Intersexual Niche Differentiation: Field Data on the Great Tit Parus major

Abstract

The effects of sexual size dimorphism of parents, diversity of delivered prey items, and laying date on reproductive success were studied in a Great Tit Parus major population near Cracow, Poland. Although males were significantly larger than females, sexual dimorphism among pairs did not correlate with the diversity of prey delivered to young, nor with the number and average body mass of 14 days old young. However, the diversity of delivered food declined during the breeding season, while the average body mass of the young increased. Furthermore, the average body mass of young was higher if parents fed them with prey of lower diversity. Partial correlation analyses showed that this increase in body mass was not only a seasonal effect but was also related to the diversity of delivered prey items. The present findings do not confirm the intersexual niche differentiation hypothesis, even though the possibility for niche extension existed in the investigated population. Possible explanations for the negative relationship between fledgling weight and prey diversity are discussed.