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| An infertile chicken egg. See the blastodisc? |
Let's assume we have a fertilized egg, discussed here. As the embryo develops, it will consume both the fatty egg yolk and high-protein egg white. By the end of day two, our little chicken will have a heartbeat. Its beak and egg tooth will begin to form on day six, and its feathers on day eight. By day nine, it will begin to look somewhat like a bird, and its mouth opening will form. On day thirteen - well, just go look at this blog for the rest.
So what does the developing embryo need to survive? We discussed the importance of temperature here, but incubating birds (or incubationist humans) also need to control humidity, move or turn the eggs, and make sure the eggs are well-ventilated.
Keeping Eggs Alive
Why were the eagles off the eggs so long on March 7th? We were asked this question on facebook, ustream, and via email. I think the eagles may have been giving the eggs a chance to dry out and breathe.
A bird's eggshell has thousands of tiny pores, which allow water and gas to pass through. Mammals like us get oxygen through an umbilicus, but developing birds receive oxygen and remove carbon dioxide through the egg shell. Gases, including oxygen, enter and leave the egg by diffusing through the pores in its shell, across the outer and inner shell membranes, and into the blood in the capillaries of a special tissue called the CAM, or chorioallantoic membrane. As the weather warmed in Decorah, the snow began to melt and the humidity soared. Condensation can form on eggshells exposed to excessive humidity, which clogs shell pores and provides a vehicle for bacteria. The result? Fatal suffocation and/or contamination. Only the eagles know for sure, but I think they may have responded to the threat of rising humidity levels by leaving their eggs uncovered. Standing or leaving entirely allows fresh air to circulate over the eggs, dropping the humidity level and giving the developing embryos fresh air. Turning or rolling assists air exchange, helps maintain an even egg temperature (especially in big egg piles, where outer eggs may be cooler than inner ones), and keeps the developing embryo from sticking to the eggshell.
In short, most bird eggs require specific temperatures, proper humidity, regular movement, and air exchange. Birds provide these by applying body heat, standing and/or leaving for periods of time, and rolling or turning their eggs. In birds, incubation is both an instinctive and learned behavior - while birds will automatically incubate eggs or egg-shaped things, they get better at incubation with experience. There are no guarantees, but Mom and Dad are skilled incubators with a long history of brooding together. Watching them meet the challenges of their environment gives me hope for their eggs. Go eagles!
Resources
Bob Anderson, personal communication.
The Chicken Chick. This is a really neat site for those of us who love chickens!
Brinsea. For all your incubation needs.
Texas A&M.
Did you know?
We usually talk about birds, but people also incubate snake eggs! This video from Snake Bytes discusses the ins and outs of snake incubation: http://youtu.be/JlSS5wap-Zc
