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 28, 2022: Happy 22 day, DH2! DH2 weighs about 3.3 pounds and its footpad is about four inches long.

April 28, 2022: Happy 22nd day, DH2! DH2 weighs about 3.3 pounds and its footpad is about four inches long.

During week three (fourteen to twenty-one days), DH2 shed most of its natal down, gained a lot of weight, hit the maximum growth period for footpads and legs, and swallowed its first indigestible materials: a fishtail, bones, and tendons. Our little eaglet is playing with nesting materials, splattering outer crib rails with expert poop shoots, and widening nest explorations as its vision, coordination, and strength all improve. Although DH2 isn’t yet standing on its feet, it has become an impressive booty scooter as it shuffles around the nest on its lengthening tarsi…weather permitting!

Look for these changes in the fourth week

As hard as it is to believe, we still have 50 or more days until DH2 fledges. 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)?

  • DH2 may start standing on its feet. This will change nest exploration and poop-shoots. Look out below!
  • Its natal down mohawk will vanish and its dark deck feather growth will accelerate. Look for DH2’s feather ‘cloak’ to start filling in.
  • Its pinfeathers will begin to grow.
  • We will be treated to the beginning of wingercizing sessions! Once DH2 can stand, it will really begin exploring its wings.

By the end of DH2’s fourth week, it could be standing. By the end of its fifth week, DH2 will be standing and could be starting to walk. I have no doubt that many of us will be mouse-clicking, shoeing, and blowing to get the inquisitive eaglet back into the center of the nest as it widens its explorations and begins broadening its horizons! We will also see changes in behavior as DH2 begins to attempt self-feeding.

One versus more: sibling competition and eagle development

Some of you have wondered if DH2 is 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. However, the singletons we watch often seem to hit their early developmental milestones – sitting up, nest exploration, crib rail spattering, and so on – a little later than broods with more than one eaglet. Why would an eaglet with more food than it can eat grow more slowly than eaglets that need to compete for food?  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.

What sex is DH2? We have no idea!

When a nest has male and female eaglets, we can make informed guesses at sex based on overall size and age at fledge. But even then, cameras can be tricky and broods can have large males and small females or be all one sex. I’m sure we’ll all have guesses and we might be able to do some size comparisons next to HD and HM, but we won’t be able to sex DH2 without measurements or a genetic test.

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: