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EAGLET DAILY PHOTOS AND MILESTONES
© elfruler 2019-2020
Click here for an Introduction to this page.
Photos here are from the 2019 breeding season in a Bald Eagle’s nest in Bluff City, TN, broadcast live by East Tennessee State University, and are used by permission. Click here for link to the live cam. The focus here is on the elder of two eaglets, BC14. Many thanks to Michelle France and Donna Young for helping to collect the screen captures and tell the story.
Measurements are derived or calculated mainly from Bortolotti 1984a, 1984c, and 1984d, and Gerrard and Bortolotti 1988 (Click here for References). Numbers given here are in the ball park but will vary from one eaglet to another. % indicates the proportion a particular measurement bears to its value at the juvenile’s full size (Click here for information about taking measurements.)
BC14 hatched at 10:32 a.m. on 3/11/19. The cam provided a rare bird’s-eye (pun intended) view of how an eaglet uncurls itself from inside the egg in the first few seconds of hatching.
I’ve slowed the stream to 10% of normal speed and added arrows to indicate the back, head, left wing, right wing, beak, tail, legs and feet, egg tooth (yes! the egg tooth!), umbilicus, and receding yolk sac of the hatchling. The eaglet has its back to us and its head is down, tail up.
In the weekly galleries below, click on photos for larger views and to scroll through images. The numbering of Days refers to the age of BC14; Day 0 is Hatch Day, Day 1 is 24 hours after hatch, etc.
The eaglet is hatched with pink skin covered by a thin layer of light gray natal down. The beak and cere are gray, facial skin is dark, legs and feet are pinkish-cream-colored, eye ring is dark and protruding. The eaglet is weak, with limited mobility, balance, and vision.
| HEIGHT | WEIGHT | BEAK LENGTH | FOOT PAD | ||
|---|---|---|---|---|---|
| Day 0 |
Skin color changes from pink to bluish-gray, feet and legs are cream-colored; cere turns from pale gray to pale yellow. Steadily gains strength and balance through the week, gaining ability to take food, “swim” on belly with wings and legs, escape from nest cup. Eyes focusing better, seeking out and imprinting on parents. Sibling competition begins. Natal down and egg tooth remain through the week.
| Day 6 |
Skin around eyes becomes lighter gray. Supraorbital ridge more prominent by mid-week. Egg tooth disappears. Cere turns gray, rictus of mouth pale yellow. Feet are beginning to grow rapidly; talons are beige and growing. Second down appears as early as Day 6. Juvenal contour feathers (pin feathers) start to emerge on wings, back, and legs by week’s end; male’s remiges emerge earlier than female’s. (The contour feathers emerge from the natal down follicles and push the natal down out as they grow.) Facial bristle feathers emerging around eyes and beak. Reaching out for food more actively; backing up to nest edge to slice; walking sturdily on hocks; preening, stretching, flapping, and scooting as far out as the nest rails.
| Day 13 |
Growth spurt begins and size differences between male and female develop, especially in weight, beak length, foot pad, and 8th primary feather. Females are larger, but males grow earlier and more quickly. By mid-week, female is gaining 70-180g per day, male 80-150g per day. Beak and feet growing rapidly. Talons turn from beige to black. Second down thickens, approaching ability to thermoregulate. Some second down is growing on the front of the upper region of the tarsometatarsi. Wing primary and covert feathers lengthen, pushing out natal down at the tips; primaries grow a little over 7mm per day. Contour feathers emerge on back, shoulders, legs, and breast. Rectrices start to emerge by mid-week. Beginning to stand on toes briefly. Stretching and rousing, pellet casting, resting on the rails.
| Day 20 | (F 57%, M 61%) |
Growth spurt continues. Remiges are increasingly measurable. Rictus of mouth is yellow. Second down covers entire body except on top of head where natal down layer is still prominent (resulting in that famous “mohawk” look). Head, neck, and side contour feathers coming in. Pecking at food, grasping and playing with nesting materials, wobbly toe-walking by the end of the week. Sibling competition transitions into play.
| Day 27 | M 115mm (87%) | ||||
| Day 27-33 |
Energy demands for metabolism and growth peak by Day 30, then weight gain tapers off. Feet nearly full size by week’s end and are turning yellow. Stretching, flapping, preening, and more confident toe-standing and -walking. Standing on nest twigs as if on a branch, practicing holding with toes and talons. Parents hold food further away to encourage reaching; eaglet may lunge for food and attempt to tear off bites with beak; hasn’t yet mastered the skill of holding food down with feet.
| Day 34 | M 3000g (72%) | M 27mm (87%) | M 129 (98%) | |
| Day 32-38 | M 110-116mm |
At week’s end the eaglet is about 3/4 of full weight. Feet are nearly full grown. Toes and tarsometatarsi fully grown (making banding possible). Legs, feet, lores, and rictus of mouth are yellow. Stretching and flapping, standing securely, grabbing and mantling food as it is delivered, tearing food more effectively. Grasping and playing with nest materials with talons and beak. Vocalizations transitioning from chirps to persistent chittering and loud “squees,” especially at parental visits and food deliveries.
| Day 41 | M 3300g (73%) | M 29mm (90%) | M 130 (98%) | |
| Day 37-43 | M 146-152mm |
From Days 40-45 growth of beak and feet slows; feet and legs will be fully grown by Day 50. Contour feathers on front of the upper region of the tarsometatarsi emerging. Wing flapping becoming more vigorous, flap-hopping higher. Standing on the rails to slice. Long stretches of standing and looking out, or sleeping on the rails.
| Day 48 | M 3500g (88%) | M 30mm (92%) | M 132mm (100%) | |
| Day 42-48 | M 181-187mm |
Contour feathers on sides and belly filling in. Whitish sheaths still visible at bases of remiges and upper- and underwing coverts. Confident standing. More effective self-feeding, but still relies on parents for most feedings; grabbing, stealing, and mantling food. Vigorous flapping, lifting off, enjoying the wind. May begin branching, perhaps with 1-2 flaps, often by stepping.
| Day 55 | M 3700g (92%) | M 30mm (93%) | |
| Day 47-53 | M 217-233mm |
After about Day 60 growth tapers off except beak, hallux claw, and flight feathers. Some remnants of sheaths at bases of wing coverts. Juvenal body feathers nearly complete except on wings and tail; contour feathers on flanks still filling in, as well as on the upper region of the tarsometatarsi. Axillary feathers (wingpits) mostly white. Aggressively grabbing and attempting to steal food from parents and siblings. Flapping results in hovering in mid-air for several seconds. Siblings watch, mimic, and play with each other. Branching likely.
| Day 62 | M 3850g (96%) | M 30mm (94%) | ||
| Day 52-58 | M 253-259mm |
|||
| Day 57-63 | M 288-294mm |
Lower leg feathers are thickening. Aggressive food grabbing, stealing, and mantling. Confident one-foot perching and preening on branches. Vigorous flapping and long hovers. Sometimes stumbles when landing, learning to use wings to regain balance. Fledging is possible from Weeks 10-13. Males usually fledge 3-4 days before females.
| Day 62-68 | M 324-330mm |
Growth of primaries slows after 72 days. Branching more confidently, learning to perch, move around, and use wings for balance on branches. Fledging can occur suddenly and without warning, although eaglet may look intently at nearby branches and appears to evaluate suitable landing spots. First landing is usually awkward, and eaglet may end up on the ground.
| Day 67-73 | M 360-366mm |
By Day 80 primaries have reached 80% and rectrices 84% of their full length (achieved in the second winter). Male primaries growth tapers off, but female primaries continue to grow after fledge. Juvenal feathers will be longer than those of mature adults and will become shorter with each molt until year 5. Beak and hallux talon not yet fully grown at fledge (they will reach full size by the second or third winter).
| Day 80 | M 50mm |
BC14 fledged unintentionally on Day 81, 5/31/19, but the eaglet was ready. The branch on which it was perched broke and the eaglet fell but quickly recovered and flew strongly in the direction of trees across the creek.
The new fledgling juvenile eagle returned to the nest 3 days later and visited several times before dispersing from the area for good. Its sibling, BC15, fledged 8 days after BC14.
PERSONAL NOTE: In my opinion the 2 eaglets at this nest in 2019 are of the same gender, either female-female or male-male (it is impossible to know which). The younger may appear to be slightly smaller, but according to Bortolotti (1986a, 1986b), a younger sibling is almost always slightly smaller than an elder of the same gender. A male develops earlier and more quickly, but a female eventually is noticeably larger, especially so if she is the elder. (Male-female broods are quite rare.) It is quite difficult to ascertain relative size because of the camera angle and lack of perspective, but at fledge I could not see a significant size difference between BC14 and BC15.
EAGLET GROWTH AND DEVELOPMENT
© elfruler 2019
The growth and development of a Bald Eaglet from hatch to fledge takes about 10-13 weeks. Along a spectrum of morphological and behavioral states from least developed (“altricial“) to most developed (“precocial“), raptors fall near the minimally developed end. Altricial hatchlings have few or no feathers, closed eyes, little to no mobility, no ability to thermoregulate, and need parental care to survive and grow. Precocial hatchlings have a full layer of down feathers, open eyes, mobility and thermoregulating ability, ability to feed themselves, and are soon able to leave the nest. Raptors are considered “semi-altricial”: at hatch they have a thin layer of down but are unable to thermoregulate, their eyes are open or partly open although not yet able to focus or follow movements, they have some mobility, and they are entirely dependent on parents to survive and grow.
Thanks to the work of Gary Bortolotti and Jon M. Gerrard in Saskatchewan in the 1970s and 1980s, we have reliable information on the development of eagle nestlings from hatch to fledge, including weight and growth of the critical beak, feet, and wing feathers. (Click here for References.) Beak and feet grow faster than other body parts because they are essential tools for survival and take several weeks to be fully developed. The “gangly” and “clownish” look of young eaglets is largely due to the disproportionate growth of feet and beaks.
Hatched with thin natal down, eaglets gain a thicker second set of down starting a week or so after hatching, and soon thereafter their body (contour) feathers begin to grow. These feathers will become the juvenal (first-year) eagle’s smooth covering by the time it fledges. They take several weeks to reach full length, especially the wing feathers which are not yet fully grown until after fledge.
Steadily emerging behaviors reflect the growth of the eaglet’s skeleton, muscles, feathers, and neurological systems. As the days go by the eaglet develops the ability to hold up its head, maintain its balance, focus on and follow the parents with its eyes, reach out and eventually lunge for food from the parent’s beak and finally pull bites of food off a fish by itself. Especially with the emergence of contour feathers, an eaglet engages in near constant preening, using its beak to remove the protective sheaths around the feathers and help the vanes unfurl and its barbs lock together.
It swiftly gains mobility, from pulling itself by its wings and pushing with its legs through the soft nest materials, to standing and walking on its hocks, stepping backwards toward the edge of the nest and tipping up onto its toes to expel wastes over the side, ultimately graduating to standing tall on its toes and walking around like its parents.
As wing feathers begin to grow the eaglet waves its arms and extends them overhead in a full body stretch (which falconers call “warbling”), then builds its breast muscles with vigorous flapping, flap-hopping, and finally catching the air to hover above the nest. Inbetween all this activity, an eaglet spends many hours sleeping and resting, apparently doing nothing but growing.
The pages here follow the daily growth and development of the two eaglets at the Bluff City nest in Tennessee in 2019, through an online camera operated by East Tennessee State University (ETSU). BC14 hatched on 11 March at 10:32 a.m., and BC15 hatched a day and a half later, 12 March by about 11:00 p.m. Photos and videos here are from the Bluff City cam and are used by permission.
Heartfelt thanks to Michelle France, camera operator and keen observer at the ETSU nests, and to my long-time eagle-watching partner Donna Young for her careful observations of Bald Eagle behavior and eaglet development over the years, and her contributions to the descriptions on these pages.
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- Daily Photo Gallery and Weekly Commentary
- References
- (More pages devoted to specific features — feathers, feet, beak, behaviors, etc. — will be added as they are developed.)
INTRASPECIFIC INTRUSIONS AT BALD EAGLE NESTS
© elfruler 2018
“intraspecific adj. : occurring within a species or involving members of one species.” (www.merriam-webster.com)
Bald Eagles choose their breeding territories and nest sites carefully, driven by factors that will lead to success in raising their young. These factors include adequate food resources, a sturdy nest platform, available shelter from dangerous weather, ease of defense, and tolerable distance from disturbances. A good location will be attractive to any Bald Eagles that come along, and it is not surprising that a resident Bald Eagle pair will be challenged by other Bald Eagles for the site, leading to competition between members of the species, or intraspecific conflict.
It is not uncommon for one or both members of a pair to be challenged even before the nesting season begins, resulting in displacement, injury, and even death. Conflicts that occur once a clutch of eggs has been laid or a brood of chicks has hatched can cause loss of eggs and chicks, despite the fierce defense that the parents inevitably mount against intruders. Often the parents are successful in repelling a challenge and their chicks fledge. In rare cases an intruder is accepted by the resident pair as a cooperative breeder (Go here for a series of pages about Cooperative Breeding and its occurrence among Bald Eagles).
The Bald Eagle nests that have been viewed on camera or monitored carefully from the ground since 1992 have provided a glimpse of intraspecific intrusions of many types and with a variety of outcomes:
- Events before, during, and after the season
- From one to many intruders
- Intruding males and females, adults, subadults, and juveniles
- Replacements, disappearances, injuries, and deaths of parents
- Unhatched or broken eggs and injured or slain nestlings
- Double clutches
- Rescued nestlings and rehabbed fledglings
- Successful fledges
- Cooperative breeding
Even careful monitoring of cams and nests don’t provide the full picture of events surrounding intrusions, which often take place out of human view. Even happenings in full view do not always have clear-cut explanations.
- How many intruders are in the area?
- When did they first appear?
- What encounters occur off-nest between intruders and residents?
- What is the sex or age of an intruder?
- Does a resident eagle disappear because it has been injured or killed, or because it has decided that it cannot prevail in a battle?
- Was an egg broken by an intruder or an agitated parent, or because it was unviable or infertile?
- Did an egg fail to hatch because intruders interrupted the reproductive cycle and prevented fertilization?
- Why would one intruder destroy eggs or chicks but another intruder leave eggs or chicks undamaged?
- Does a resident adult respond primarily defensively to an intruder, or might there be a trigger that precipitates an offensive response?
- Why would an intruder become a helper and cooperative breeder rather than a threat?
Answers to such questions would provide a much better understanding of events we can see, but too often the answers elude us.
The number of Bald Eagle nests for which reliable daily reports have been made more than doubled from 2008 to 2018 – from 24 to 57. They range from southern California to New England, from south Florida to Alaska, and many points on the continent inbetween. The nests are found in a variety of habitats, including rural farmland, along rivers and streams, lakes and bays, in woodlands in parks and wildlife refuges, on coastal islands, college campuses, and in city neighborhoods. Yet it cannot be claimed that these nests are a representative sampling of all of the thousands of Bald Eagle nests in North America.
Nor can we be sure that we have witnessed every conceivable behavior or outcome associated with intraspecific intrusions. The information that I present here is illustrative of certain types of Bald Eagle behavior, but should not be taken as a statistical report on intraspecific intrusions.
This first chart (at this link) describes the intraspecific intrusions observed at Bald Eagle nests from 1992 through 2018, the nest locations and habitats, what is known about the intruders, the events, and the outcomes. (Link opens in a new browser tab.) Losses (of parents, eggs, or chicks) can result directly or indirectly from intrusions, or they may occur for other reasons not related to intraspecific conflict. Not every intrusion leads to a loss.
This second chart (follow this link) summarizes the details and gives percentages to enable comparison of intrusions, losses, and fledges from one year to the next. (Link opens in a new browser tab.) The percentages are number of intrusion events (nest intrusions, clutch intrusions, eggs lost, etc.) compared to the total number observed (nests, clutches, eggs., etc.) in that season. (The years 1992-2007 at the California West End nest are not included in this summary because of the outsized effect of DDE contamination on egg production there.) Notably, for these nests there is no clear trend in the percentages over time.
- While 2018 clearly was a bad year for nest intrusions (including both before and during a clutch), at 24.6% of observed nests, 2008 was almost as bad, at 20.8% of observed nests.
- On the other hand, 2008 was a worse year for clutch intrusions (after eggs were laid), at 19.2% compared to 17.9% in 2018.
- And in 2008, 4.5% of chicks were lost, compared to a much smaller 1.3% in 2018.
- The number of eggs lost was a staggering 11.3% in 2018, but the 8.6% of eggs lost in 2008 is the second highest percentage.
- The year 2013 was difficult, with 13.6% of clutch intrusions and 6.7% of egg losses.
- Some years were relatively benign: 2011 saw only one intrusion and 2012 only two. A dip in losses occurred in 2015 and in intrusions in 2016.
- From 2013-2017 most percentages were relatively stable – nest intrusions, clutch intrusions, losses – with an overall dip in 2016. Intrusions and losses in 2018 were severe, and it remains to be seen in coming seasons whether that year was an outlier.
BRIEF UPDATE ON THE 2018-2019 SEASON: I have not yet added new information to the charts, but the number of intraspecific intrusions declined significantly from the year before. Only 6 such intrusions occurred, only 2 of which happened after eggs were laid. 2 nests ended up with no eggs laid, and a total of 4 eggs were lost. No eaglets were lost.
There is no question that suitable habitat for nesting Bald Eagles is on the decline across the continent because of human development and encroachment. But the numbers we have for these particular nests do not necessarily mean that increasing numbers of nest intrusions point to an approaching saturation of carrying capacity for Bald Eagles across the board. Each territory has its own conditions that may or may not be either conducive or resistant to nest intrusions. Increasing population density in a particular area may simply drive some Bald Eagles to adapt by seeking out previously unclaimed territories, by gradually shrinking the size of their territories (over time) to allow for more nests (if the food supply allows for it), or by allowing more instances of cooperative breeding. It remains to be seen whether intraspecific intrusions will have a negative impact on the Bald Eagle population in the long run. Some have argued that a rise in population density ultimately could result in a state of population equilibrium by slowing the breeding productivity to offset the long period of increase that followed the banning of DDT in 1972.
While the snapshots that these observed nests provide give us some narratives about intraspecific intrusions at Bald Eagle nests and make comparisons possible, a broader understanding of the causes and effects of such intrusions, as well as a glimpse of what they may entail in the future, must await more detailed and systematic studies (such as Mougeot et al. 2013 in Saskatchewan and Turrin and Watts in the Chesapeake Bay, 2014 and 2015).
For perspective on the Bald Eagle population in North America and trends over time, Partners in Flight (PIF) estimates the number of breeding-aged Bald Eagle individuals in 2017 at around 250,000, based on data from the North America Breeding Bird Survey, an approximate 131% increase since 1970. The U.S. Fish & Wildlife Service’s oft-cited number of about 10,000 breeding pairs (or 20,000 individuals) in the lower 48 United States in 2007 does not include numbers from Canada or Alaska (both of which exceed the number in the lower 48 states), and it represents only eagles in pairs that are actively breeding. The PIF estimate encompasses all individual Bald Eagles throughout North America of breeding age whether they have formed breeding pairs or not. None of these numbers include juvenile or subadult Bald Eagles, which could more than double the totals.
There is as yet no sign that the Bald Eagle population is declining, whether because of habitat changes that lead to overpopulation and intraspecific conflict in a territory, or other causes such as contaminants, trauma, electrocution, disease, poisoning, and poaching. In 2010, following the removal of the Bald Eagle from the list of threatened and endangered species, the U.S. Fish & Wildlife Service produced a Post-delisting Monitoring Plan for the Bald Eagle. The Plan establishes a 20-year monitoring period (roughly four generations of breeding Bald Eagles) in the lower 48 states, with data analyzed and reported to the public every 5 years. The Plan will yield information on changes in numbers and their causes, and it includes provisions for responding to a 25% or greater decline with corrective action by federal, state, and local agencies, Native American Tribes, and other interested partners. The Plan specifically references the possibility of re-listing the Bald Eagle as threatened and/or endangered as a remedy to an unacceptable level of decline.
REFERENCES
Dzus, E.H. and J.M. Gerrard 1993. Factors influencing Bald Eagle densities in northcentral Saskatchewan. The Journal of Wildlife Management 57: 771-778.
Elliott, K.H, J.E. Elliott, L.K. Wilson, I. Jones, and K. Stenerson 2011. Density-dependence in the survival and reproduction of Bald Eagles: linkages to chum salmon. The Journal of Wildlife Management 75: 1688-1699.
Farmer, C.J., L.J. Goodrich, E. Ruelas I., and J.P. Smith 2008. Conservation Status of North America’s Birds of Prey. In K.L. Bildstein, J.P. Smith, E. Ruelas I., and R.R. Veit (eds). State of North America’s Birds of Prey. Nuttall Ornithological Club and American Ornithologists. Union Series in Ornithology No. 3. Cambridge, Massachusetts, and Washington, D.C., 303-420.
Grubb, T.G., L.A. Forbis, M. McWhorter, and D.R. Sherman 1988. Adaptive perch selection as a mechanism of adoption by a replacement Bald Eagle. The Wilson Bulletin 100: 302-305.
Hancock Wildlife Foundation Forum.
Hornby Eagle Group Projects Society. Our Nature Zone.
Hunt, W.G. 1998. Raptor floaters at Moffat’s equilibrium. Oikos 82: 191-197.
Institute for Wildlife Studies. Channel Islands EagleCAM Forum.
Jenkins, J.M. and R.E. Jackman 1993. Mate and nest site fidelity in a resident population of Bald Eagles. The Condor 95: 1053-1056.
Mahaffy, M.S. and L.D. Frenzel 1987. Elicited territorial responses of northern Bald Eagles near active nests. The Journal of Wildlife Management 51:551-554.
Markham, A.C. and B.D. Watts. Documentation of infanticide and cannibalism in Bald Eagles. Journal of Raptor Research 41: 41-44.
Mougeot, F. 2004. Breeding density, cuckoldry risk and copulation behaviour during the fertile period in raptors: a comparative analysis. Animal Behaviour 76: 1067-1076.
Mougeot, F., J. Gerrard, E. Dzus, B. Arroyo, P.N. Gerrard, C. Dzus, and G. Bortolotti 2013. Population trends and reproduction of Bald Eagles at Besnard Lake, Saskatchewan, Canada 1968-2012. Journal of Raptor Research 47: 96-107.
Turrin, C. and B.D. Watts 2014. Intraspecific intrusion at Bald Eagle nests. Ardea 102: 71-87.
Turrin, C. and B.D. Watts 2015. Nest guarding in Chesapeake Bay Bald Eagles. Journal of Raptor Research 49: 18-28.
U.S. Fish and Wildlife Service.
Watts, B.D., G.D. Therres, and M.A. Byrd 2007. Status, distribution, and the future of Bald Eagles in the Chesapeake Bay area. Waterbirds 30: 25-38.
Watts, B.D., G.D. Therres, and M.A. Byrd 2008. Recovery of the Chesapeake Bay Bald Eagle nesting population. The Journal of Wildlife Management 72: 152-158.
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
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