The video below shows embryonic chicken development. Eaglets and chicks are very different: chickens are precocial birds that run around and feed themselves shortly after hatching, while eagles are altricial birds that require intense parental care. Chicken egg yolks are larger, their eggs contain less albumin, and they are more developed when they hatch. But this video still gives a good look at what goes on inside the egg.
While the math doesn’t quite math, divide chicken time by 1.55 to yield an approximate embryonic eaglet stage of growth and development. Since chickens hatch after about 21 days and eaglets hatch after about 38, one day of chicken time is about 1/2 day of eagle time…more or less.
What do embryonic eagles look like as they develop and grow inside their eggs? Dr. Peter Sharpe of the Institute for Wildlife Studies created a table of bald eagle embryonic development, based on research by Hamburger and Hamilton (1951). While not all bald eagle eggs hatch in exactly 35 days, their development generally follows these stages…
Development of an avian embryo
How does it all begin?
As a fertilized egg begins its journey through a female eagle’s reproductive system, a single cell, formed by the union of sperm and egg, divides into multiple layers. The foundations for the head and tail take shape, the emerging embryo forms segmented blocks called somites, and essential life-support systems begin developing, including the nervous system, skin, circulatory system, gastrointestinal system, and optical system. Around 72 hours after incubation begins, the tiny heart starts to beat, and the embryo shifts onto its left side.
At four days of age, the embryonic eagle looks nothing like a bird, but it has inner and outer layers, a developing circulatory system capable of transporting materials, and an anterior-to-posterior template in place. While its brain and nervous system will continue to grow and change, the stage is now set for the formation of its skeletal system, limbs, beak, tongue, wing and foot digits, and internal organs.
An eaglet’s inner and outer layers aren’t yet separated by its integument. Part of the embryo’s network of blood vessels lies outside its body and runs under the eggshell, collecting oxygen and calcium and releasing carbon dioxide. That network will supply oxygen to the embryonic eagle until shortly before hatch.
An embryonic bird in very early stages of development. It has a top, bottom, front, back, left and right sides, and layers.
How big is it?
Based on scaling from chickens, a four-day old embryonic eaglet is likely 1–2 mm in length: almost invisible except as a thickening or blood spot on the egg yolk. That’s tiny by our standards, but massive compared to eagle eggshell pores, which are estimated to range from 20 to 30 microns in diameter.
Birds can adapt the size of their pores to air pressure (chickens nesting at altitude versus chickens nesting at sea level). I would love to know whether Bald Eagles – say, DNF – can adapt egg pore sizes to local weather conditions from year to year. If they can, does this capacity increase as they become experienced parents? Decrease as they age? So many questions!
To everything, turn, turn, turn…
DNF and Mr. North are rolling and footing their eggs multiple times a day. Turning promotes the development of the embryo’s external blood vessel network, facilitates the diffusion of nutrients and water within the egg, and helps ensure that the embryo remains properly positioned relative to the yolk and albumen. Without gas exchange, nutrients, water, and fuel, the embryo will fail to develop and likely die.
Egg turning is especially critical in the first few days of egg/embryonic development. We’ll see whether this behavior diminishes as the embryos get larger and the bottom of the egg becomes heavier. I also wonder whether the ‘Decorah shimmy’ that positions eggs up against a parent’s brood patch shuffles them enough to facilitate nutrient and water distribution…not all that dissimilar to shaking a can of paint.
Why is there blood on the eggs?
Do you see blood on the eggs? While it looks alarming, the embryonic eagle is not vascularized enough to produce substantial amounts of blood and may not even have a heart. The contents of the egg are primarily yolk and albumen, so damage will appear not in the form of blood, but as a crack and leak of clear or yellowish fluid. Eagles often have messy lunches and sometimes a little bit drips on their eggs. It’s all good in the North oak woods!
A messy lunch can result in a messy egg!
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.
Things that helped me learn about this subject:
The Raptor Resource Project