Food Supply, Territory Quality, and Reproductive Timing in the Blue Tit (Parus Caeruleus)

Abstract

We performed supplemental feeding experiments during three breeding seasons of the Blue Tit (Parus caeruleus L.) in order to establish the importance of food in reproductive timing. In particular, we addressed the question of why the response to supplemental food is restricted to 3—6 d in many bird species. Supplemental food advanced the start of egg—laying in all three breeding seasons. The mean difference between fed and control pairs was 4 d in two years and 6 d in another. The amount and quality of the supplemental food used in the experiment were not limiting, since pairs receiving a further increase in amount and quality of food did not start laying earlier than pairs with a standard feeding regime. The amount of food that was consumed increased during the period from 15 d before egg formation until the beginning of egg formation and then remained at a high utilization rate until clutch completion. Although fed pairs, on average, started egg—laying earlier than control pairs, the earliest control females started to lay eggs as early as the first females. Fed pairs were also relatively late in a year when control pairs were late. In 1992, fed pairs did not produce larger clutches or more fledglings, but had nestlings with lower mass at 13 d post—hatching than those of control pairs. Analysis at the territory level revealed that supplemental food affected laying date to a greater extent in low—quality territories, in which laying could be advanced by up to 9—10 d, compared to high—quality territories in which laying was not advanced at all. We conclude that laying date in most Blue Tit females is limited by low food supply early in spring, but some females, occupying high—quality territories, will not advance laying in response to food. Thus, above a certain critical limit, laying date is unrelated to food supply, and birds use other cues in their decision to commence breeding. Such cues are probably also responsible for the between—year variation in laying dates of food—supplemented pairs, since these were provided with the same amount of food during the different years.