Eaglet Growth and Development: Week Four

We’re writing a series of blogs about the first few weeks of an eaglet’s life. An eaglet spends roughly 75 to 80 days in the nest. For about the first half, it grows and gains weight. For about the second half, it grows flight feathers and starts developing the skills it will need post-fledge. We will focus on week four in this blog.

April 10, 2024: DN17 and DN18. They turned 17 and 16 days old today.
April 10, 2024: DN17 and DN18 at 17 and 16 days old.

During week three (fourteen to twenty-one days), the dynamic duo shed most of their natal down, gained a lot of weight, hit the maximum growth period for footpads and legs, and swallowed their first indigestible materials: a fishtail for both. They are playing with nesting materials, splattering outer crib rails with expert poop shoots, and widening nest explorations as their vision, coordination, and strength all improve. They began standing on their feet with wing assists on April 11 and relatively unassisted on April 13!

Look for these changes in the fourth week

DN17 and DN18 turned 25 and 26 days old today. As hard as it is to believe, we still have roughly 50 or more days until they fledge. Eagles grow very rapidly in their first thirty-five to forty days of life, gaining weight and building bones, muscles, tissue, and features like tarsi, footpads, toes, and claws. But during an eagle’s fifth week of life (28 to 35 days), feather growth starts to overtake structural growth. Pinfeathers sprout on eaglet wings, tails, and backs; beak, leg, and footpad growth all slow; and wing growth speeds up. So what can we look forward to in week four (21 to 28 days)?

  • Their natal down mohawks will vanish and their dark covert or deck feather growth will accelerate. Look for their feather ‘cloaks’ to start filling in.
  • More pinfeathers! I don’t think we’ll see plushy tushy tail feathers, but we’ll see.
  • We will be treated to practice for the wingercizing Olympics!

Depending on the weather, they might be walking around the nest by the end of this week. I have no doubt that many of us will be mouse-clicking, shoeing, and blowing to get the inquisitive eaglets back into the center of the nest as they widen their explorations and begin broadening their horizons! They’ve already attempted self-feeding and may be successful by the end of the fifth week, if not the fourth.

One versus more: sibling competition and eagle development

Last year, some of you wondered if DH2 was affected by the lack of a sibling. Like everything else about eagles, it’s complicated! Researchers have analyzed data on brood size and nestling period for several species of eagles and found that those with broods of one grow slower than those with broods of two or more. However, they don’t agree about the role of sibling competition in selecting for rapid growth or earlier achievement of developmental milestones.

Eagle siblings compete for food and establish a size-based pecking order very early in nest life. As Bortolotti writes of bald eagles: “For the Bald Eagles I watched, the size difference between siblings was important in determining the outcome of bouts of sibling aggression. When the size difference was large early in the nestling period, the first-hatched chick (C1) almost invariably pecked, bit, and thrashed the second-hatched chick (C2) into submission. Such aggression…was more like a contest, often instigated by C2.” He adds: “As the size difference between the siblings decreased, so did the frequency of aggression, and more important, C1 was less frequently dominant and C2 retaliated with aggression rather than adopting submissive postures“. Although eaglets compete for food throughout their time in the nest, this is completely in line with our observation that baby bonking mostly ceases in the third week.

Bortolotti didn’t believe that sibling competition was an important factor in selecting for rapid growth, evolutionarily speaking. But DN17 and DN18 are hitting their developmental milestones a little earlier than DH2 did last year. So why would two eaglets that compete for food grow more rapidly and reach developmental milestones earlier than one eaglet that had more than it could eat?  We know that bonking strengthens muscles, aids coordination, and helps improve eyesight. Does food competition lead to greater food intake, helping to fuel early rapid growth in nests with multiple eaglets and plenty of food? Does it lay the ground for future social interaction, which includes plenty of body language, vocalization, and dominant/submissive interaction? More research is needed. (Amy’s note: I’ve been reading about filial imprinting and interaction. We’ll be blogging on it soon.

What sex are the eaglets?

We don’t know DN17 and DN18’s sex, but we do know that DN17 is more likely to be female and DN18 is more likely to be male. Sex is linked to hatching order; a large study at Besnard Lake Canada found that, in 2-egg clutches, females represented 63% of first eggs hatched and 31% of second eggs hatched. We’ll look for size differences (if the two are different sexes, their growth should be beginning to diverge now), feather development differences (males develop feathers faster), vocal differences (females have lower-pitched or deeper voices), and behavioral differences (males generally fledge early than females) to see if we can tell.

The general stages of eagle development are:

Stage 1 – Structural growth. In their first thirty-five to forty days of life, eagles grow very rapidly, gaining weight and building bones, muscles, tissue, and features like tarsi, footpads, toes, and claws. This phase of development slows down about halfway through an eaglet’s time in the nest, even though individual features might continue some level of growth.

Stage 2 – Feather and flight-related growth. Eagles grow four sets of feathers – natal down inside the egg, thermal down, juvenile feathers, and adult feathers. Thermal down starts growing at about ten days, juvenile deck feathers at about 20-23 days and juvenile flight feathers at about 27 days, but feather growth doesn’t overtake structural growth until thirty-five to forty days after hatch. Flight muscles also begin growing as eaglets wingercize, flap, hover, and eventually branch and fledge.

Stage 3 – Neurological Coordination. Eagle watchers know how ungainly eaglets can seem! As they grow, they become more adept at controlling beaks, legs, wings, and feet. They learn to stand on their own feet, tear food, self-feed, and flap their wings, going from cute but clumsy clown clompers to graceful young eaglets poised at the edge of fledge.

So where is our cortical homunculus in week four? I think that legs, feet, and wings are accelerating in importance this week, which will lead to important behaviors like standing, tearing, and flapping! I also wonder what impressions are being made now that they are beginning to pay attention to the outside world. The nest and eagles always have more to teach us!

Things that helped me write this blog, with a few considerations: