Eaglet Growth and Development, Week Two

It is April 10 as I write this, and our eaglets are growing rapidly! In Decorah, D26 is 10 days old, D27 is 9 days old and D28 is 6 days old. At Decorah North, DN4 and DN5 are 12 and 11 days old. And at Fort St. Vrain, FSV34 and FSV35 are 15 and 13 days old.

D26. See the earhole?

In their second week of development, the eaglets will gain roughly two pounds between their 7th and 14th day of life. They will 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 yellow, not pink. 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. This rapid weight growth is fueled by their nutrient-rich diet of meat. Over the past week or so, we watched the eaglets chow down on fish, roe (fish eggs), rabbit, squirrel, unidentified birds, and prairie dog. Poop is beginning to streak the poopcasso tree and crib rails as the eaglets become more proficient at shooting poop out of the nestbowl. While babylet battling hasn’t entirely subsided, it has become less intense as pecking orders are established and eaglet crops are repeatedly stuffed until they look ready to burst!

Ma FSV provides shade for FSV34 and FSV35

The Fort St. Vrain eaglets are the oldest of the group. Watcher Donna Young wrote that “We have two eaglets that are already quite adventurous. They are moving about the big nest. One climbed up onto the fence rails yesterday, but found its way down too. It may become a true Colorado mountaineer!” This is in line with past years at Fort St. Vrain, where a large nest and warm temperatures seem to lead to earlier wandering. Cold is a challenge to eaglets under 10-15 days of age, but so is heat! With little ability to control body temperature and no way to lose heat except by panting, the eaglets do their best to retire to what little shade tree limbs and the nest itself offers. Parents also provide shade for the eaglets by moving from one to the other and standing between them and the sun.

In the week to come, we can expect (continued) rapid growth in footpads, talons, and legs. Beak growth will rapidly slow as the eaglets’ beaks approach adult size and we may see dark juvenile feathers start to sprout from their grey down. Overall weight and height gain will continue, most likely reaching their steepest curves some time this week. By the end of their second week of life, our little bobbleheads at Decorah, Decorah North and Fort St. Vrain 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.

Let’s talk a little bit about ‘parenting styles’. Last year, watchers observed that Dad North was less involved in feeding his eaglets directly, although he participated in a lot of bucket brigade feedings. The North eagles didn’t tend to stockpile prey and food often seemed scarce at the North nest. Given the differences between Dad Decorah and Dad North, some watchers speculated that Dad North was on his first round of eaglets. While we didn’t weigh in on that discussion, we have seen changes this year. The North’s nesting chronology moved ahead by almost one month to match that of the Decorah eagles. While Dad North still offers the bucket brigade from time to time, he is participating in more tandem feedings with Mom North. When feeding solo, Dad North often offers food to both eaglets, picks up dropped food and re-offers it, and removes grass from their beaks. Food seems plentiful compared to last year, with fish after fish coming into the nest for DN4 and DN5. Using feeding and food availability as benchmarks, Dad North has undeniably become more skilled at some aspects of eagle parenting. Like flying, parenting is instinctual – but proficiency is learned.

Tandem feeding, Dad and Mom North. Dad North (at left) is feeding DN5. Mom is feeding DN4

As John pointed out, food availability in the nest reflects food availability on the ground. In late March, suckers are spawning, trout are actively feeding on emerging and hatching insects, rabbits and other mammals are leaving their winter dens and grounds (often with young in tow), and flocks of birds are migrating through the area. This rush of food comes at the perfect time for newly hatched eaglets – something Mom and Dad North appear to be taking full advantage of this year! The Fort St. Vrain nest is also piled high with prey, including prairie dog. If the weather stays warm, we might see turtles join the list as streams and smaller water holes shrink. Look for turtle plastrons at the bottom of the Fort St. Vrain nest!

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.

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! I’d tend to think that visual acuity suddenly ‘lit up’ this week, leading changes in coordination as the eaglets began sitting up and moving around.

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