Microbes have the potential to be important selective forces in many aspects 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 microbes on fledgling condition. We set out to (1) characterize the time course of microbial colonization of the cloacae of nestling Tree Swallows (Tachycineta bicolor), (2) examine the relationship between cloacal microbes and fledgling condition, and (3) determine if nest mates had similar assemblages of cloacal microbes. We repeatedly measured nestlings and sampled their cloacal microbes on nestling days 2, 3, 5, 7, 12, 16, and 19. We detected cloacal microbes in nestlings as early as nestling day 2. Colonization of nestlings by microbes began soon after hatching. Nestlings were colonized by more types of microbes and carried heavier loads of most types of microbes as they got older. Cloacal microbes did not affect fledgling success. However, plate scores for gram-negative enteric lactose fermentors, 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 assemblages of cloacal microbes within broods suggested host-genetic influences on the colonization of nestlings by microbes, but they also may have reflected the facts that nest mates were fed by the same adults and were raised in the same nests.


Tree Swallows, Tachycineta bicolor, birds, aves, microbes, microorganisms


Biology | Microbiology | Poultry or Avian Science | Zoology