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 two in this blog.
In their second week of development, eaglets gain roughly two pounds, experience rapid growth in features like beaks, culmens, and footpads, start replacing their white natal down with thicker grey thermal down, and begin exploring the nest. Although they aren’t yet standing on their toes, they are able to sit up – way up! – for feeding and shuffle around on their metatarsi. Their eyes are wide open and fit more comfortably in their eyesockets, features like brow ridges are beginning to appear, and their legs and footpads are turning from pink to yellow.
Gary Bortolotti wrote that bald eagles might gain more weight per day than any other North American bird, although the majority of their weight gain occurs within the first 30-40 days. Their rapid weight growth is fueled by their nutrient-rich diet of meat and we’re watching TE3 chow down on fish, more fish, a coot, a bluejay, and yet more fish! Its natal grey down is growing in, its egg teeth is almost worn away, its beak is noticeably larger, and its eyes have receded into their eye sockets. They are also getting better at shooting poop: not surprising, given all the meals they are packing away!
Growing Eaglets = Bottomless Pits!
- Between April 19 and April 25, Mrs. T fed TE3 a total of 65 times and Mr. T fed the eaglet once. Remember, TE3 went from roughly one pound to two pounds last week, doubling its weight. That takes a lot of food!
- Mrs. T did all of the hunting and almost all of the feeding. She provided 16 fish and one opossum head. Mr. T took fish or fish parts from the nest six times.
During the second week, we observed:
- Preening: When birds preen, they remove dust, dirt and parasites from their feathers and align each feather in its optimum position. Their exploratory downy nibbles mark an important first step on the road to feather care.
- The emergence of thermal down: Thermal down began to emerge! A hatchling eaglet’s fuzzy white natal down doesn’t assist thermoregulation, aka controlling one’s temperature. Denser thermal down provides more insulation and helps nestling eaglets keep their body temperatures at a relatively constant 105’ish degrees.
- Cropzillas! As eaglets are able to handle bigger bites, little crops swell to what looks like bursting! Eaglets store food in their crops until their stomachs are ready for more. This mini ‘pantry’ helps assure that rapidly growing eaglets get the nutrients and calories they need, when they need them.
- Painting the crib rails: For an eaglet to really shoot poop, it needs to be able to ‘stand’ on its tarsi, point its little rump up in the air, and squirt! As silly as it sounds, the ability to shoot poop out of or almost out of the nest marks an important developmental milestone as the little poopcassos become more coordinated and stronger!
- Eaglet exploration! Nestlings explore the world by wandering and nibbling. We saw TE3’s stick obsession fire up yesterday and it began eaglet exploration in its second week. Enjoy your new views and stay away from the crib rails, okay?
By the end of its second week of life, TE3 was eight to ten inches tall! Its larger size, greater weight, and emerging thermal down helped it to survive the cold rain and wind that killed its siblings. Mrs. T is an excellent mother and TE3 benefits from getting all of the food (well, most of it given that Mr. T takes some). We’re watching nature, which means we never know what’s going to happen. But I feel very good about TE3’s prospects.
Enjoy eaglet earholes and egg teeth while you still can – TE3’s earholes will soon be covered by down and its egg tooth is wearing away.
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.
- Neurological coordination occurs throughout an eaglet’s time in the nest. During week two, their eyesight and basic coordination skills are improving as they grab food from Mom and Dad, sit up tall for feedings, shuffle around on their tarsi, and explore the world with their sensitive beaks. As they grow, they will become more adept at controlling beaks, legs, wings, and feet. They will 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. While useful and extremely cool, most cortical homunculi 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! I’d tend to think that visual acuity and eaglet beak and tongue sensitivity suddenly ‘lit up’ this week, leading changes in coordination as the eaglets began sitting up, grabbing food, moving around, and preening and nibbling at things.
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
- This chart, taken from Bortolotti’s work (with permission):
- 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.
The Raptor Resource Project