How great tits maintain synchronization of their hatch date with food supply in response to long-term variability in temperature

Breeding birds increase their fitness by synchronizing their production of chicks with a peak of food abundance. Synchronization is primarily achieved by varying first egg date, but yearly temperature variations may delay or accelerate the food peak after the first egg has been laid. We tested the extent to which great tits (Parus major L.) can strategically change their synchronization of hatch date with the food peak after the first egg has been laid by changing clutch size, laying interval and the amount of incubation. We also tested whether these possible synchronization change mechanisms resulted in changes in breeding success, because if the food peak is late relative to first egg date, we would predict more of the population to have larger clutches and/or a hatch date synchronous with the food peak date. Great tits modified synchronization strategically by varying clutch size and the onset of incubation after clutch completion. When first egg date was early but then followed by cold weather so that the food peak was late, clutch sizes were larger, and when first egg date was late and temperatures were low, mean incubation periods were shorter if the food peak was early. Over the 39-year period of the study, the period between the first egg date and food peak increased significantly by 1+7 days more than the period between the food peak and hatch date. Great tits have maintained synchronization by significantly increasing their incubation period.

Will Cresswell and Robin Mccleery, Journal of Animal Ecology 72 (2), 356-366