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Seasonal changes in egg‐mass within and among clutches of birds: general explanations and a field study of the Blackbird Turdus merula
Author(s) -
MAGRATH ROBERT D.
Publication year - 1992
Publication title -
ibis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.933
H-Index - 80
eISSN - 1474-919X
pISSN - 0019-1019
DOI - 10.1111/j.1474-919x.1992.tb08394.x
Subject(s) - yolk , biology , avian clutch size , zoology , bird egg , ecology , seasonality , clutch , laying , reproduction , physics , thermodynamics , astronomy
In many populations of birds there is a seasonal change in the mean mass of eggs in the clutch. This might be caused by seasonal changes in the costs of egg‐production, or changes in the benefits of laying eggs of different size. In the Blackbird Turdus merula the mean mass of an egg correlated specifically with the air temperature during the period when it was predicted to be undergoing rapid follicular growth. There was no residual effect of date or day‐length on egg‐mass when statistically controlling for temperature during the period of rapid yolk synthesis, yet temperature still had a significant effect when controlling for date or day‐ length. Thus the seasonal increase in egg‐mass appears to be due to changes in the cost of egg‐production, not changes in the benefits of laying larger eggs. However, I could find no effect of food supply during laying on the mean mass of eggs in the clutch, either using an indirect measure of food availability, rainfall, or in a food‐supplementation experiment. This could be because females responded to extra food by laying earlier, and probably larger, clutches, rather than by increasing egg‐mass. The effect of temperature on egg‐mass increased through the laying sequence and there was a small seasonal increase in the mass of the last‐laid egg compared with the mean of the other eggs in the clutch. I propose that the mean mass of the last‐laid egg relative to the clutch mean, which can characterize a species or population, could evolve in response to the environmentally‐caused variance in the mass of the last‐laid egg: when the variance is high, the mean may have to be high to avoid producing unviable eggs.