As of this blog post, DNF’s two eggs are 22 and 19 days old. We have a three-day-old egg in Trempealeau and four eggs ranging from 11 days old to two days old at Xcel Energy’s Fort St. Vrain nest.
What do embyronic eagles look like as they develop and grow inside their eggs? Dr. Peter Sharpe from the Institute for Wildlife Studies developed a table of bald eagle embryonic development based on work done by Hamburger and Hamilton (1951). While not all bald eagle eggs hatch in 35 days, the stages of development look something like this…
Development of an avian embryo, drawing from Frank Lillie photos. Artist William Sillin
What happens between the 11th and 17th day? When we last peeked inside the egg, our embryonic eagle had a head, the beginnings of an eye, stiff differentiated limbs, the beginnings of a beak, and rudimentary digestive organs. It required a little imagination to see a bird, but our eaglet was beginning to take shape.
Day Eleven to Day 17
Between eleven and seventeen days, our eaglet grew a beak and tipped it with an egg tooth! Its limbs bent as they became longer and more defined, feather germs formed, and a new bony plate around its eyes provided protection, structure, and a place for muscles that control focusing and eyelid movement to attach. Its nictitating membrane became visible and its eyelids closed on its still-developing eyes.
This chicken embryo is roughly equivalent to an eaglet at about 17 days. We can see the bony plate around its eyes, developing eyelids, egg tooth, long folded limbs, feather germs, and oversized head. Next up: feathers and scales!
At 17 days, we are roughly halfway to hatch. Our embryo’s head is disproportionately large, but it is definitely a bird. It has a beak, distinct toes, bent limbs, and eyes that take up most of its head. But its eyes and eye cavities aren’t done forming and it needs to develop scales, nails, rough pads and spicules, and down feathers. Its yolk sac and small intestines are still outside its body cavity, and it has a lot of growing to do!
Illustrations were taken from Popular Science Monthly/Volume 71/September 1907/The Problem of Age, Growth and Death III: Link. Thanks to artist William Sillin for allowing us to use his lovely illustrations: http://www.willsillin.com/ (check it out – his illustrations are very cool!). Also take a look at this cool plate by Keibel and these lovely photos of chicken embryos: http://www.microscopy-uk.org.uk/mag/artnov04macro/mlchicken.html.
Image Credit
Embryo: Oxford Scientific – https://www.gettyimages.com/detail/photo/chick-embryo-gallus-gallus-9-days-old-royalty-free-image/90051284?adppopup=true
Things that helped me learn about this subject:
The Raptor Resource Project