Due to the similarities in habitat requirements, tree swallows (Tachycineta bicolor) live in close proximity to other secondary cavity nesters, such as the mountain bluebird (Sialia currucoides). While these birds do not compete directly after territory establishment, relationships between neighbours can impact the success of swallows. Theoretical and empirical work suggests that neighbours can increase success through processes of reciprocal altruism in nest defence or decrease success through density-dependent competition. The goal of this study is to understand the impact that conspecific and heterospecific neighbours have on tree swallow reproductive success.To do this, we analyzed 659 tree swallow nest success over an 8-year period. We found that the number of nestlings per nest increased as the distance to the nearest swallow neighbour increased and the number of swallow neighbours within 1000m decreased. Also, we found that the number of fledglings per nest increases as the number of swallow neighbours within 500m increased but decreased when there were more swallows within 250m. Lastly, we found that mountain bluebird neighbours had a marginal negative effect on reproductive success. These results indicate that the relationship between neighbours is scale-dependent, but consistent with reciprocity, density-dependent competition, and the dear enemy phenomenon.
