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Free from a watery environment, bird eggs survive on land, and therefore, can develop in a vast variety of environmental conditions. Bird eggs are large. The bird embryo develops within the egg entirely independent of their parents for resources. Bird eggs are a complicated structure. Egg development and the growth of an embryo within the egg is a fascinating process and story. For now we'll address just one bird egg topic.
Last summer we wrote a piece on California and Mountain Quail, a part of which discussed synchronized hatching which is typical of most gallinaceous (ground) birds. Although as many as 14 eggs may laid over a period as long as 14 days or more, the entire clutch hatching occurs within hours. With quail, the clutch hatching is 21 days after the bird started incubating. Galliformes are precocious and leave the nest within a day.
How do eggs remain viable for an extended period before incubation starts? When this question comes up, a sense of puzzlement on the part of the inquirer is evident, as it was with me for some time, and, in part, is to this day. Combing through the literature did not reveal much on this question. So we sent off an inquiry to the Cornell Laboratory of Ornithology in Ithaca, New York. Edited a bit, here is Claudia Zan's reply.
"I'm not sure that we know what mechanism, or process, maintains the viability of fertilized eggs. We do know that temperature is the most important factor determining the viability of unincubated eggs. Embryos are most susceptible to exposure to high and moderate temperatures.
"At cool temperatures, freshly laid eggs remain in a state of stasis (stagnation of bodily fluids – a condition of balance. Ed). No development will occur in eggs maintained in temperatures below physiological zero, or 80.6F. Thus, freshly laid eggs maintained at temperatures below physiological zero can remain viable for an extended period of time.
"Embryo development commences as soon as the eggs are warmed above physiological zero. However optimal development occurs at incubation temperatures typically between 96.8F and 100.4F. Thus, in temperate climates, cool ambient temperatures allow eggs to remain dormant until the onset of incubation.
"On the other hand, exposure to temperatures above physiological zero, but below normal incubation temperature, can cause abnormal development and embryo mortality. As a result, in warm climates, birds may be constrained to commence incubation early in order to maintain egg viability and increase hatching success.
"Few studies have been conducted to test the egg viability hypothesis in passerine (perching or song birds. Ed) birds. However, among passerines, there is a general correlation between hatching asynchrony (eggs hatching day by day over several days. Ed) and latitude. Which suggests that egg viability might provide a strong selective pressure favoring hatching asynchrony.
"For example, in House Sparrows, eggs left unicubated for 3 or more days and exposed to temperatures above physiological zero exhibited reduced hatching success compared to eggs that were incubated earlier."
Just to finish off Claudia's last comments, laying dates within the breeding season do vary with clutch size. That is, small clutches (2-3 eggs) lay later. Large clutches ( 4-6 plus) lay earlier. So, our large clutch synchronized hatching Quail nest and lay earlier.
This appears to contradict her last suggestion which was that egg viability might provide a strong selective pressure towards asynhronized hatching. This certainly appears correct. To that though, we'll just add that success in natural selection considers more than one, or even two, traits at a time.
Here, out of context, using the above clutch size definitions, are some interesting clutch size facts. Generally speaking, small-clutch birds are fed by the adults, occur in the tropics, found in lower altitudes, oceanic islands and coastal areas, use open nests in high nest available sites, the female will be younger, and laying date will be later.
Again, generally speaking, large-clutch birds will feed themselves, occur in temperate/arctic regions, are found at higher altitudes, mainland and continental interiors, in low nest available sites use cavities, while the female will be older, and will lay earlier.
Thank you Claudia and Cornell. The key point is that freshly laid fertilized eggs, maintained at cool temperatures, that is, below physiological zero, will remain viable for an extended (not indefinite) period. What process maintains this stasis (balance) we may not know until a researcher discovers it. On the other hand, maybe there is no mechanism. Perhaps it is just the biological nature of eggs and is no mystery at all.
Good birding – gone birding.
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