The Eastern Bluebird is a common species of open and semi-open habitats. Easily recognized by their brilliant blue plumage, bluebirds are a familiar sight in my Canton neighborhood, perch-hunting for insects from tree snags, mailbox poles and stonewalls.
My own yard not only provides excellent feeding opportunities, it provides bluebirds with a place to nest and raise young. In a typical breeding season, our lone nest box will produce two clutches of eggs.
After a successful first nesting this summer, a second attempt was soon underway. As activity inside the box increased, I confirmed that a new clutch of five eggs were placed. However, in the days to follow, the lively activity that had preceded the egg laying had all but ceased. Suspicious, I decided to check the box one more time. What I discovered was truly upsetting. The bluebirds had abandoned the nest box and five eggs due to a large infestation of sugar ants.
Disappointed, I removed the nest and prepared the box for another possible nesting attempt. I brought the infested nest down to the woods to discard it properly and bury the eggs. As I was carefully picking out each individual bluebird egg to set it down on the ground, one of them had broken in my hand. The photograph below is what I found inside the broken egg.
Admittedly knowing nothing about the development of an egg, I turned to the book; Manual of Ornithology (Noble Proctor and Patrick Lynch, 1993) for some enlightenment. I’d like to share with you some information from the book which helped me better understand the structure and development of an egg.
The avian egg is a miracle of natural engineering. Light and strong, it provides everything a developing bird embryo needs from just after the ovum is fertilized until the chick hatches.
The avian egg is a self-contained womb in which the embryo is protected and provided with the food and nutrients it needs to develop and hatch. But the egg is not a completely closed system, shutting off all outside contact. Birds, even as they are developing in the egg, are warm-blooded animals, with all of the metabolic activity that homoiothermy demands. The chick within the egg must be able to respire, to exchange gases and water vapor with the outside world, or it will suffocate.
The bird egg must allow an efficient exchange of gases to support the growing chick, allowing waste gases out and oxygen in. The avian eggshell is not simply a coating of calcium carbonate but a complex laminate of mineral crystals embedded within a web-like matrix of material similar to collagen fibers. Reptiles, birds and mammals are called amniotes because the eggs of all three groups develop within an amniotic membrane. The amniotic membrane forms an enclosed, watery environment that protects the developing fetus and allows it to exchange gases with the surrounding world (Carey 1980, 1983; Romanoff and Romanoff 1949).
The yolk of the egg supplies the embryo with nutrients, storing food until the chick hatches. The yolk sac surrounding the yolk is another embryonic membrane. The yolk is approximately 30 percent lipids (fats), 15 percent protein, and 55 percent water. Fat, not protein, is the primary food in bird eggs. This gives bird eggs a significant advantage over reptile eggs, in which the primary nutrients are stored as protein, because when broken down, fats yield more metabolic energy and water per unit than do proteins. They can thus survive in much drier environments than reptile eggs.
The age of the embryo, from what information I could gather, is anywhere between eight to twelve days old, given the size of the formed digit and toe plates.
Despite the loss of our five bluebird eggs, I feel there is always a way to take a bad situation and extract something positive from it. Learning about egg structure and the developing embryo was certainly an eye opening experience, as was the actual embryo in my hand.
I also realized that I needed to be a better steward to the birds I provide for. Whether it be a hummingbird feeder or a nest box, periodic inspections and cleaning helps ensure a birds health and nest productivity.
Finally, I would like to thank Noble Proctor and Patrick Lynch for publishing this outstanding book. The writing is excellent and easy to understand, and the drawings are exceptional. Roger Tory Peterson’s foreword to the book says it best; “A gold mine of facts…Every library and biology department, as well as every birder, should have a copy close at hand.” I couldn’t agree more.
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