EGG SIZE AND CLUTCH SIZE IN THE REPRODUCTIVE INVESTMENT OF AMERICAN KESTRELS

We studied causes and consequences of egg-size variation among clutche s of American kestrels (Falco sparverius). Eggs from 275 clutches were measured from 1990 to 1992. To test the hypothesis that the size of e ggs was constrained by food availability in the pre-laying period, we censused small mammal populations in the three years and performed a f ood supplementation experiment in 1990 and 1991. Kestrels did not adva nce the date they laid their first egg but did lay significantly large r eggs in response to extra food. The size of eggs was correlated with small mammal abundance on the territory, and females in good body con dition tended to lay large eggs. Body size did not affect egg size, an d there were no relationships between egg size and laying date except in 1990, the poorest food year. Clutches with a large mean egg volume had better hatching success than clutches containing small eggs. We ar gue that there is a phenotypic component to egg size in kestrels, and that kestrels use egg size to fine-tune reproductive investment to ava ilable resources.