Food abundance and clutch size of Tree Swallows Tachycineta bicolor

We studied Tree Swallows Tachycineta bicolor during eight breeding seasons (1977‐84) at three sites that differed in food availability, which we measured as an index of the biomass of flying insects derived from daily catches in suspended nets. Mean clutch size differed significantly between sites (6.2, 5.8, and 5.6 eggs) and was related to average food abundance at the site. Within two sites, clutch sizes were significantly larger in years when food was more abundant during egg‐laying (r= 0.83 and 0.97 at the high and low food abundance sites, respectively; r= 0.63, not significant, at the medium abundance site). Both clutch size and laying season food abundance were positively correlated with food abundance during the nestling stage at the ‘high’ site, but not at the other two sites. Our results support the hypothesis that female Tree Swallows respond proximately to abundant food during egg‐laying by laying more eggs, but they do not elucidate the ultimate evolutionary factors involved in determining clutch size.