The role of a synanthropic bird in the nest niche expansion of a secondary cavity nester to man-made structures.

Species with similar ecological characters often compete with each other; however, a species may also facilitate the survival or reproduction of another ecologically similar species, although such interaction is rarely documented in birds. Here, we reported a facilitative species interaction between Asian house martins () and russet sparrows (), both passerines using closed nests, in a montane farming area of Taiwan. We found that Asian house martins constructed dome-shaped nests in human houses that provided additional nest sites for russet sparrows, secondary cavity nesters with greatly declining populations in Taiwan. Russet sparrows that used house martin nests had reproductive success comparable to those that used artificial nest boxes. However, Asian house martins avoided reclaiming sparrow-used nests, which reduced their available nest sites. Interestingly, our results imply that man-made structures may be used as a conservation tool to improve the breeding of the endangered russet sparrows via this facilitative interaction.