Friend or foe? The influence of neighbours on reproductive success of mountain bluebirds and tree swallows

During the breeding season, secondary cavity nesting birds are often limited by the availability of suitable nesting locations. The placement of artificial nest boxes on the landscape not only provides potential nesting sites, but it can also lead to interactions within and between species. This thesis examines the influence of conspecific and heterospecific neighbours on the reproductive success of mountain bluebirds (Sialia currucoides) and tree swallows (Tachycineta bicolor) and the occurrence of extra-pair offspring at mountain bluebird nests. Increased tree swallow neighbour abundance early in the season negatively influenced the hatching success of mountain bluebirds. However, an increase in tree swallow neighbour abundance had a positive influence on the fledging success of tree swallows and mountains bluebird later in the season. While improved fledging success with increased tree swallow neighbour abundance may be due to shared nest defense or habitat quality, repeated interruption of incubating female bluebirds by tree swallow neighbours may put eggs at risk and reduce hatching success. Together, these results indicate that the influence of conspecific and heterospecific neighbours on reproductive success varies by species and phase of reproduction. The abundance of conspecific neighbours also influences extra-pair paternity in mountain bluebirds. The occurrence of extra-pair nestlings at bluebird nests increased as neighbour abundance within 500m and 1000m increased. In contrast, we detected a decrease in the occurrence of extra-pair offspring in bluebird nests as the abundance of conspecific neighbours increased within 250m of the nest. Although an increase in neighbour abundance increases the opportunity of extra-pair copulations, more neighbours relatively close by likely increases mate-guarding to mitigate extra-pair copulations. Indeed, bluebirds in our study travel an average of over 2000m in search of extra-pair mates. These findings are consistent with other studies of increasing neighbour densities leading to increased extra-pair mating opportunities and increased mate-guarding activity to reduce paternity loss. Together, these results highlight the importance of exploring conspecific and heterospecific neighbour interactions throughout the nesting period. Future behavioural and tracking studies may provide insight into the mechanisms that produce variation in mountain bluebird and tree swallow reproductive success and strategies employed by both male and female bluebirds when seeking extra-pair copulations.