Clutch Size of Kestrels: Seasonal Decline and Experimental Evidence for Food Limitation under Fluctuating Food Conditions

Abstract

We studied the clutch size of Eurasian kestrels, Falco tinnunculus in western Finland during 1985-1996 (the total number of nests 546). Voles, the main food of kestrels, fluctuated in three-year population cycles. 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. Yearly 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 phase 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 clutch size; kestrels produced smaller clutches in the low than in the increase and decrease vole years. Supplementary feeding before and during 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 resulted in larger clutches. These results show that both laying date and abundance of main food at the early stage of breeding are crucial determinants of clutch size in kestrels. Our result, that variation in food abundance can alter the relationship between clutch size and laying date, supports the quality hypothesis (that differences in quality between individual birds and/or their territories may result in the seasonal decline of clutch size). We suggest that food is limiting clutch size in kestrels both during the egg-laying and nestling periods, and not only at low but also at high levels of natural food abundance.