Eaglet Growth and Development: Week One

We are watching eaglets at three nests right now: Decorah, Decorah North, and Xcel Energy Fort St. Vrain. The oldest (FSV34) is five days old, and we're still waiting for DN6 and all of the Decorah eaglets as I write this. What can we expect in the first week of watching? Weight Gain!

DN4 at the Decorah North Nest

Like humans and other animals, growing nestlings have developmental milestones. The eaglets spend roughly the first week of their life gaining weight. They aren't able to thermoregulate yet, so depending on the weather and temperature, they may spend a lot of time under Mom and Dad. We'll see them eat, sleep, scuffle, and grow stronger as they interact with one another.  They will go from roughly 3.2 ounces - about the weight of 18 nickels - to roughly 16 ounces or one pound, increasing their weight five times over in just seven days.

Weight gain (g/day) as a function of age for male and female nestling Bald Eagles
This chart was reprinted from The Wilson Bulletin 96: 527 from an article published by G. R. Bartolotti (1984) with the written permission of the Wilson Ornithological Society. 

Many structural features, including foot pads, tarsi, and hallux claws, won't start rapid growth until 10-15 days after hatch. But the hatchlings' mid-toes and culmen - the dorsal ridge of the upper mandible - are already growing longer! Food is the root of all else besides, so it isn't surprising that the culmen achieves maximum growth in the first ten days. I suspect that the mid-toe aids balance, a crucial element of sitting up and exploring the nest. While our eaglets won't truly stand on their feet until they are roughly four weeks old, they will begin to shuffle around the nest on their tarsi long before that.

Enjoy the downy bobbleheads this week! By next week, they will already be growing their longer 'wooly' second or thermal down and alternately worrying and thrilling us with their interactions and sojourns around N2B.

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. While thermal down starts growing at about ten days and juvenile flight feathers at about 27 days, 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.

I'm not sure how familiar many of you are with the cortical homunculus, an image-based tool that maps tactility. We discussed it very briefly in this blog and I'll include links below. While useful and extremely cool, most cortical homunculii are static - that is, they reflect just one phase (usually adult) of an organism's life. But an eaglet's cortical homunculus will differ from an adult's as body parts and associated skills are gained and neural pathways developed. Our eaglets' brains and bodies are rapidly growing and changing as they gain the skills they need for life outside the egg!

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

• Eaglet hatch weight 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. Thanks to The Wilson Ornithological Society for their permission to use this chart. 
• 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.