Meijer T., Daan S. & Dijkstra C. (1988) Female condition and reproduction: effects of food manipulation in free-living and captive Kestrels. ARDEA 76 (2): 141-154

Experiments on free-living and captive Kestrels were set up to investigate the effect of food and condition on laying date and clutch size, and to distinguish between the income and capital models of Drent & Daan (1980). Kestrels showed the same seasonal decrease in clutch size in different areas (Flevoland and Lauwersmeer) and in different years (early and late). Juveniles and adults had the same distribution of laying dates and laid the same number of eggs in any particular date interval. The seasonal decline in clutch size found between individuals was also found within individuals, breeding in different years. Feeding experiments in the field, early and late in the breeding season, advanced the mean date of laying by 10 and 6 days respectively, compared with controls. However, the advance depended on the actual food situation: in low vole years experimental birds laid three weeks earlier than controls, in high vole years experimental and control birds had no significant difference in laying date. The advance in laying date was associated with a change in the number of eggs, and clutch size did not deviate from the expected size for the date. Food experiments in the laboratory supported these findings: food supply had a direct effect on the date of laying but not on the number of eggs. Body mass of laying females with surplus food was higher than in their controls (specially of late breeders) and did not vary with date over two months, still late surplus fed breeders laid smaller clutches. The data do support neither the income model (rate of energy supply during laying determines clutch size) nor the capital model (condition of the laying female determines clutch size). We propose that food availability determines the laying date and the number of eggs is determined by date itself. Whether body mass as an indicator of condition is causally involved in determining laying date or is merely correlated with food supply can not presently be established.