/08 07152/27/08 17004/2/08 1615?
or <4/3/08 0631
CA trt2/12/11 164936d16h54m3/21/11 10432/15/11 164536d17h30m3/24/11 111503d00h32m
BC hrn3/22/11 194436d17h55m4/28/11 13393/26/11 152336d00h28m5/1/11 155103d02h12m
WI val3/27/14 160836d18h52m5/3/14 1100
GHOW pred		3/30/14~1950~35d23h40m5/5/14 193002d08h30m
FL swf11/19/14 140736d22h52m12/26/14 125911/22/14 161634d19h12m12/27/14 112800d22h29m
BC wht3/13/12 153236d23h26m4/19/12 14583/16/12~2010~35d17h30m4/21/12 134001d22h42m
VA ccb2/8/12 174437d00h46m3/16/12 19302/11/12 181735d12h14m3/18/12 073101d12h01m
MD blk1/26/08 155037d01h47m3/3/08 17371/29/08 or
1/30/08<0656		3/5/08 063301d12h56m
IA dav2/7/13 143037d02h10m3/16/13 17402/10/13 18083/18/13 am
TN har2/18/15 170437d02h30m3/27/15 20342/21/15 182636d11h52m3/30/15 071802d10h44m
BC sid3/4/10 183537d10h45m4/11/10 06203/7/10 1956
raven pred
CA cTH2/17/11 203137d10h53m3/27/11 08242/21/11 2323
broke
CA cTH2/17/10 184637d12h29m3/27/10 08152/21/10 190034d23h43m3/28/10 194301d11h28m
TN har2/10/13 174537d12h38m3/20/13 07232/13/13 1856~35d22h04m3/21/13~1800
ME br23/8/10 160637d12h47m4/15/10 05533/11/10 1807?35d20h29m?4/16/10 153601d09h43m
BC wht3/13/15 165737d13h28m4/20/15 06253/16/15 2019<35d11h37m4/21/15<0756
FL swf
new M		12/19/15 162537d14h58m1/26/16 072312/22/15 174036d04h59m1/27/16 223901d15h16m
CA hum3/17/15 150337d16h50m4/24/15 07533/20/15~1930~36d04h44m4/26/15 001401d16h21m
NC jor2/28/14 185837d18h04m4/7/14 14023/4/14 18414/9/14 pm?
CA cWE2/28/09 173337d18h48m4/7/09 13213/3/09 1758~35d21h42m4/8/09 1640?
WV shp2/6/13 180937d19h29m3/16/13 14382/9/13 181535d20h18m3/17/13 153301d00h55m
WV shp
new M		2/5/12 173437d21h04m3/14/12 15382/8/12 2311<36d07h09m<3/16/12 0720
NC crc1/18/12 160737d21h05m2/25/12 13121/21/12 pm?
no hatch
MD blk1/11/12 144437d21h38m2/18/12 1222
ad BAEA pred		1/14/12 16092/18/12 pm
ad BAEA pred
GA ber1/6/15 170037d23h27m2/13/15 16271/9/15 1906<36d11h10m2/15/15<0616
TN har1/27/16 175238d01h38m3/5/16 19301/30/16 185535d21h36m3/6/16 163100d21h01m
GA ber1/7/16 172838d02h6m2/14/16 19341/10/16 191836d18h54m2/16/16 141201d18h38m
BC dl33/1/15 142338d02h8m4/8/2015 17313/4/15 1427
no hatch
BC wht3/13/11 163138d02h23m4/20/11 18543/16/11 202736d00h41m4/21/11 210801d02h14m
BC dl23/28/14 160838d05h35m5/5/14 21433/31/14 155735d18h40m5/6/14 103700d12h54m
CA cWE2/11/16 224238d11h36m3/21/16 11182/15/16 184236d12h45m3/23/16 082701d21h09m
BC dl2
new nest		3/10/16 155538d13h26m4/18/16 06213/13/16 133536d06h30m4/18/16 200500d13h44m
PA han2/18/16 151339d03h12m3/28/16 19252/21/16 1615
no hatch
TN jns2/10/16 070839d04h57m3/20/16 13052/13/16 1710?
FL nef11/16/14 1252~36d19h43m12/23/14~083511/19/14 132636d01h30m12/25/14 1456
NJ duk2/18/16 1615~36d23h45m3/26/16~17002/21/16 173535d14h6m3/28/16 0841
CA zPH2/25/10 1736<36d12h58m<4/3/10 07342/28/10 145235d19h27m4/5/10 1119
ME br13/16/14 1355<37d16h09m<4/23/14 06043/19/14 170535d23h37m4/24/14 1642
starv?
MT lib3/16/09 1825<38d12h55m<4/24/09 07203/20/09 165036d17h34m4/26/09 1024
BC dl23/3/11 am4/11/11 13133/6/11 111036d22h4m4/12/11 1014
CA cTH2/19/12 1831< 3/26/12
hatch fail?		2/22/12 202735d13h14m3/29/12 1041
fox pred
BC dl2~3/3/12< 4/11/12 06153/6/12 171735d17h12m4/11/12 1129
CA zPH
new nest		3/6/12 1358
broke		3/9/12 140135d02h6m4/13/12 1707
CA cTH2/15/13 2257
broke		2/18/13 220034d17h28m3/25/13 1628
BC dl23/7/13 pm
no hatch		3/10/13 153436d21h10m4/16/13 1244
GA ber1/14/14 1512
no hatch		1/17/14 190135d16h19m2/22/14 1120
CA cWE
new F		2/23/15 1633
broke		2/26/15 193337d11h29m4/5/15 0802
infection
© elfruler 2017

3-Egg Clutches

NESTEGG 1Time E1-H1HATCH 1EGG 2Time E2-H2HATCH 2EGG 3Time E3-H3HATCH 3Time H1-H2Time H2-H3Time H1-H3
IA dav2/11/12 145336d15h32m3/19/12 07252/14/12 1333~35d19h03m3/21/12~09362/17/12 163535d16h55m3/24/12 103005d03h05m
MN bnd3/7/14 164936d21h22m4/13/14 15113/10/14 182835d23h48m4/15/14 18163/13/14 220036d08h20m4/19/14 0620
weather		02d03h05m03d12h04m05d15h09m
IA dnn3/11/16 142936d21h50m4/17/16 13193/14/16 1428<35d13h51m4/19/16 <0419
poison		3/18/16 084136d00h33m4/23/16 091405d19h55m
IA dec2/25/10 192636d23h07m4/3/10 19332/28/10 221335d16h58m4/5/10 16113/5/10 18504/9/10 1730 or
4/10/10 0834		01d20h38m
IA dec2/18/15 180737d00h32m3/27/15 19392/21/15 190135d12h15m3/29/15 08162/25/15 185735d13h46m4/2/15 094301d12h37m04d01h27m05d14h04m
IA dec2/23/11 173337d08h06m4/2/11 02392/26/11 184235d11h20m4/3/11 07023/2/11 184734d11h01m4/6/11 064801d04h23m02d23h46m04d04h09m
BC sid3/1/09 171137d13h01m4/8/09 0712<3/5/09 07174/10/09 07593/8/09?4/14/09 075602d00h47m03d23h57m06d00h44m
IA dec2/23/14 165537d15h27m4/2/14 09222/26/14 173336d04h56m4/3/14 23293/2/14 184335d15h02m4/7/14 104501d14h07m03d11h16m05d01h23m
BC sid3/7/11 154437d17h04m4/14/11 09483/10/11 164836d13h38m4/16/11 07263/14/11 180436d00h12m4/19/11 181601d21h38m03d10h50m05d08h28m
NJ duk2/28/11 140037d17h04m4/7/11 08043/3/11 150936d15h05m4/9/11 07143/6/11 <161601d23h10m
CA trt2/6/15 153637d17h25m3/16/15 10012/9/15 154436d01h27m3/17/15 18112/12/15 165635d13h00m3/20/15 065601d08h10m02d12h45m03d20h55m
MN dnr1/25/16 151837d17h32m3/3/16 08501/28/16 134536d05h49m3/4/16 19341/31/16 1632~35d13h48m3/7/16~062001d10h44m
VA nbg2/3/11 144937d19h36m3/13/11 11252/6/11 162536d18h35m3/15/11 12002/9/11 175535d18h35m3/17/11 133002d00h35m02d01h30m04d02h05m
BC laf3/13/13 161037d20h26m4/20/13 12363/16/13 162535d22h27m4/21/13 14523/19/13 162634d20h11m4/23/13 1237
hatch fail		01d02h16m
VA riv2/16/16 152937d21h11m3/25/16 13402/19/16 152536d02h26m3/26/16 18512/23/16 <2330
no hatch		01d05h11m
CA trt2/15/10 161538d02h07m3/25/10 19222/18/10 163236d00h33m3/26/10 18052/21/10 184535d13h10m3/29/10 085500d22h43m02d14h50m03d13h33m
BC wht3/16/14 163738d12h17m4/24/14 04543/19/14 184035d21h49m4/24/14 16293/22/14 2003
accid broke		00d11h35m
MN dnr2/14/14 150038d14h47m<3/25/14 0647
injury		~2/17/14<3/26/14 1716~2/20/143/30/14 0630
IA dec2/17/12 194738d16h33m3/27/12 13202/20/12 210636d10h44m3/28/12 08502/24/12 200535d06h10m3/31/12 031500d19h30m02d18h25m03d13h55m
BC laf3/15/14 163938d18h27m4/23/14 11063/18/14 1600
hatch fail		3/21/14~1900~35d13h28m4/26/14 082802d201h22m
WV shp2/17/14 175438d20h56m3/28/14 15502/20/14 1819<36d12h53m< 3/29/14 0812<2/24/14?05454/1/14 am
CA trt2/6/09 180038d21h09m3/17/09 16092/9/09 171737d13h26m3/19/09 07432/13/09 pm or 2/14/09 am3/22/09 am01d15h34m
VA nbg2/10/09 162538d21h41m3/21/09 15062/13/09 170536d14h43m3/22/09 08482/17/09 120835d18h03m3/25/09 071100d17h42m02d22h23m03d16h05m
VA nbg1/31/10 141438d22h56m3/11/10 13102/3/10 1150~37d12h11m~3/13/10 00012/6/10 1229~35d11h32m~3/14/10 0001
OH snd2/27/16 165039d15h07m4/7/16 08573/1/16~1800~39d13h05m4/10/16 08053/4/16<21294/15/16~111502d23h08m
CA cWE2/23/11 174139d17h16m4/4/11 11572/26/11 1840<37d10h59m?4/5/11<06393/2/11 2030?<35d08h49m?4/7/11<0619
CO fsv2/14/15 181739d18h16m3/26/15 13332/17/15 181736d22h13m3/26/15 1730
weather		2/20/15 195836d10h54m3/29/15 075200d03h57m02d14h22m02d18h19m
CA cWE2/22/13 181040d12h17m4/4/13 07272/25/13 205737d17h32m4/4/13 15293/1/13 1655<36d12h20m4/7/13<061500d08h02m
PA pit2/19/14 1645~36d20h51m3/28/14~14362/22/14 161835d13h59m3/30/14 07172/25/14 183935d21h15m4/2/14 165403d09h37m
OK seq12/17/11~1600~37d18h47m1/24/12 104712/20/11 162935d20h27m1/25/12 125612/23/11 pm1/29/12 1638
weather		01d02h09m04d03h42m05d05h51m
CO fsv<2/17/09 0658~37d22h41m3/27/09 0539
weather?		2/19/09 180035d21h00m3/27/09 1600
weather?		<2/24/09 06203/31/09 1549
weather?		00d10h21m03d23h49m04d10h10m
NJ duk2/17/14 1534~39d13h26m3/29/14~06002/20/14~1400~36d18h01m3/29/14 09012/23/14 164636d13h08m4/1/14 065402d21h53m
CO fsv2/16/16~2100~40d11h23m3/28/16 0923
weather		2/19/16 2017~37d21h43m3/28/16~1900
weather		2/23/16 184736d12h58m3/31/16 0845
weather		02d23h22m
CO fsv2/17/13 1809<39d11h12m3/29/13<06212/20/13 175436d18h01m3/29/13 1255
weather		2/23/13 184636d19h01m4/1/13 1447
weather		03d01h52m
CA cWE2/18/12 2024<39d14h04m3/29/12<11282/22/12 183736d10h58m3/30/12 06352/26/12 1752~36d11h28m4/3/12~0620
WV shp<2/4/08 09173/13/08 02392/6/08 200036d10h25m3/14/08 07252/10/08 053036d00h22m3/17/08 065201d04h46m02d23h27m04d04h13m
VA nbg
clutch 2		3/16/08
broke		3/19/08
broke		3/22/08 122036d03h18m4/27/08 1538
CA zSC2/1/16 <1409
broke		2/4/16<19013/12/16 am2/7/16 181935d16h27m3/14/16 1146
OH avn2/26/16 20454/5/16 am3/1/16 183536d15h22m4/7/16 1057<3/5/16 am4/11/16 am
© elfruler 2017

© elfruler 2016, 2017

EGG-LAYING TIMINGS

The Bald Eagle nest cams from 2007-2016 are giving us priceless new information about breeding in the wild (as opposed to captivity). We now have some real numbers to crunch. Data about egg-laying collected from JudyB’s charts, the Hancock Wildlife Forum, the Channel Islands EagleCAM Forum, nest cam websites, and my own observations give reliable dates and times for 61 breeding seasons at 29 nests with 2-egg clutches, 38 seasons at 21 nests with 3-egg clutches, and 1 nest with a 4-egg clutch. Only nests with precise timings are included in this report.

Three charts below show the information collected. You can sort on a column by clicking its heading. Nest codes used here are listed in this chart. All times are local nest time and are given in 24-hour format without a colon (0000=midnight, 1200=noon).

Abbreviations:
h = hour
m = minute
d = day
< = before or by (not included in calculations)
~ = approximately (not included in calculations)

These data yield some general findings:

  • Most first eggs of a clutch are laid between about noon and midnight.
  • The eggs come at roughly 3-day or 4-day intervals. This is consistent with afternoon-evening layings: if another egg is not laid by the end of the third day, it is likely to come about a day later.
  • The shortest interval between one egg and the next was 68h40m or just under 3d (BC dl2 in 2016). The longest interval was 116h37m or nearly 5d (IA dec in 2010).
  • The interval from one egg to the next almost always increases with subsequent layings.
  • A particular female generally has a consistent pattern of timings. Some tend to lay eggs at 3-day intervals, others at 4-day intervals. A particular female also tends to lay at roughly the same time of day from year to year, although there are exceptions. If you sort on the Nest column in each chart you can observe the patterns and inconsistencies. See comments below the charts for further details.

2-egg clutches

  • About 82% (50 of 61 breedings) had a 3-day interval from the first to the second egg, with a mean average time of 73h21m.
  • About 18% (11 of 61 breedings) had a 4-day interval from the first egg to the second, with a mean average time of 94h34m.
  • Most females are consistent 3-day or 4-day layers from year to year (sort on the Nest column).

NESTEGG 1Time E1-E2EGG 2
BC dl23/10/16 155568h40m3/13/16 1335
CA cTH2/27/15 205769h12m3/2/15 1809
CA zPH2/25/10 173669h16m2/28/10 1452
PA han2/14/15 175470h51m2/17/15 1645
CA cTH2/15/13 2257
broke		71h3m2/18/13 2200
CA zSC2/24/13 2030
ravens pred		71h27m2/27/13 1957
ravens pred
BC dl23/28/14 160871h49m3/31/14 1557
VA riv2/12/15 1724
hatch fail		71h54m2/15/15 1718
CA trt2/12/11 164971h56m2/15/11 1645
CA zPH
new nest		3/6/12 1358
broke		72h3m3/9/12 1401
BC dl33/1/15 142372h4m3/4/15 1427
no hatch
WV shp2/6/13 180972h6m2/9/13 1815
FL nef11/14/13 134272h6m11/17/13 1348
TN har2/4/14 183572h21m2/7/14 1856
CA cWE2/28/09 173372h25m3/3/09 1758
WI e4k3/10/16 1557
juv BAEA pred		72h27m3/13/16 1724
juv BAEA pred
VA ccb2/8/12 174472h33m2/11/12 1817
FL nef11/16/14 125272h34m11/19/14 1326
BC dl23/10/15 1543
incub fail		72h38m3/13/15 1621
incub fail
MN bnd3/1/16 174072h46m3/4/16 1826
TN pgf
F & I		3/23/13 195572h51m3/26/13 2046
PA han2/18/16 151373h2m2/21/16 1615
no hatch
TN har1/27/16 175273h3m1/30/16 1855
TN har2/10/13 174573h11m2/13/13 1856
FL swf
new M		12/19/15 162573h15m12/22/15 1740
NJ duk2/18/16 161573h20m2/21/16 1735
BC sid3/4/10 183573h21m3/7/10 1956
raven pred
OR des3/24/10 2010
weather		73h21m3/27/10 2131
weather
TN har2/18/15 170473h22m2/21/15 1826
MD blk1/11/12 144473h25m1/14/12 1609
FL swf11/17/13 163773h41m11/20/13 1818
MN bnd2/28/15 155873h50m3/3/15 1748
GA ber1/7/16 172873h50m1/10/16 1918
CA cTH2/19/12 183173h56m2/22/12 2027
GA ber1/6/15 170074h6m1/9/15 1906
FL swf11/19/14 140774h9m11/22/14 1616
WI e4k3/2/12 163074h24m3/5/12 1854
MN bnd3/9/13 155274h30m3/12/13 1922
FL nef11/16/15 134974h53m11/19/15 1642
CA cWE
new F		2/23/15 1633
broke		75h2/26/15 1933
CA zPH2/25/09 142375h8m2/28/09 1731
ME br13/16/14 135575h10m3/19/14 1705
BC wht3/13/15 165775h22m3/16/15 2019
CA zPH2/24/08 132975h31m2/27/08 1700
IA dav2/7/13 143075h38m2/10/13 1808
GA ber1/14/14 1512
no hatch		75h49m1/17/14 1901
BC wht3/13/11 163175h56m3/16/11 2027
FL swf11/26/12 134476h54m11/29/12 1838
WV shp2/5/12 173477h37m2/8/12 2311
CA zSC3/2/12 184177h45m3/6/12 0026
BC hrn3/22/11 194491h39m3/26/11 1523
CA cWE2/11/16 224292h2/15/16 1842
BC har4/3/16 202192h46m4/7/16 1707
BC har3/24/15 204893h21m3/28/15 1809
MT lib3/16/09 182594h25m3/20/09 1650
CA cTH2/25/16 2206
no hatch		94h52m2/29/16 2058
no hatch
BC hrn3/30/14 2019
no hatch		94h53m4/3/14 1912
BC har4/4/13 201095h34m4/8/13 1944
NC jor2/28/14 185895h43m3/4/14 1841
CA cTH2/17/10 184696h14m2/21/10 1900
CA cTH2/17/11 203198h52m2/21/11 2323
broke
© elfruler 2017

3-egg clutches

  • About 92% (35 of 38 breedings) had a 3-day interval from the first to the second egg, with a mean average time of 72h16m.
  • About 8% (3 of 37 breedings) had a 4-day interval from the first egg to the second, with a mean average time of 93h57m. (Note the contrast with 2-egg clutches, where a larger percentage (18%) had a 4-day interval.)
  • The proportion between 3-day intervals and 4-day intervals from the second to the third egg is more balanced than with 2-egg clutches:
    • About 55% (16 of 29 nests with precise timings) had a 3-day interval, with a mean average time of 74h12m.
    • About 45% (13 of 29 nests) had a 4-day interval, with a mean average time of 96h46m.
  • As with 2-egg clutches, eagles at particular nests usually had a consistent pattern of laying from year to year (sort on the Nest column), either 3 days both between first and second eggs and between second and third eggs, or 3 days between first and second egg and 4 days between second and third eggs. Where the pattern is not consistent, possible reasons may be a change of mate (as at CA trt in 2014), inclement weather, intruders, etc.
  • The 2010 season at IA dec was unusual in that the interval between the second and third eggs, 116h37m, was  more than 20 hours longer than the next longest interval, at CA trt in 2013 (97h58m).  If the third egg-laying at IA dec in 2010 is excluded from the calculations, about 57% (16 of 28 breedings) had a 3-day interval, with a mean average time of 74h12m, and 43% (12 of 28 breedings) had a 4-day interval, with a mean average time of 94h55m.

NESTEGG 1Time E1-E2EGG 2Time E2-E3EGG 3
VA nbg1/31/10 141469h36m2/3/10 115072h39m2/6/10 1229
CA trt
new M		2/6/13 1714
disappeared		70h20m2/9/13 153497h58m2/13/13 1732
MN dnr1/25/16 151870h27m1/28/16 134574h47m1/31/16 1632
IA dav2/11/12 145370h40m2/14/12 133375h2m2/17/12 1635
IA dnn3/11/16 142970h59m3/14/16 142890h13m3/18/16 0841
CA trt2/6/09 180071h17m2/9/09 17172/13/09 pm
or 2/14/09 am
IA dec2/18/16 202871h18m2/21/16 194695h13m2/25/16 1859
infertile
BC laf3/15/14 163971h21m3/18/14 1600
hatch fail		~75h3/21/14~1900
PA pit2/19/14 164571h33m2/22/14 161874h21m2/25/14 1839
CO fsv2/17/13 180971h45m2/20/13 175472h52m2/23/13 1846
IL umr
1F & 2M		2/1/16 155771h48m2/4/16 154575h4m2/7/16 1849
VA riv2/16/16 152971h56m2/19/16 15252/23/16<2330
no hatch
CO fsv2/14/15 181772h2/17/15 181773h41m2/20/15 1958
MD blk1/13/14 1623
no hatch		72h5m1/16/14 1628<1/20/14 0847
TN pgf
I & J		2/18/16 0823
no hatch		72h7m2/21/16 0830
no hatch		96h50m2/25/16 0920
no hatch
CA trt2/6/15 153672h8m2/9/15 154473h12m2/12/15 1656
TN pgf
F & I		3/21/16 1911
no hatch		72h14m3/24/16 1925
no hatch		< 3/29/16 0730
no hatch
BC laf3/13/13 161072h15m3/16/13 162572h1m3/19/13 1626
CA trt2/15/10 161572h17m2/18/10 163274h13m2/21/10 1845
OK snr2/6/09 1547
no hatch		72h23m2/9/09 1610
no hatch		2/13/09 pm
WV shp2/17/14 175472h25m2/20/14 1819<2/24/14?0545
WV shp2/12/15 153972h27m2/15/15 160674h19m2/18/15 1825
IA dec2/23/14 165572h38m2/26/14 173397h10m3/2/14 1843
MN bnd3/7/14 164972h39m3/10/14 182875h32m3/13/14 2200
VA nbg2/10/09 162572h40m2/13/09 170591h3m2/17/09 1208
IA dec2/18/15 180772h54m2/21/15 190195h56m2/25/15 1857
CA cWE2/23/11 174172h59m2/26/11 184097h50m?3/2/11 2030?
BC sid3/7/11 154473h4m3/10/11 164896h16m3/14/11 1804
IA dec2/23/11 173373h9m2/26/11 184296h5m3/2/11 1847
NJ duk2/28/11 140073h9m3/3/11 15093/6/11 <1616
IA dec2/17/12 194773h19m2/20/12 210694h59m2/24/12 2005
VA nbg2/3/11 144973h36m2/6/11 162573h30m2/9/11 1755
BC wht3/16/14 163774h3m3/19/14 184073h23m3/22/14 2003
accid broke
CA cWE2/22/13 181074h47m2/25/13 205791h58m3/1/13 1655
IA dec2/25/10 192674h47m2/28/10 2213116h37m
4d20h37m		3/5/10 1850
TN pgf
F & I		3/27/15 2041
no hatch		93h47m3/31/15 1828
no hatch		71h7m4/3/15 1735
no hatch
OH avn2/26/16 204593h50m3/1/16 1835<3/5/16 am
CA cWE2/18/12 202494h13m2/22/12 183795h15m2/26/12 1752
WV shp<2/4/08 09172/6/08 200081h30m2/10/08 0530
© elfruler 2017

4-egg clutches

  • These are rare among Bald Eagles. From 2006-2016 only three nests with 4 eggs have been documented by direct observation (WV shp in 2010, MT cfr in 2011, and OK snr in 2013). Of those only WV shp has precise observed timings.

NESTEGG 1Time
E1-E2		EGG 2Time
E2-E3		EGG 3Time
E3-E4		EGG 4
WV shp2/2/10 2215
weather		87h57m2/6/10 1412
weather		75h34m2/9/10 1746
disappeared		92h56m2/13/10 1442
© elfruler 2017

  • The first three of the eggs were lost in snowstorms; the fourth ended in a successful fledge. Note that the timings between eggs alternate between 4-day and 3-day intervals.

© elfruler 2016, 2017. Revised 1/5/17

BREEDING

This menu item tracks the stages of the Bald Eagle breeding season from the formation of a pair bond through the incubation and hatching of eaglets.  Pages will cover (links are added as pages are published):

Click here for pages about eaglet growth and development from hatch through fledge.  (See Menu item “Eagle Growth.”)

© elfruler 2017-2018

BALD EAGLE NEST CAMS

The pages I have published under this tab (Bald Eagle Nest Cams) as well as elsewhere on the website, present information I have gathered from  Judy Barrows’ charts, the Hancock Wildlife Forum, the Channel Islands EagleCAM Forum, the Hornby Eagle Group Projects Society forum, individual nest cam websites, numerous Facebook pages and groups, and my own observations.

Here you will find: