CLUTCH SIZE OF KESTRELS - SEASONAL DECLINE AND EXPERIMENTAL-EVIDENCE FOR FOOD LIMITATION UNDER FLUCTUATING FOOD CONDITIONS

We studied the clutch size of Eurasian kestrels, Falco tinnunculus in western Finland during 1985-1996 (the total number of nests 546). Vole s, the main food of kestrels, fluctuated in three-year population cycl es. In low vole years, kestrels started to lay eggs in mid-May and on average 8 to 10 days later than in the increase and decrease years. Ye arly clutch sizes were also smaller in low vole years (yearly mean 4.4 to 5.3) than in the increase and decrease years (5.4 to 5.9). Clutch size showed a similar seasonal decline (0.05 egg per day) in each phas e of the vole cycle. After controlling for the effect of laying date, the phase of the vole cycle appeared to be the main determinant of clu tch size; kestrels produced smaller clutches in the low than in the in crease and decrease vole years. Supplementary feeding before and durin g the egg-laying periods in 1986-1988 did not advance the start of egg -laying and did not prevent the seasonal decline in clutch size, but r esulted in larger clutches. These results show that both laying date a nd abundance of main food at the early stage of breeding are crucial d eterminants of clutch size in kestrels. Our result, that variation in food abundance can alter the relationship between clutch size and layi ng date, supports the quality hypothesis (that differences in quality between individual birds and/or their territories may result in the se asonal decline of clutch size). We suggest that food is limiting clutc h size in kestrels both during the egg-laying and nestling periods, an d not only at low but also at high levels of natural food abundance.