Seasonality of clutch size determination in the Kestrel Falco tinnunculus: An experimental approach

Egg numbers in Kestrel clutches decline from 6.5 to 3.2 with progressive laying date. Laying Kestrels did not respond to eggs added to the nest after the second egg. Egg removals starting on day 3, 5 or 7 of laying demonstrated that responsiveness (production of extra eggs) ceased ca 4 d before the normally final egg. Thus clutch fixation occurred sooner after the first egg in late than in early nests. Continuous records of nest temperature were used to establish incubation frequency of the female throughout laying in experimental and control nests. Incubation rose from 0 to 98% of time in the course of the laying phase. This rise started sooner and was more rapid in late nests and in nests with added eggs, and delayed in response to egg removal. The higher level of incubation tendency in late breeders at the onset of laying may be instrumental in more rapid follicle resorption and hence in clutch size reduction.