Eaglet Growth and Development: Week Four

Top to bottom:
Decorah N2B, Decorah North,
Xcel Fort St. Vrain

Most of our eaglets are in their fourth week of life: 26, 25, and 22 days old at N2B in Decorah, 27 and 26 days old at the Decorah North nest, and 31 and 29 days old at the Xcel Energy Fort St. Vrain nest. Over the past 16 days, we've seen eaglet footpads and legs growing and turning yellow, talons darkening from taupe to black, grey thermal down replacing white natal down, and pinfeathers emerging from eaglet wingtips. The eaglets have started coughing up pellets, playing 'house' (moving grasses and other nesting material around), and taking their first steps towards self-feeding (https://youtu.be/IPkJ6kgYFHs). As their vision, coordination, and strength have improved, the eaglets have expanded nest explorations and started to track events outside their nests, although they also spend a lot time sleeping off big meals and cuddling or even hiding under piles of grass in the cooler, wetter weather at both Iowa nests.

Several watchers have asked if the eaglets are going to fledge soon given their size.  No - as hard as it is to believe, we still have roughly 50 days until fledge at both Decorah nests and 45'ish days until fledge at Fort St. Vrain! 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 the coming week? Remember, the eaglets we are watching range from 22 days (D28 is just starting its fourth week) through 31 days (FSV34 is about halfway through its fifth week).

• The eaglets should 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 sticks, 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 around 50-60 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!

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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 3-4? 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. 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.