Microbial colonization of the cloacae of nestling tree swallows

Microbes have the potential to be important selective forces in many aspect s of avian biology. Microbes can affect fitness as a result of either their pathogenic or beneficial effects on host health. Little is known about the chronology of microbial colonization of nestlings or the effects of microb es on fledgling condition. We set out to (1) characterize the time course o f microbial colonization of the cloacae of nestling Tree Swallows (Tachycin eta bicolor), (2) examine the relationship between cloacal microbes and fle dgling condition, and (3) determine if nest mates had similar assemblages o f cloacal microbes. We repeatedly measured nestlings and sampled their cloa cal microbes on nestling days 2, 3, 5, 7, 12, 16, and 19. We detected cloac al microbes in nestlings as early as nestling day 2. Colonization of nestli ngs by microbes began soon after hatching. Nestlings were colonized by more types of microbes and carried heavier loads of most types of microbes as t hey got older. Cloacal microbes did not affect fledging success. However, p late scores for gram-negative enteric lactose fermenters, which include E. coli, Salmonella spp., and Shigella spp., were positively correlated with a greater degree of wing asymmetry. This relationship suggests that microbes affect fledgling survival because wing asymmetry hinders flying ability, a critical survival skill for these aerial insectivores. Patterns in the ass emblages of cloacal microbes within broods suggested host-genetic influence s on the colonization of nestlings by microbes, but they also may have refl ected the facts that nest mates were fed by the same adults and were raised in the same nests.