Monday, April 11, 2005

Blakeman on Egg Development

For everyone impatiently awaiting the hatching of this year's brood(s), let me make a few comments on what's happening in the eggs. (This is not any sort of treatise on the subject. Those have been written by the raptor researchers who perfected the captive breeding of peregrines, where copious numbers of eggs and eyasses were produced for placement in wild nests and release at hack boxes. But those are other stories.)
As observers have noted, the incubating adult will periodically rise above the eggs and carefully turn them with her bill. This keeps egg membranes and tissues from fusing together in inappropriate configurations. This is a crucial parental duty.
We are now late into the incubation period. In the first week or so, the embryo was tiny and consumed very little dissolved oxygen, and it produced very little waste carbon dioxide. But as the embryo matured, with completely functioning organ systems, the consumption of oxygen increased significantly, and the production of carbon dioxide increased commensurately . Carbon dioxide dissolved in blood and tissue fluids renders them acid, and increased acidity (decreased pH) warps ("denatures") proteins, especially delicately-configured enzymes, which control virtually all biochemical reactions. There is a delicate balance between oxygen consumption and carbon dioxide production. And excess of the later is certain death.
Once out of the egg, the little eyass can breath freely just like we do. But while in the egg, things are gaseously rather tight. By the third and forth week, it's getting critical in there, as ever more oxygen must diffuse into the egg, and equivalent quantities of carbon dioxide must exit. Where? There is no breathing hole.
When laid, the egg shell is thick and strong, but later, the shell chemically begins to thin out and become microscopically more porous as the embryo develops, allowing increasing volumes of gases to pass through. By now, gases are passing rapidly through the porous egg shell itself, and also across the membranes beneath.
This shell thinning will be crucial, as the little eyass would not be able to puncture the egg were it to retain its initial thickness and strength. As most know, the little eyass has an "egg tooth," a small projection on the tip or top of the beak that is used to puncture the egg. This process, called pipping, takes about 24 hours or more, and it allows the lungs to slowly become accustomed to the increased oxygen of the open atmosphere. But more importantly, it allows the lungs to slowly adapt to the drying of real air. Inside the egg there is 100% humidity, and to be thrust quickly into open air causes the eyass's lungs to rapidly dehydrate.
So don't be alarmed if ever a camera is placed above the nest and an egg is seen to be broken, but the little eyass doesn't emerge. It shouldn't, until at least 24 hours after the egg tooth makes its first, tentative puncture. We needn't wish for haste here, our mammalian, parental instincts notwithstanding. Incubation and hatching will proceed at their slow, ancestral pace. It's all controlled by humidity and gas concentrations within the egg.
Today, changes within the eggs are happening at a rapid rate. Much of the egg "white," a concentration of proteins provided by the mother's single fallopian tube during egg development, is being converted into muscles, skin, and feathers. The egg's "yolk,' a supply of energy-dense fats, is also being depleted as the eyass grows. The white provides the body-building materials, and the yolk provides the energy to power it all. At the end, only the little hawk and a thinned egg shell will be left, and then it will be time to start the pipping process.
In short, the eggs aren't sitting there just being warmed. Some remarkable, hidden physiology is taking place. We'll be seeing the results of this not long from now.
I'm beginning to get some anticipatory excitement. Who could decide to disregard all of this? I've watched it now for over 35 years, and it's still always a thrill. I'm glad so many in New York City will be able to once again watch the appearance and development of young red-tailed hawks . It's good for everyone to have some visible, personal connections to things in nature, and what could be more noble and inspiring than watching another family of red-tailed hawks be produced? What a privilege this is. I thank all of those in Central Park who are watching and describing these events for all of us.
John A. Blakeman