Eaglet Growth and Development, Week Two

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.

April 3, 2024: DN17 turned 10 days old and DN18 turned 9 days old today!
April 3, 2024: DN17 turned 10 days old and DN18 turned 9 days old. Look at their thermal down, growing wings, pouchy rear ends, and big beaks! Squeeee!

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 the terrific two chow down on fish, more fish, an opossum, a raccoon, and yet more fish! Their natal grey down is growing in, their egg teeth are almost worn away, their beaks are noticeably larger, and their eyes have receded into their eyesockets. They are also getting better at shooting poop: not surprising, given all the meals they are packing away!

Growing Eaglets = Bottomless Pits! 
  • Mr. North and DNF fed a total of 120 times between March 24 and April 2. DN17 ate 110 meals and DN18 ate 87 meals. Remember, the terrific two went from roughly three ounces to one pound last week, increasing their weight about five times in just seven days. That takes a lot of food!
  • DNF did most of the feedings (91 to Mr. North’s 29), but Mr. North brought in more meals (19 to DNF’s 8). Although there were more fish than anything (20 of 27 prey items were fish), the Norths also brought in two squirrels, one racoon, an opossum, and three UFOs!
During the second week, we’ll be looking for:
  • 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 begins 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! We’re seeing little cropzillas already. As eaglets are able to handle bigger bites, little crops swell to what looks like bursting! They store the 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 and babysitting branch: 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! DN17 ‘painted’ DNF yesterday: https://youtu.be/tOpbnTQXLow?si=2_HH-B7fK9YZtSki
  • Eaglet exploration! Nestlings explore the world by wandering and nibbling. We saw DN17’s stick obsession fire up yesterday and will likely see intentional eaglet exploration by the end of this week. Enjoy your new views and stay away from the crib rails, okay?

By the end of their second week of life, DN17 and DN18 will be almost a foot tall! Enjoy eaglet earholes and egg teeth while you still can – their earholes will soon be covered by down and their egg teeth are 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: