Raptor Resource Project Forum

Chat Moderators => Decorah Eagle Mods Want You To Know -- Eagle Education => Topic started by: jfrancl on April 07, 2014, 11:17:53 AM

Title: Growing Up...Developmental Milestones
Post by: jfrancl on April 07, 2014, 11:17:53 AM
We are starting a new thread for discussion about the growing process of bald eaglets and the developmental milestones to watch for.  A quick reminder:  As has been stated in previous "Posting Guidelines," we ask that your posts and comments are limited to this topic in this particular thread.
Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on April 07, 2014, 11:20:44 AM
I am entering FinnBMD's starting post on Finn's behalf.

In the first week of life, the hatchling becomes a nestling.  The growth and development process starts.  Initially the little ones are virtually helpless and their activities are minimal:  eating, sleeping and pooping.  For the first couple of days after hatch, they will be confined to the egg cup but by the end of that first week their growth will have started.

The late Canadian researcher, Gary Bortolotti, was a key figure in studying bald eagles and their offspring.  In one of his earlier articles, "Criteria for Determining Age and Sex of Nestling Bald Eagles," he wrote the following about the appearance of chicks in the first week after hatching:  "For the first few days after hatching the chicks can perhaps be aged by appearance.  On the day of hatching the area of the body where the allantois is attached is swollen, circular shaped and somewhat protruding.  Remnants of the allantois can be seen on the chick for up to 3 days after hatching, but after day 0 it is desiccated and thread-like.  On the day of hatching the chicks generally appear very weak and spend most of their time lying in the nest.  The skin is initially loose, wrinkled, and a bright pink, but it fills out, is noticeably tighter, and fades to a soft pink on day 1.  The legs too are very pink on day 0, but are a faded flesh color on day 1.  Beginning as early as days 2 and 3, but usually on day 4 or 5, the skin (upon close examination) on the ventral side [i.e., the front side of the body] is tinged with blue.  In the next 4 or 5 days the skin over most of the body is blue-gray."

In another article, Bortolotti wrote:  "On the day of hatching, eaglets had dark brown eyes, the gape and legs were pink, and the skin was bright pink.  The cere was a very pale gray and the culmen [the central midline ridge running from tip of upper bill back to base of the bill] was dark gray-black with a white tip.  The talons are largely flesh-colored. Between day 4-8 the skin turned from largely pink to largely blue except for a small area under the wings.  When the eaglets were 4-8 days old, the cere was pale yellow, the bill was dark gray-black and the legs were pink yellow.
Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on April 08, 2014, 08:05:37 AM
Though this is a poultry site, my guess is that it is anatomically similar.  The day by day steps include the allantois and allantoic vessel

Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on April 08, 2014, 11:55:15 AM
Finn, thanks for the great explanation of the first week of a young bald eagle hatchling. 
Going into the second week of life, the young nestlings start to become more curious though still completely dependent on their parents for warmth and protection against the elements.  They are not yet able to thermoregulate ( the ability to keep body temperature within certain boundaries regardless of ambient temperatures).   Commonly, the chick will rest at its parents breast or under it for warmth during this time and parents will never leave it unattended, though during more comfortable weather, the parent may not be physically on the nest constantly.
At about one and a half to two weeks of age most young weigh between one and a half to two pounds.   From nine to twelve days old, their cere (fleshy covering at the base of the upper mandible) is a pale olive color and legs are a pale yellow color with areas still pink-yellow.  When properly brooded, cold temperatures are not as significant a problem as heat is.  Often during warmer days the parent will spread its wings with its back to the sun to shade its young.  At about 10 days of age, a darker medium-gray thermal down (second down) begins to replace the natal down.  Thermal down has very good insulating qualities and by 15 days old the chicks are typically able to thermoregulate on their own.
Eagles have what is called an asymptotic pattern of growth in that they begin slowly, have a rapid phase of growth, followed by a slowing of growth to adult size.  Researcher Gary Bortolotti stated that male eaglets had an average growth rate of roughly 3.5oz per day while females average 4oz per day.  ( The bald eagle's growth rate is faster than any other bird in North America).
At two weeks old the eaglet is strong enough to hold its head up for feedings, and look around the nest area, and begging calls become louder and stronger.  Before two weeks of age the nestling will defecate in the nest but at about the two week mark, it will start to instinctively back up to the edge of the nest and release its excrement (which we affectionately refer to as PS) over the side. From early on in nests with more than one chick, sibling rivalry becomes apparent, partly due to competition for food and partly for parental attention with one bird, usually the oldest, out-competing or dominating the others.  The younger bird(s) learns to keep a low profile until the oldest has been fed but quickly learns to grab food or doing run arounds to the dominating eaglet to get its share.  This behavior occurs more within the first two weeks of life, declining somewhat as the siblings grow and learn.  They don't always eat every meal offered to them and one may eat while another (or 2) don't eat each time but they all  will normally get enough food during the course of the day.  On several occasions, we've seen dad feed the youngest while mom takes care of the other two!
Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on April 10, 2014, 02:01:18 PM
Abbie, since male eagles and other male raptors are smaller, they reach their terminal size faster than females (at younger ages). Males make 1st flight at about 78 days (range 68 to 84) and females usually fledge later at about 82 days (range 75-88) - Gary Bortolotti.

He also stated that males mature faster, become more proficient hunters sooner than females and are more likely to disperse from natal nest area sooner than females.
Title: Re: Growing Up...Developmental Milestones
Post by: Faith on April 10, 2014, 08:59:39 PM
The way I understand it - and please correct me if I'm wrong, Jf and Finn - is that since females are destined to be larger than males, they will catch up and maybe even surpass the males in physical size in a relatively short amount of time.  But since males mature faster than females, they will attain the milestones sooner, such as reaching their terminal growth, hovering, branching, fledging, and maybe even dispersing.  An example is D1/E2.  She was the 2nd hatched, but in 2 or 3 weeks time she appeared larger than both E1 and E3.  However, she was slower in branching, fledging, and leaving the area to be on her own.
Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on April 11, 2014, 01:31:50 PM
Right Faith, Mark Stalmaster stated that after 20-30 days, the weight of the ywo sexes differ, females gain weight faster than males but males develop plumage faster than females.

Males mature faster, become more skilled hunters sooner than females and most likely disperse from natal nest area sooner than females.
Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on April 21, 2014, 06:38:17 AM

D18 and D19 are now going into week four!   D20 is 5 days younger than D18 so D20 will reach these developmental milestones a little later than Ds18 and 19.  During this time, their appetites are huge!  They may attempt to peck at food in the nest at about 1 month old but they are not yet ready to self feed.  They might now start to grab their parents beak for food, and may not wait to be fed but instead may snatch enormous chunks of food and gulp it down. 

At about 4 weeks old, eaglets will begin to cast out pellets (a ball of indigestible food such as bones, fur and feathers) from their mouths  which are formed and compacted in their gizzards.   At this point they are ingesting more of the prey and in larger pieces.  On or about 27 days or so, the pin feathers including contour and flight feathers begin to emerge and males will develop their plumage sooner than females. Pin feathers are often called blood feathers because they're connected to the blood supply which emerges from the skin's feather follicle and allows them to grow. The emerging feathers will initially be surrounded by the heavy layer of down.  The dark feathers are arranged in tracts, beginning with the head (capital) and back (dorsal) tracts.  The ventral or belly feathers are the last group to appear.  Their feet, eyes and beak are growing fast and nearing full size during this stage and will give them a clumsy disproportional appearance.  Researcher Gary Bortolotti stated that  sexual dimorphism in size was distinct at about 20-24 days old for beak depth and 25-29 days old for foot pad.   
Here is a link that shows the different plumage stages starting from 10 days old to maturity at 5 years old:


Most of the Bald Eagle's weight gain occurs early on in development.  A male eaglet gains weight at a rate of about 3.6 oz per day and a female eaglet will gain about 4.59 oz per day. The bald eagle's growth curve in its first month has an "S" shape when charted, starting out slow but in just days becomes rapid.  After 20 - 30 days the weights of the two sexes diverge or separate and reverse sexual dimorphism begins to appear.  Females gain weight faster than males which results in lifelong differences in size.  So most of the growth in the early stages of development occur in body tissues.

What we are noticing now with Ds18 and 19, (D20 soon to follow) that they are sitting up nice and tall, they can barely fit under mom, they have started climbing out of the nest cup more (what's left of it) waddling around the nest and enjoying the fresh air and sunshine, and all three have been eating larger chunks of food! 
Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on May 05, 2014, 07:12:45 AM
Entering the six week mark,  the attendance of parents usually drops off considerably (with the parents often perching in trees nearby).  The young eaglets will preen often, pick up and manipulate sticks, play tug of war with each other, practice holding things in their talons, and stretch and flap their wings.  Exercise gradually becomes more intense as each week passes in the nest. They now peck at food and attempt self feeding though they are still not proficient at it.  Parents will continue to feed them during this stage while allowing them to refine some of their self feeding skills. At about six weeks old they will nibble up and down the prey and finally after some unsuccessful tugs they will pull off a piece and self feeding begins!  We might also see them snatching food from the parent's beak and swallow it whole and as we have seen with Ds 18, 19 and 20, they will eat as much as they can at a single feeding storing food in their crops!  Their visual acuity improves daily as eaglets will watch their parents flying to and fro.  As Finn mentioned, they are almost adult size at six weeks reaching T 90 (90% total physical growth) at about 42 days old.

To give you an idea of the size of a 6 week old bald eaglet, here are some photos of a six week old eaglet taken from Al Cecere's FB page, credit is given to: Mr. Al Cecere and the "American Eagle Foundation, wwwEagles.Org"




In one study, avian researcher Gary Bortolotti  stated that the sexes were clearly separated at about 40-45 days old when growth was almost complete.  In another study, his observations on bald eagle nestling feather development were "feathers on the lateral ventral surface became noticeable between days 26 and 45."  During this age range, their brownish black feathers are becoming apparent in rows.  The shaft of their feathers are a blue-ish color due to the blood supply entering the shafts of the feathers. 
At 5 to 6 weeks old an eaglet's tarsi has reached full size and thickness and this is when leg banding can be safely applied.   The foot pad, length of middle toe (#3) and bill depth have also reached nearly full size at about 40 days old or about 1/2 way through the nesting period which is when the sex identification can be calculated mathematically.  Nick Fox states that around this age, they have imprinted on their parents, siblings, fear response and nest site.

Stay tuned for Finn's next post on week 7 developmental milestones!
Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on May 19, 2014, 05:40:15 AM
Wow! D 18, D19 and D20 are developing beautifully and right on target.  At week eight, the downy grey bald eagle chicks have grown into  full statured and proudly standing eagles. 

But their wings are not yet fully grown. Their growth is so rapid that a standing eaglet might occasionally droop its wings since the developing feathers are blood filled and the wing can be a little heavy.  The entire set of primary and secondary feathers is growing at the same time, each adjacent feather follicle is filled with the necessary blood  to grow the shaft and veins.  Each grows with the side protection of the neighboring feather.  One of the most reliable outward signs that eaglets have moved past the period of most rapid growth and peak energy demand is when they begin to appear mostly feathered. This appearance shows that the transition between structural growth and feather growth has been made.   Flight feathers continue to grow until fledge.  In one study, researcher Gary Bortolotti stated that flight feathers and flight muscles did not fully develop until after their fledge but developed enough for flight.  The Center for Conservation Biology states that three general phases of development as nestlings including 1) structural growth, 2) feather growth, and 3) neurological and behavioral development.  These phases overlap but do show a steady sequence from structural through neurological development. The last phase of development as nestlings is neurological which is learning of coordinated movements.  This includes walking, feeding, and flying.  The final month of development is lead by acquiring coordination, which is needed for movement and flight.

Exercising their wings in a limited space can be tricky but fun to watch!  The eaglets will jump around the nest, pick up every little gust of wind to practice "hovering" which is sustaining air briefly but staying  above the nest (think helicopter)  and  practice important landings.  Wing flapping and hovering also strengthens their pectoral muscles necessary for flying.  Other exercises they  can perform are treading, trampling and prancing which might help them later in subduing prey. At this stage they can be referred to as "wingers" though eaglets and nestlings also apply.   

At 8 weeks old the eaglets will have a blue-black beak and dark brown eyes and will also be stealing food from their siblings and even their parents, given the opportunity.  At the self feeding stage the young eaglets become aggressive towards the parents and will rush toward a parent in hopes of grabbing some food.  If a parent shows up at the nest without prey, overeager eaglets might even grab at whatever it can get, usually a parental toe!  When an eaglet does grab the prize from a parent, it might either mantle the prey with its wings and body to claim it and/or swallow the prey whole.  When mantling they often vocalize to warn off its nestmates ("This is mine, go away").

Researchers have said that at this stage, nest bound eaglets become more aware of their surroundings outside the nest and may react unfavorably to other birds nearby as well as humans nearing the nest.   Dr. Scott Nielsen stated that from about  the age of 8 weeks old and on, they are more likely to jump from the nest if any sort of danger or disturbance comes their way.   Caution should be taken so as not to disturb them in any way in order to avoid any eaglet from accidentally falling or jumping out of the nest.   The USFWS categorizes weeks 8 through fledging as "Very Sensitive Periods"  and may flush from the nest prematurely due to disruption.


Stay tuned for Finn's post on week 9 coming up next week! 
Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on June 08, 2014, 05:36:32 PM
10 to 11 weeks old!  Now it's all about preparing to fledge!  As we have mentioned, along with flight feather development, the last stage of development for a bald eaglet is neurological and behavioral, learning coordinated movements.  At 10  -11 weeks old the eaglet will soon be ready for flight.  Typically fledge occurs between 10 to 13 weeks of age.  Researcher Gary Bortolotti stated that males 1st flight average 78 days (68-84 day range), females 1st flight average82 days (78-88 day range).  They are now exercising their wings regularly, hovering and may have started to branch.  These active behaviors strengthen their pectoral muscles needed for flight.  As the eaglets approach fledging the adults spend less time around their young and the eaglets themselves do not seem very interested in their parents unless they arrive with a meal.  When the nestlings spot the parents coming toward the nest, they usually start their loud screaming whines and squeeing!  We have been asked in chat about their changing feeding schedules.  When the chicks are young, peak energy demand occurs in the late stage of rapid growth when metabolic demand is high.   After terminal size is reached, energy demand declines to maintenance alone and resembles that of adults feeding schedule. We have also been asked if parents will disallow food for a few days around this age to encourage fledge.  Some researchers state that this is not true and the eaglets fledge when they feel confident in doing so. 

The juvenile eaglet's eyes are a dark brown, beak and cere are a black tone and feathers are a dark brown to black in color and the feet and foot pads are yellow.   Weight varies but a ballpark of 10 lbs with a range of approximately 2 lbs in either direction would be safe to say. Determining gender with a 100% accuracy at this point can only be done through DNA analysis or blood work.

Mark Stalmaster reports that at 10 to 11 weeks old , defensive behavior towards other birds or humans nearing the nest might include retreat to the opposite side of the nest, or jumping from the tree or attempting to fly away.  This can lead to severe consequences for a still flightless bird.  He also categorizes 3 stages following fledge.  These include early, intermediate and late stages.  In the early stage (10 - 12 weeks old), once most of their flight feathers have fully developed at about 95%, they may attempt leaving the nest.  The timing of fledge can be affected by several circumstances.  Disturbance as I mentioned earlier in the post, gender - males more likely to fledge earlier than females, and a single eaglet may fledge sooner than one who has nestmates.  Birds that fledge at a younger age are more likely to land on the ground before trying again.  Parents do not force fledge and as stated above, fledging occurs only when the eaglets are physically prepared to leave the nest. 

During these final weeks before fledge, the eaglets flap, jump and generally move around the nest a great deal and spend an increasing amount of time on support branches of the nest.   According to Gary Bortolotti, males tend to be noticeably more active in this respect than females as is typically the case with raptors.  The period just before fledge, nest site imprinting is on the upswing and will peak after their first flight.

Stay tuned for Finn's informative post on week 12!
Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on June 21, 2014, 04:11:33 AM
One quick observation I'd like to add to Finn's wonderful post on week 12 and fledge.
During the fledglings time on the wing observing their parents hunt and fish, we have also seen dad teach them how to soar!  This seems to occur a couple weeks after fledge, at least at this nest site.  in 2013, Our observations:  Dad starting low circling his way up in thermals, purposefully and deliberately.  Rising up strong and steady and effortlessly.  One of his fledglings followed just beneath him, not knowing exactly what to do, flapping then a little soaring, flapping, soaring, while still rising up under dad.  Then another fledgling did the same under its sibling.  Flapping, soaring, flapping, soaring!  Now with 2 eaglets under dad and rising, the third eaglet joined in under sibling #2.  Flapping, soaring, flapping, soaring!  Now all under dad in a column formation flapping, soaring.  As they rose higher, more soaring than flapping.  By George, I think they've got it!  It was breathtaking to see how dad taught his eaglets how to Flap, Glide, Soar!!
Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on July 22, 2014, 08:13:46 AM
Our last stage in the milestones thread is dispersal!

Dispersal can be defined as movements that have no fixed direction or distance, resulting in a mixing of individuals from different areas but don't necessarily bring about any change in overall distribution.  This occurs when the young eagle is no longer dependent on its parents for food. Dispersal can lead to range extension and can have important genetic consequences (reducing inbreeding, promotes gene exchange).  There are several types of dispersal.  Natal dispersal, breeding dispersal and non-breeding dispersal.  In following our Decorah juvenile, we will cover natal dispersal, which generally encompasses much larger areas than the other forms of dispersal.   

The degree of nest site fidelity varies between species and between sexes in which females usually disperse further than males and larger species such as bald eagles tend to disperse further than smaller species.  Movements of juvenile bald eagles has not been well documented in the past as their movements are more nomadic than purposeful at first.  Many juvenile eagles will migrate south if their natal nest is in the north or time to go north if raised in southern latitudes. A flyway is a path birds congregate along to travel. There are four major flyways in the US. Pacific Flyway, Central Flyway, Mississippi Flyway and Atlantic Flyway. Tracking using telemetry offers researchers the opportunity to document the daily movements of individuals for many consecutive years.

Mark Stalmaster stated that approximately six to ten weeks after fledging young eaglets will begin to break family ties and leave the nesting area.  Dispersal times vary depending on the individual bird, some leaving sooner, some later. By this time they are more or less sufficient, able to fly with ease and acquire prey on their own.  In some populations that are not migratory, they may remain in the vicinity for several years.  And even migratory juveniles may return to the general area of their natal nest location, and perhaps establish a territory of their own and continue the cycle of breeding.  Colder weather increases eagles food requirements and shorter days give them less time to obtain sufficient food. In addition to learning hunting techniques the juvenile will learn what type of prey to hunt and what not to pursue though some prey recognition is obtained while in the nest. First year eagles are notorious for attacking just about anything that floats or moves.  Whether they are successful or not in capturing prey is a different story.  Most first year eagles will feast mainly on carrion or stealing prey from other eagles and other species of birds.

Closer to home, D1 and Four are both part of the largest longitudinal study of bald eagles ever undertaken.  We know that D1 has traveled North to Hudson Bay in Canada starting in 2012 and back to the greater Decorah area each year.  Here is a post pertaining to this study that Decorah Eagles Chat room moderator FinnBMD entered in the Flap, Glide, Soar thread in RRP forum.  Page 11,  post number 151:


We know that young eagles may move in more random directions than adults who have developed strong habits, returning along the same routes to the same wintering and nesting areas year after year. Movement pathways also depend on the season. Gary Bortolotti stated that: "For immature eagles in their first fall, migration and movements appear to be, to a considerable extent, a reflection of whither the wind blows. For adults, wind probably has no influence on the eventual target of their migration, generally summer and winter habitats with which they are familiar. Nevertheless, the wind strongly affects the route they take and other aspects of the pattern and timing of their movements."

As D1 has been following a similar path going north and returning south, it will be interesting to see in which direction Four will travel in the coming years!  Will she follow her sister's migration pattern or will she venture in a totally different direction?  Only time will tell!
Title: Re: Growing Up...Developmental Milestones
Post by: T40cfr403 on August 06, 2014, 06:59:38 AM
This quote comes from the Raptor Education Foundation's Facebook page:

"I have been told by a long-term eagle biologist that typically the very EARLIEST a young golden eagle will start hunting on her own is approximately 90 days after fledging. However, most eagles rely upon carrion and the kills of older, more experienced eagles, until they are around 2 1/2 to 3 years of age. The status of local prey populations makes a huge difference in the learning curve."

This sounds reasonable to me but do you think it could also apply to bald eagles?
Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on August 06, 2014, 08:03:50 AM
Hi T40 and thanks for your comment.

According to Gary Bortolotti, male bald eagles fledge at about 78 days old and female bald eagles fledge at about 82 days old.  They stay in the area with their parents until they are no longer dependent on parents for food.  That learning period can vary greatly but an average is between 4 weeks and up to 2 months after fledge. 

The eaglet first learns to recognize foods in the nest; when it fledges and follows the parents, it learns by watching them and other eagles. It continues to learn by observation and through trial and error. The predator's instinct to pursue prey is innate and is fine-tuned through learning and experience.

Their first year of independence is the toughest for them and they will mostly rely on carrion and stealing tecniques for their meals.  Fine tuning these techniques takes time and proficiency in hunting make not be accomplished until they reach 2-3 years old.  Of couse bald eagles are mainly fish eaters and goldens rely more on ground prey but the availability of prey would probably play a part in the success or failure of honing their survival skills. 

In a study done in Labrador, Canada, it was stated that juvenile migration might be initiated by changes in foraging opportunities with birds departing an area if prey is not readily available.  It stated: "While the four eaglets (out of 5 being studied) travelled independently of one another some went in a similar direction moving between lakes and rivers and exploiting aquatic environments.  They made several stopovers along the way, most near larger water bodies such as the Gulf of St Lawrence, Ontario Lake and Lake Champlain.  Gerard believed that juvenile BEs followed rivers and lakes as these areas represented a more favorable habitat.  We suspect that the Labrador eaglets gained experience during their autumn migration and their flight routes and strategies were more defined and less nomadic in the spring."
Title: Re: Growing Up...Developmental Milestones
Post by: T40cfr403 on August 06, 2014, 09:14:06 AM
Thanks jf!  Good info, as always.

Read a few other posts in this thread and your "Flap, Glide, Soar" observations were stunning.  What a life-changing event to witness.  Jealous.

I was also impressed by the information that nestlings imprint on prey.  Watching Dad bring in a dizzying array of prey those first few days after hatch has even more meaning now.  I thought his sole purpose was to provide a wide variety of protein that was easily digestible.  But now I realize it was also a teaching opportunity to show the eaglets appropriate prey.  Genius.  Almost too much to comprehend at times.
Title: Re: Growing Up...Developmental Milestones
Post by: shaggydog1 on August 06, 2014, 12:17:51 PM
Another big thanks to you, jf, for this information.  You write so well and it's so easy to read and comprehend.  Not at all the stodgy, documentary style of many writers!!! Thank you!!!
Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on August 06, 2014, 01:06:39 PM
You're welcome T40 and Shaggy! 

It's great that so many of us are interested in learning more and more!!    :D
Title: Re: Growing Up...Developmental Milestones
Post by: myschatzi on September 04, 2014, 06:32:06 PM
Another big thanks to you, jf, for this information.  You write so well and it's so easy to read and comprehend.  Not at all the stodgy, documentary style of many writers!!! Thank you!!!

Agreed Shaggy... It brought tears to my eyes just reading about our E's milstones..  I will continue to read more.  Info is very informative, I'm still learning!!  ;D

Thank You, jf for the link   ;D
Title: Re: Growing Up...Developmental Milestones
Post by: T40cfr403 on April 26, 2015, 04:29:11 AM
I spent a couple of hours watching the autorecords of the close-ups on 4/24/15 and was particularly interested in D23.  My main problem is I don't know the biology of feather growth, so what I thought I saw could be way off base.  It seemed to me when D23 was preening the thermal down, s/he was just breaking up clumps of the down.  It reminded me of humans nibbling on an ear of corn (not that corn!) because its beak wasn't pulling on anything and the beak always came away clean, with no down attached.

So I was thinking maybe the purpose of that action of quick, short strokes over a large area might be useful in simply making way for the remaining natal down to come to the surface (here's where my feather education fails me).  The preening nibbles might be triggered by them feeling pin feathers starting to grow below the surface of the thermal down.

My theory doesn't explain why natal down pops up through the thermal down on the head almost overnight with no preening possible and in such even patterns, but maybe head feathers/follicles are different from those on the rest of the body?

Or, again with the lack of knowledge (!), but they keep the thermal down, right?  So maybe D23 really was preening the thermal down because it's also growing and morphing into those soft, white, tiny, downy feathers?  Those white down feathers just don't grow with the same type of sheath the pin feathers have, right?  Or maybe there is some type of protective coating on the thermal down that needs to be nibbled away?  Oh, I've dug myself a deep hole and sorely need a textbook!  ;)
Title: Re: Growing Up...Developmental Milestones
Post by: jfrancl on April 26, 2015, 12:53:02 PM
Thanks for your post T40.  I didn't see what you were referring to but I'll give this a shot.

 Down feathers normally lack a rachis and do not form a vane.  Contour feathers will push out the natal down ahead of it and second down has no tracts as the natal down does have.  The second down follicles aren't evenly distributed but are randomly spaced amongst the tracts of the contour feathers.  So the second down, replaced in molting, is a permanent underlayer of the down throughout the bird's life.  D23 is 24 days old, and at about 3 weeks old, eaglets are largely covered with their second coat of down.  By day 18-22 very little natal down is visible, except on the top of the head.

Down feathers in adult birds is called Definitive Down and vary from thick to restricted distributions.

Hope this helps.

Title: Re: Growing Up...Developmental Milestones
Post by: T40cfr403 on April 26, 2015, 01:38:45 PM
ty jf