Sources of variation in growth of Tree Swallows

ABSTRACT.--We studied two populations of Tree Swallows (Tachycineta bicolor) that differed primarily in the amount of food available to the breeding birds. We obtained an index of food abundance and performed field experiments to distinguish factors affecting variability in growth of nestlings. The experiments were designed to detect the influence of the location of egg laying, incubation, and nestling rearing, type of parent (natural or foster), and year of breeding on nestling growth. Some broods were transferred between nests and raised by foster parents, and some clutches and broods were transferred between populations. Variables were analyzed in two-and three-way factorial analyses of variance. The insect biomass index during the nestling period differed about 7-fold between locations, regardless of year of breeding. Nestlings with more food grew and survived better