Energetic and fitness costs of mismatching resource supply and demand in seasonally breeding birds

By advancing spring leaf flush and ensuing food availability, climatic warm ing results in a mismatch between the timing of peak food supply and nestli ng demand, shifting the optimal time for reproduction in birds. Two populat ions of blue tits (Parus caeruleus) that breed at different dates in simila r, but spatially distinct, habitat types in Corsica and southern France pro vide a unique opportunity to quantify the energetic and fitness consequence s when breeding is mismatched with local productivity. As food supply and d emand become progressively mismatched, the increased cost of rearing young pushes the metabolic effort of adults beyond their apparent sustainable lim it, drastically reducing the persistence of adults in the breeding populati on. We provide evidence that the economics of parental foraging and limits to sustainable metabolic effort are key selective forces underlying synchro nized seasonal breeding and long-term shifts in breeding date in response t o climatic change.