A colleague and former boss of mine, Tom Martin, wonders about these things all the time. He recently published a paper in the Proceeding of the National Academy of Sciences:
Martin, T.E. 2008. Egg size variation among tropical and temperate songbirds: An embryonic temperature hypothesis. PNAS 105:9268-9271.
I was excited to read the paper because it presents an interesting hypothesis about variation in egg size among birds and because I measured many of the eggs!
In the paper, Tom reports that, after controlling for adult body size, eggs are larger in South America than in North America. Birds in South America spend less time incubating eggs than their North American cousins and as a result, their average egg temperatures are cooler. The pattern exists among birds within a study site as well. Tom's hypothesis is that, by providing an egg with additional resources, parents offset the metabolic costs of egg cooling. Since this is an observational study, experiments are needed to test the hypothesis.
The jury is still out as to why South American birds spend little time on the nest when they should be incubating. Tom thinks that adults are vulnerable to predation while on the nest and southern birds have greater incentive to decrease this danger. Whatever the reason, it is an interesting way to explain why one egg is bigger than the other.
While working in one of the field sites in Venezuela, I carried a small electronic scale with me and weighed eggs of every species of bird whose nest I found including hummingbirds, trogons, and turkey-sized tinamous. At the time, I was not sure why I had to do this, but now that the paper is out I have a clearer understanding. The paper and others from our studies can be found at http://www.umt.edu/mcwru/temwebsite/Martinpublications.htm#BBPubs
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