ENERGETIC BOTTLE-NECKS DURING BREEDING AND THE REPRODUCTIVE COST OF BEING TOO EARLY

1. Experimental provision of extra food before egg-laying resulted in blue tit females starting to breed significantly earlier than unmanipulated females. 2. When the females had laid their first egg, the experimental food source was withdrawn and females were thus manipulated to lay, incubate and feed nestlings at lower food levels than unmanipulated females. 3. Although tricked into breeding earlier, the experimental females laid the same number of eggs, had no more laying gaps and produced eggs of equal size as the unmanipulated females. 4. Females manipulated to start breeding early started to incubate significantly later in relation to clutch completion than did controls. Experimentally induced differences in food availability, together with date, explained 60% of the variation in onset of incubation. 5. The frequency of total brood loss was higher for experimental females than for control ones. This led to a significantly reduced fledging success of females manipulated to start egg-laying early. 6. Among females, local survival was reduced following experimental advancement of egg-laying, indicating the existence of a reproductive cost of breeding too early. 7. The dependence of post-fledging survival and future breeding prospects on hatching date, results in Strong selection pressure on breeding as early as possible. Reduced fledging success and female survival oppose this selection pressure.