Arrival date and microorganisms in barn swallows

Migration between breeding sites and winter quarters constitute a major life history strategy in birds. The benefits of such migrations must at least equal the costs for such behavior to evolve and be maintained. We tested whether there was a relationship between abundance and diversity of microorganisms on nest lining feathers and timing of arrival by barn swallows Hirundo rustica. Nest lining feathers are chosen and transported by adult barn swallows to their nests just before and during egg laying, at a time when barn swallows have arrived weeks earlier, implying that any heterogeneity in abundance and diversity of microorganisms on feathers in nests must arise from feather preferences. There was a negative relationship between arrival date and the total number of fledglings showing that early arrival is advantageous. The arrival date of adult barn swallows was significantly positively correlated with the abundance of specific bacteria (Bacillus licheniformis) and positively correlated with the abundance of the fungus Trichoderma reesei and negatively correlated with the abundance of the fungus Mucor circinelloides. Moreover, we found a significant positive relationship between arrival date and mean total number of bacterial colonies in TSA medium. There was a significant negative relationship between arrival date and mean total number of bacterial colonies in FMA medium, and Simpson’s diversity index of the abundance of bacteria in FMA medium. Such heterogeneity may arise from some microorganisms being beneficial, others detrimental and yet others benign and of no significant importance. In contrast, there was no significant relationship between arrival date and age of individuals. These findings are consistent with the hypothesis that early arriving barn swallows differ in abundance and diversity of microorganisms from late arriving conspecifics, and that they choose feathers for their nests that differ in terms of microorganisms from those chosen by late arrival individuals.