INTRACLUTCH VARIATION IN EGG SIZE OF AMERICAN COOTS

I measured within-clutch egg-size variability for 3,219 American Coot (Fulica americana) eggs from 357 completed clutches, and I determined relative egg volume for 882 eggs of known laying sequence from 233 completed clutches ranging from five to 13 eggs. Within-clutch variation was unrelated to mean egg size, clutch size, laying date, year, and nesting attempt, but was slightly affected by supplemental food. Relative egg size increased with laying sequence for the first two to five eggs, then declined for the remainder of the clutch. Variation in relative egg volume with laying sequence was essentially unaffected by year, laying date, and supplemental feeding, but differed among initial nests, renests, and continuation nests. Most previous work on within-clutch variation in egg size has focused on the last-laid egg, and patterns of variation in the size of this egg typically have been evaluated in the context hatching asynchrony. However, no single adaptive hypothesis appears capable of explaining the pattern of intraclutch egg-size variation observed in American Coots, and I conclude that this pattern may result form physiological constraints.