Tag Archives: bald eagle

EGGS, NESTLINGS, AND FLEDGLINGS

AT WILD BALD EAGLE NESTS,
2006-2020

© elfruler 2020

The table presented on this page gives numbers of eggs, nestlings, and fledglings observed at wild Bald Eagle nests on streaming video cams or by credible ground observers from 2006-2020. (Click here for a list of nests providing data.)

Table 1 gives precise counts of eggs laid, chicks hatched, and eaglets fledged over the 15 breeding seasons, broken down by year and by clutch size (1-egg, 2-egg, 3-egg, 4-egg, 5-egg), with totals and percentages.

TABLE 1

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Here are some highlights:
    • The data incorporate numbers from a total of 388 nesting seasons, resulting in 401 clutches of eggs including 12 second clutches after loss of the first clutch (see Notes at the bottom of the table).
    • Of the total clutches for the 15-year time frame,
      • 1-egg clutches made up 6.7% of total clutches;
      • 2-egg clutches made up 60.8% of total clutches;
      • 3-egg clutches made up 31.4% of total clutches;
      • 4-egg clutches made up 0.7% of total clutches.
    • A total of 910 eggs were laid.
    • The average number of eggs laid per clutch was 2.3.
    • 721 of the eggs hatched, or 79.2% of the eggs laid.
    • Of the clutches in which at least one egg hatched,
      • in 1-egg clutches, 44.4% of the eggs hatched;
      • in 2-egg clutches, 78.9% of the eggs hatched;
      • in 3-egg clutches, 83.3% of the eggs hatched;
      • in 4-egg clutches, 75% of the eggs hatched.
    • A total of 588 eaglets were confirmed to have fledged, either directly from the nest (571), or after rehab and release (17). This is 81.6% of nestlings hatched, or 64.7% of eggs laid.

In a handful of cases it is unknown whether a particular eaglet fledged, sometimes when the cam went down, or the cam angle made it impossible to follow an eaglet’s movements, or an eaglet had a misstep and fledged before it seemed ready and ground searches could not confirm it was safe.

Numbers can fluctuate up and down from one year to the next, and there is no clear trend in either direction.  For example:
    • 14.6% of the clutches in 2018 had 1 egg, none in 2019, and only 7% in 2020. The numbers of 3-egg nests jumped from 28.6% in 2013 to 41.4% in 2014, then dropped to 21.4% in 2015.
    • The total number of eggs hatched ranges from 66.7% in 2006 to 96.3% in 2012.
    • Sheer numbers can be deceptive. In 2020 a whopping 29 eggs were lost, but that is 70.7% of the total number of eggs laid that year, slightly below the average of 79.2% for all eggs hatched.
    • Confirmed fledges (directly from the nest and rescue/rehab) varies between 69.2% in 2012 and 93.5% the year before, 2011.

The fluctuations in the numbers actually reflect what observers have seen happen on the nests. Many factors affect the success of a given nest in a given year. These include weather, change of nest, change of mate, food availability, intruders and predators, and unusual events such poisoning and accidents. The variations also could be reflective of the relatively small sampling of nests.

NUMBERS FROM THE NESTS

WILD BALD EAGLES, 2006-2020

© elfruler 2020

The video cameras that have been trained on Bald Eagles’ nests since 2006 have provided a treasure trove of information about the breeding behavior of these apex raptors. In the universe of the more than 100,000 active Bald Eagle nests in North America, the data that these particular nests yield is minuscule. A few published scholarly reports on Bald Eagle nesting success focus mainly on a circumscribed area (e.g. Florida) for 1 or a few breeding seasons. The data here from the nests on cam span 15 years of breeding from 2006-2020 across a wide geographical expanse throughout the continent, and they represent the full range of climates and habits in which Bald Eagles reproduce. (Nests included in the data are listed here.)

Over the period, adult pairs at these nests made 401 breeding efforts at 85 locations, producing 910 eggs, 721 hatchlings, and at least 588 fledglings. These numbers might be considered a fair sampling of breeding data for the species.

The pages and tables that follow break down the data collected via these cameras on multiple levels. The raw numbers of eggs laid, nestlings hatched, and juveniles fledged, from nest to nest and year to year, yield statistics and percentages that give an overall view of breeding success over the 15 years. Burrowing more deeply into these numbers reveals how many clutches are successful over time, and which clutches of a particular size (1 egg, 2 eggs, etc.) are more successful than others. The numbers open a window into losses of eggs and eaglets, and what we can learn about reasons for those losses. And the numbers help flesh out some perceptions of behaviors of nesting Bald Eagles, such as coping with bad weather, predators, and intraspecific intruders (by other Bald Eagles), and replacing a lost clutch.

The data reinforce some facts that are already known:  Bald Eagles typically lay clutches of 2 eggs, with clutches of 3 eggs less common, clutches of 1 egg unusual, and clutches of 4 eggs quite rare. A fair number of eggs do not hatch, but a healthy majority end in successful fledges.

Other details to emerge from these analyses are perhaps more surprising:  While overall averages seem consistent with what is generally believed, there is often a wide range of values across seasons and from nest to nest.  In some years the number of eggs lost far exceeds the average, while in other years few eggs remain unhatched. Similarly, the number of nestlings that die before fledging covers a wide range among the years. Three-egg nests produce a higher percentage of fledges than either 2-egg nests or 1-egg nests; the latter are least successful in producing fledges.

These pages represent a complete revision of data that I published here in 2018, which consisted of a single page and 1 spreadsheet. For this new report I have pared down the nests to include only those with the most reliable observations, mainly the ones with streaming video cams, plus a small number of nests with reliable ground observers. I have also expanded the detail and breadth of information and analysis, resulting in 8 spreadsheets, and I have provided a narrative discussing each one. I have also compiled a lengthy list of References to literature on breeding, eggs, incubation, and survival.

These tables and narratives are presented in sequence in the pages that follow:
Additional new pages also make use of the nest data:
Full references for citations in the following pages are given here:

I began collecting data when I started watching web cams in 2009. Thanks to the Hancock Wildlife Foundation, the Institute for Wildlife Studies, spreadsheets compiled by Judy Barrows, nest cam websites and Facebook pages, and numerous individuals with whom I have communicated, I have been able to stretch the data back to 2006 when streaming cams first began operating. These sources also have been invaluable in filling in gaps in my own observations. I owe all of them a great debt of thanks.

 

BALD EAGLE NEST CAMERA BREEDING SEASON 2019-2020

I use short codes to refer to each nest. Click here for a key.
Click here for links to the nest cams.
Click here for calendars of egg-laying dates in past years.
A question mark ? indicates an approximate date or best guess.
Strikout indicates unhatched eggs.
Italics indicate nestlings that died before fledge.
< = by this date.

More detailed information can be found in
JudyB’s annual report on the nests.

NESTEGGSHATCHESFLEDGESNOTES
FL swf
clutch 1		11/12/19
11/16/19		12/19/19
unhatched		d. 1/15/20
---		eaglet ingested anticoagulant poison, broken blood feather led to fatal blood loss
TX web
new M?		12/12/19
12/15/19		1/17/20
1/19/20		4/6/20
fell 4/7/20,
rehab,
released 4/23/20		cam not streamed live, videos posted on Facebook
FL ece12/28/19
12/31/19		2/3/20
2/5/20		5/1/20
5/7/20
OK seq
new M?		1/4/20
1/7/20		1 broke 2/8/20
1 abandoned <2/15/20		---
---		intruder adult attacked 2/8/20, Mom possibly injured; unk which egg broke, which abandoned
CA bbl1/8/20
1/11/20		unhatched
unhatched		---
---		ravens ate eggs 3/15/20
GA ber1/11/20
1/14/20		1 broke 2/22/20
1 broke 3/11/20		---
---		unk which egg broke first
FL nef
new F & M		1/13/20
1/16/20		2/19/20
2/21/20		5/7/20
5/17/20
NJ duk
new F		1/20/20
1/24/20		2/26/20
3/1/20		both seen 6/25/20cam down
4/30/20 thru
end of season
TN dal1/22/20
1/25/20
1/28/20		2/29/20
3/2/20
3/4/20		fell 3/4/20
1 seen <6/8/20
1 seen <6/15/20		tree fell 4/29/20, cam down
TN blf1/27/20
1/30/20
2/3/20		3/5/20
3/7/20
3/11/20		5/29/20
5/29/20
6/5/20
MD tob2/3/20
2/6/20

2/9/20		3/14/20
hatch fail, 3/14/20
3/16/20		6/4/20?
---

6/5/20?
TN jns2/4/20
2/7/20
2/10/20		3/15/20
3/17/20
unhatched		6/3/20
6/5/20
---		male (Noshi) disappeared 4/24/20; intruder M appeared on cam 4/26/20
MN dnr
new F & M		2/6/20
2/9/20
2/12/20		3/15/20
3/17/20
3/19/20		6/16/20
6/20/20
d. <4/13/20
E3 failed to thrive, possibly starved
AZ gfd
new nest
new M
clutch 1		2/9/20?------new M didn't incubate; nest
abandoned 2/18/20
MI arb<~2/16/20
?		3/20/20?
?		6/9/20
?
CA red
no cam		2/11/20?
?		3/22/20?
<4/12/20		6/17/20?
?
PA frm
new cam		2/13/20
2/16/20

2/19/20		3/25/20
hatch fail, 3/25/20
3/26/20		6/14/20
---

<6/16/20
PA han2/13/20
2/17/20		broke 3/29/20
broke 2/17/20		---
---
PA pit2/13/20
2/16/20		3/21/20
3/23/20		6/11/20
6/6/20
IL umr2/14/20
2/17/20		3/23/20
3/24/20		6/10/20
6/16/20		tree fell in derecho 8/10/20
CA robat least 2
<2/27/20		2 hatched
<3/25/20		<6/10/20
<6/10/20
CO fsv2/14/20
2/17/20

2/21/20		1 hatch 3/29/20
1 hatch <3/31/20
1 unhatched		1 d. ~4/16/20
<6/12/20		cams off 3/16/20 due to coronavirus; unk which egg unhatched; unk which chick d., probably weather related
WV shp2/17/20
2/20/20		3/25/20
3/27/20		fell 3/27/20
6/13/20
CA cWE
new M
clutch 1		2/20/20
2/23/20		ravens predated
both 2/24/20		---
---		new M didn't incubate consistently
IA dnn2/21/19
2/24/20		3/30/20
3/31/20		d. 4/10/20
6/9/20		DN11's death cause unk
CA zSC
new nest
clutch 1		2/21/20broke at lay---
FL swf
clutch 2		2/22/20
2/25/20		3/31/20
4/2/20		6/15/20 accid
7/1/20		102d after clutch 1 first egg, 38d after clutch 1 eaglet d.
NY ctr2/22/20
?
?		3/31/20
<4/2/20
<4/5/20		6/18/20?
6/20/20?
<6/26/20
IA urb
new nest		2/25/20
2/28/20
3/2/20		4/3/20
4/6/20
4/7/20		6/22/20
6/25/20
6/29/20
IA dec2/26/20
2/29/20
3/4/20		4/5/20
4/5/20
4/8/20		6/18/20
6/21/20 accid
6/21/20
IN ndl2/26/20
2/29/20
3/4/20		4/4/20
4/6/20
4/9/20		6/23/20?
6/26/20
d. 5/14/20
BC sur2/27/20
3/1/20		4/5/20
4/6/20		6/28/20
6/28/20
CA cTH2/27/20
3/1/20		4/5/20
broke 3/20/20,
infertile		6/26/20
---
MT mil3 by 3/6/20<4/5/20
<4/7/20
<4/14/20		<6/18/20?
<6/20/20?
<6/28/20?
CA zSC
clutch 2		2/29/20
3/4/20
3/7/20		broke in 3 hrs
broke <3/5/20
broke <3/11/20		---
---
---
OH avn3/1/20

3/4/20
3/7/20		4/10/20

4/11/20
4/12/20		6/18/20 accid (BDOW)
1 d. 4/26/20
6/29/20		unk which chick d. or cause
CO std3/1/20
3/4/20
3/7/20		unhatched
4/12/20
unhatched		---
d. 4/14/20
---
intruder F attacked 4/6/20; eaglet d. of hypothermia or starved; magpie took eaglet's body 4/15/20; M stopped incubating
BC dl23/1/20
3/4/20
3/7/20		1 unhatched
4/11/20
4/12/20		---
7/1/20 accid
7/4/20 accid		unk which egg unhatched
OH ash
new cam		3/1/20
3/4/20		4/7/20
4/10/20		6/24/20
6/24/20
IA dav3/6/20
3/9/20
3/12/20		unhatched
4/14/20
4/17/20		---
<7/4/20
<7/4/20		egg #1 gone 3/7/20?, unk cause
TN har3/9/20?
3/12/20?		<4/16/20
4/18/20		<7/6/20
<7/6/20
BC wht3/10/20
3/13/20		unhatched
4/18/20		---
7/2/20
AZ gfd
clutch 2		~3/10/20------new M didn't incubate; abandoned
<3/25/20
CA cWE
clutch 2		3/18/20

3/21/20		raven predated,
3/20/20
raven predated, 3/29/20		---

---
BC hrn3/28/20
3/31/20 ?		5/5/20
<5/9/20		7/28/20
8/2/20
AK ken4/22/20
4/25/20		5/31/20
6/2/20		8/19/20
d. from fall, 6/22/20		M (Redoubt) disappeared 6/18/20; possible intruder

EAGLET GROWTH AND DEVELOPMENT

It has long been a dream of mine to collect a daily log of screen caps and videos that illustrate the growth and development of one eaglet from hatch through fledge, and which can serve as a framework for discussion of the many milestones in an eagle’s first 3 months of life.  This year, thanks to the superb camera setup at the Bluff City nest in Tennessee, operated by East Tennessee State University (ETSU), I have been able to realize this dream.

I have created a series of pages that includes a general introduction to eaglet growth and development, daily photos of young BC14 from hatch on 3/11/19 to fledge on 5/31/19, and a list of references.  I will be adding more pages to the series, covering specific eaglet features like feathers, feet, and various behaviors as it grows.

These pages would not have been possible without the assistance and support of Michelle France, camera operator and keen observer at the ETSU nests, who not only expertly zoomed, panned, and tilted the sometimes touchy cam, often catering to my requests, but also created a treasure trove of screen caps and videos every day which has been invaluable in my quest for daily shots.

I also must thank my long-time eagle-watching buddy Donna Young, whose careful observations over a decade have greatly enriched my understanding of eagle behavior.  Our countless conversations have helped shape this project and contributed enormously to the information I have gathered and presented.

The series I am launching today is found under the Menu item “Eaglet Growth” and so far includes:

More pages are in the works.

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.

HEIGHTWEIGHTBEAK LENGTH
BEAK DEPTH
FOOT PAD
Day 0
7.6cm
91g (2%)
11mm (21%)
9mm (26%)
16mm (11%)

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.

HEIGHT
WEIGHT
BEAK LENGTH
Day 6
16cm
300g (6%)
17mm
(F 29%, M 31%)

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.

WEIGHT
BEAK LENGTH
BEAK DEPTH
FOOT PAD
Day 13
950g (20%)
24mm
(F 45%, M 48%)
19mm
(F 58%, M 63%)
F 88mm (60%)
M 87mm (66%)

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.

HEIGHT
WEIGHT
BEAK LENGTH
BEAK DEPTH
FOOT PAD
Day 20
30cm
F 1750g (34%)
M 1500g (37%)
31.6mm
(F 57%, M 61%)
F 24mm (69%)
M 23mm (70%)
F 109mm (74%)
M 108mm (82%)

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.

WEIGHT
BEAK LENGTH
BEAK DEPTH
FOOT PAD
8th PRIMARY
Day 27
F 3000g (59%)
M 2400g (60%)
40mm
(F 72%, M 77%)
F 27mm (81%)
M 25mm (83%)
F 125mm (85%)
M 115mm (87%)
Day 27-33
F 64-70mm
M 74-80mm

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.

WEIGHT
BEAK DEPTH
FOOT PAD
8th PRIMARY
Day 34
F 3500g (69%)
M 3000g (72%)
F 29mm (85%)
M 27mm (87%)
F 142mm (97%)
M 129 (98%)
Day 32-38
F 100-106mm
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.

WEIGHT
BEAK DEPTH
FOOT PAD
8th PRIMARY
Day 41
F 3800g (75%)
M 3300g (73%)
F 31mm (88%)
M 29mm (90%)
F 146mm (99%)
M 130 (98%)
Day 37-43
F 136-142mm
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.

WEIGHT
BEAK DEPTH
FOOT PAD
8th PRIMARY
Day 48
F 4500g (88%)
M 3500g (88%)
F 32mm (90%)
M 30mm (92%)
F 147mm (100%)
M 132mm (100%)
Day 42-48
F 171-177mm
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.

WEIGHT
BEAK DEPTH
8th PRIMARY
Day 55
F 4600g (90%)
M 3700g (92%)
F 33mm (93%)
M 30mm (93%)
Day 47-53
F 207-213mm
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.

WEIGHT
BEAK LENGTH
BEAK DEPTH
8th PRIMARY
Day 62
F 4800g (94%)
M 3850g (96%)
F 54mm (98%)
M 49mm (95%)
F 33mm (94%)
M 30mm (94%)
Day 52-58
F 243-249mm
M 253-259mm
Day 57-63
F 278-284mm
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.

8th PRIMARY
Day 62-68
F 314-320mm
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. 

8th PRIMARY
Day 67-73
F 350-356mm
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).

WEIGHT
BEAK LENGTH
BEAK DEPTH
8th PRIMARY
Day 80
F 5100g (100%)
M 4000g (100%)
F 54mm
M 50mm
F 35.8mm
M 32.2mm
F 378mm
M 353mm

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.

FEATHERS!

At the Bluff City nest in Tennessee this week (March 17-23, 2019) we are witnessing the three phases of feather development on a growing eaglet. This nest is live-streamed online by East Tennessee State University, and photos are used with permission. Link to the cam.

On Sunday the two eaglets (BC14 and BC15) were covered with their light gray natal down. These fluffy feathers, called “plumulaceous,” grow in tracts, or rows, that are layered more or less evenly across the skin. The feathers are thin and are not good insulators.

On Wednesday for BC14, and then on Thursday for BC15, we began to see the secondary down feathers emerge. This dark gray layer of down can appear as early as Day 6 after hatch. It is thicker and provides better insulation than the natal down, hence is sometimes called thermal down, as it helps the eaglets begin to self-thermoregulate. The secondary down soon will cover the natal down (but does not replace it because it grows from a different set of follicles). In this photo you can see the dark gray feathers appearing between the neat light-colored rows of natal down feathers.

Within a few days, even while the thermal down is still growing, the follicles that produced the natal down will begin to produce “pennaceous” feathers, ones with a central shaft that makes them stiff, and barbs and barbules that lock together. The shaft has blood vessels running through it as it grows, hence the name blood feather, sometimes also called a pen- or pin-feather (adapted from “pennaceous”). These contour feathers, which include the wing and tail flight feathers, will grow over several weeks to create a protective covering of the eagle’s entire body.

Pin-feathers begin to emerge around Day 12 after hatch, and today at Bluff City, right on time, BC14’s and B15’s wings show a thin line of dark brown feathers at the back edge of the wing. The flight feathers on the wings, the remiges, are the first to begin to grow because they are the longest and take the most time to reach their full length.

In this photo you can see the light gray tips of the natal down feathers that are being pushed out by the growing remiges.

So for the next few days we will be able to see all three types of feathers on the Bluff City eaglets: natal down, thermal down, and contour feathers. The contour feathers will eventually replace the natal down completely, leaving only the thermal down underneath. Contour feathers will appear slowly on the back, then the tail, shoulders, neck, head, sides, chest and belly, and finally the legs and feet. The feathers on the backs of the feet will be the last to grow in because the eaglets will only gradually develop the strength and balance to walk on their toes like their parents do, and feathers would not withstand the wear and tear of constant contact with nest materials.

Hatching Eaglet

Here is a rare bird’s-eye (pun intended) view of how an eaglet uncurls itself from inside the egg in the final stage of hatching. This video is of the hatch of BC14 on 3/11/19 at 10:32 at the Tennessee Bluff City cam, operated by East Tennessee State University.  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 new hatchling.  The eaglet has its back to us and its head is down, tail up.