Chat Moderators > Decorah Eagle Mods Want You To Know -- Eagle Education

Growing Up...Developmental Milestones

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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.

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.


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! 

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!

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! 


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