Eaglet Growth and Development: Week Three

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 three in this blog.

DH2 turns 18 days old today. During week two (seven to 14 days), its footpads and talons grew, its feet and legs yellowed, its talons started turning black, and its grey natal down started sprouting! During week one – especially the first few days – HD and HM did their best to carefully place tiny morsels of meat into gaping, sometimes unsteady beaks. During week two, they dangled larger strips of flesh for their hungry eaglet to lunge at, improving visual acuity and feeding skills. DH2 had a fine buffet to chose from, including trout from the stream (look for finer scales and a front pointing mouth with thin lips), sucker (look for rougher scales and a bottom pointing mouth with fleshy lips), rabbit, and duck. Of course, all that food resulted in a whole lot of eaglet poop. DH2 got better at sitting up, bending over, and shooting poop, although sometimes parents got in the way!

Is DH2’s development slower, even though it has no siblings to compete with? When we compared its developmental milestones with eaglets from previous years, and with eaglets from Xcel Energy’s nest in Fort St. Vrain, Colorado, DH2’s developmental stages appeared to be proceeding just a little bit slower. Chicks of eagle species that raise just one young tend to have slower growth rates early in nest life, although we don’t why. However, we suspect that the overall need to compete might spur growth and developmental rates more than extra meals do. And now that HM and HD have dialed in feeding, DH2 is getting a lot of food! https://youtu.be/kdGnpLUNXTM.

We saw whole lot of eating and sleeping during week two.  While big changes are on the way, week three (14 to 21 days) will involve a whole lot more eating and sleeping as DH2 continues to gain wait and starts sprouting pinfeathers!

Here are some things to watch for:

  • Mohawks! Natal down disappears from the head last, which will give DH2 a distinctly punk rawk ‘do in the next seven days or so.
  • A Cropzilla so heavy that DH2 has a hard time sitting up! Given that bites now include scales and fur, we will likely see pellet casting soon.
  • Pinfeathers! Look for pinfeathers to start emerging along the edges of DH2’s wings.
  • Eaglet explorer! While DH2 can’t stand on its feet yet, it’s getting stronger and more coordinated: https://youtu.be/xlVgcfax6Yc!  It will begin to expand its explorations as it grows older and (hopefully) the weather becomes a little warmer and drier.
  • Clown clompers! Feet pads and legs/tarsi will grow like crazy, as this chart from Gary Bortolotti shows! You think those footpads are big now? Just wait!Eaglet Footpads
April 24, 2023: DH2. Look at that footpad!

April 24, 2023: DH2. Look at that footpad!

By the end of week three, DH2 will be about 25% of the way through life in the nest. Our adorable little fuzzball will have been (mostly) replaced by a large grey tween eagle. Enjoy white down and earholes while you can!

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 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! Areas related to visual acuity, strength, and coordination all ‘lit up’ in week two as the eaglets got better at perceiving things at a distance, improved their poop shooting, reached for food, took bigger bites, and started exploring!

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