The Relation Between Food Abundance And Reproductive Performance Of Tree Swallows

Few field studies have examined experimentally the importance of food availability on adult activity and breeding performance in birds. I studied two populations of insectivorous Tree Swallows Tachycineta bicolor, 3 km apart, that differed primarily in the amount of food available to the breeding birds. I obtained an index of food abundance and conducted experiments involving the transfer of clutches and broods between populations. Tree Swallows showed substantial phenotypic plasticity in breeding characteristics and parental activities. My results demonstrated that food abundance during the the breeding season influenced clutch size, egg quality, nestling growth, behavior, survival to fledging and adult activities such as frequency of feeding young. It was important where Tree Swallows reproduced, where their nestlings were raised and probably, by whom they were raised. Food abundance, year of breeding and sequence in the clutch explained significant amounts of variation in egg weights. Growth of nestlings was not influenced by type of parent (i.e. natural or foster) or by pre-hatch factors such as location of incubation. However, the location where nestlings were raised explained much of the variation in growth of nestlings, and regardless of where parents nested, genetic variation in offspring and variation in provisioning abilities of parents may have been important components of within area variation in growth. These results support the hypothesis of food limitation of clutch size that also predicted slower nestling growth and poorer fledging success of young raised where food was less abundant