THE PROXIMATE EFFECTS OF FOOD-SUPPLY ON INTRACLUTCH EGG-SIZE VARIATION IN AMERICAN KESTRELS

Egg size can be an important determinant of offspring survival in bird s. We measured eggs from 275 clutches of wild American kestrels (Falco sparverius) to study the degree of intraclutch variability in egg siz e. We also performed two food-supplementation experiments to investiga te the proximate role of food supply during laying in determining egg size. Females with relatively abundant food and those in good body con dition did not lay eggs that were more uniform in size than those laid by control females. This result is contrary to hypotheses that propos e an adaptive explanation for intraclutch egg-size variation and also to ideas of energy depletion during laying. Patterns of egg size versu s laying order were different between years, suggesting that females d id not adaptively manipulate laying order and egg size within a clutch . The food-supplementation experiments showed that laying female kestr els probably depend on both stored energy reserves and on daily energy surpluses to form eggs. It appears that slight intraclutch variations in egg size occur in response to short-term food shortages during lay ing, but that these variations are probably nonadaptive. This is in ma rked contrast to interclutch (among females) variation in egg size, wh ich we have shown varies significantly with food supply.