Eaglet Growth and Development: Week Four

April 30, 2019: D32 and D33

D32 left, D33 right

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.

D32 and D33 are 26 and 23 days old. During week three (fourteen to twenty-one days), the eaglets shed much of their natal down, gained a lot of weight (reaching as much as five pounds by around 21 days), began sprouting pinfeathers, and hit the maximum growth period for footpads and legs. While D33 is still smaller than D32, the difference is not as striking as it was just a week ago. The two are playing with nesting materials, splattering outer crib rails with expert poop shoots, casting impressively large pellets, and widening nest explorations as their vision, coordination, and strength improve. Although they aren’t yet standing on their feet, both eaglets have become impressive booty scooters as they shuffle around the nest on their lengthening tarsi…weather permitting!

Watchers are starting to ask if the eaglets are going to fledge soon given their size. No – as hard as it is to believe, we still have over 50 days until D32 and D33 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 grow from 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)?

  • The eaglets may start standing on their feet. This will change nest exploration and poop-shoots. Look out below!
  • Natal down mohawks will vanish and dark deck feather growth will accelerate. Look for the eaglets’ feather ‘cloaks’ to start filling in.
  • Still enclosed in their keratin sheaths, eaglet pinfeathers will grow longer.
    We may be treated to the beginning of wingercizing sessions! Once the eaglets can stand, they can really begin exploring their wings.

By the end of their fourth week, the eaglets could be standing. By the end of their fifth week, they 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 inquisitive eaglets back into the center of the nest as they widen their explorations and begin broadening their horizons! We will also see changes in behavior. Although the eaglets continue to compete for food, baby bonking has mostly ceased. This always makes me wonder what functions it serves. We know bonking strengthens muscles, aids coordination, and helps improve eyesight. Does food competition lead to greater food intake, helping to fuel an eaglet’s rapid growth? Does it lay the ground for future social interaction, which includes plenty of body language, vocalization, and dominant/submissive interaction? Does it give parents information about an eaglet’s overall heath, or help prompt provisioning? Or is it simply replaced by a new suite of physical behaviors as the eaglets begin to explore the nest and enter the next phase of nestling life? Bonking may have ended, but the eaglets are starting to play with nest materials, move towards a full stand, and expand their explorations of the nest.

While we’ve been making guesses at gender, the weight of the two sexes begins to separate as females gain weight faster than males. Sex takes over from age as a size determinant between 50 and 60 days. But cameras can be tricky and clutches can have large males and small females or be all one sex, making ID impossible without measurements or a genetic test. We’ll have a lot of fun seeing if size conforms to our observations based on what we have seen of beak size, commissure extension, and other traits, and I can hardly wait for food tearing and wingercizing!


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, leading 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: