Eaglet Growth and Development: Week Four

D24 and D25, April 25, 2016

D24 is 28 days old today, while D25 turns 26 days old. At this point, feather growth is starting to overtake structural growth as the eaglets approach the half-way point in the nest. Pinfeathers emerged this week, growing at an astonishing rate as they poked out from their keratin sheaths along the edge of the eaglets' wings. Feathers also started to emerge along their backs - the 'cloak' - and sprout from their tails. As beak, leg, and footpad growth began to slow, wing growth sped up, leaving the eaglets with noticeably longer, larger wings by the end of week four.

We also saw changes in behavior. Although the eaglets will continue to compete for food, baby bonking has ceased. This always makes me wonder what functions it serves. We know it 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 sticks, move towards a full stand, and expand their explorations of the nest. They've also begun noticing the area around the nest, tracking with Mom and Dad as the adults pay attention to the outside world.

So what can we expect to see in week five? Watch for an explosion of dark feathers, leading to an evenly mottled grey and white appearance some time in the next week. The eaglets will begin standing and walking, leading to many mouse clicks as we try to shoo them back to the center of the nest! We may see them begin to tear their own food and we will get to see them 'play house' as they begin moving sticks around themselves and with Mom and Dad. If past years are any judge, Mom and Dad will give them plenty of materials to work with as they pile up the crib rails with larger, heavier sticks!

We've talked a lot about physical changes. We are also entering a period of rapid developmental changes as the eaglets acquire new skills and grow feathers. The eaglets will play cooperatively and competitively, learn to stand, walk, and tear food, and begin moving their wings. Following the appearance of Mom's teakettle whistle, I'm waiting for baby vocalizations to turn into juvenile screes for food! I'm looking forward to watching D24 and D25 move into the next phase of their lives as feather growth takes over from body growth and greater mobility leads to changes in behavior.

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 weeks 4-5? I'd tend to think that legs, feet, and wings are accelerating in importance this week, leading 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 and have moved from playing with grass to nibbling at larger, heavier sticks. The nest and eagles always have more to teach us!

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

• Eaglet weight and growth is based on Gary Bortolotti's work with eaglets at Besnard Lake in Canada. It is possible that our eaglets are a little smaller than his, since Bald eagles get bigger the farther north one travels: a phenomena known as Bergmann's Rule. Bortolotti's paper makes for interesting reading and provides a great look at the work involved in field science. Citation: Physical Development of Nestling Bald Eagles with Emphasis on the Timing of Growth Events, The Wilson Bulletin, Vol. 96, No. 4 (Dec., 1984), pp. 524-54. https://www.usask.ca/biology/bortolotti/pubs/wb96-4-524-542.pdf
• Mouseunculus: http://phenomena.nationalgeographic.com/2013/07/24/mouseunculus-how-the-brain-draws-a-little-you/
• Homunculus: http://io9.com/5670064/how-your-brain-sees-your-body-meet-the-cortical-homunculus
• Input from Bob Anderson, who imparted much wisdom and information before he passed.
• RRP moderators and their calendars, lists, books, charts, and personal observations.