ENERGY CONSTRAINTS AND ULTIMATE DECISIONS DURING EGG-LAYING IN THE BLUE TIT

In a population of Blue Tits, Parus caeruleus, we performed two different food provisioning experiments; one starting before clutch initiation and one starting when the first eggs were laid. We assessed the effect of these feeding experiments on four fitness-related factors, viz., laying date, clutch size, egg mass, and onset of incubation. Given that breeding birds are food and energy constrained, extra food during the breeding season should be invested in the fitness-related factor(s) that is most important for maximization of overall fitness. In the first feeding experiment, when breeding pairs had access to extra food both before and during egg-laying, the female converted this extra energy into earlier laying dates. When food was provided after clutch initiation only, the above preferred option was no longer available. In this situation, females used the extra energy to start incubating earlier in the laying sequence, with no influence on the other factors studied. Thus, both experiments resulted in surplus energy being used to increase the quality of the offspring instead of their number. In both cases the quality was connected with time savings leading to earlier hatching dates.