THE SEASONAL DECLINE IN CLUTCH SIZE - AN EXPERIMENT WITH SUPPLEMENTARY FOOD IN THE KESTREL, FALCO-TINNUNCULUS

To test whether the seasonal decline in clutch size is proximately determined by laying date or by parental quality an experiment with supplementary food was performed in the kestrel Falco tinnunculus. The experiment showed that supplemented pairs laid earlier and larger clutches than control pairs. Comparison of clutch size between supplemented and non-supplemented pairs laying on the: same dates, showed that supplementary Food affects clutch size independently of laying date. The seasonal decline in clutch size seems to occur when there are among-pair differences in food supply, since clutch size declines among non-supplemented pairs bur not among supplemented pairs. I reviewed Daan et al.'s model on optimization of clutch size and laying date. Contrary to what their authors defended, the model predicts that laying a clutch size in accordance with parental quality (or food supply) is more advantageous than doing it in accordance with laying date because there may be a change in the clutch size-laying date relationship. Independently of laying date, birds will maximize their fitness laying a clutch size as large as the number of young that can be adequately fed.