Cooperative defence of colonial breeding house martins (Delichon urbicum) against nest-usurping house sparrows (Passer domesticus)

The usurpation of house martin nests by house sparrows has previously been reported. However, our study demonstrates how neighbouring house martins cooperatively defended against nest-usurping attempts by house sparrows. House martins collectively helped a conspecific pair build their nest at a much faster pace than would be possible for the breeding pair alone, within several hours as compared to a couple of days, in order to overcome the continued attempts of house sparrows to usurp the partially built nest. In our study, between the two breeding seasons of 2018 and 2019, the number of breeding house martins at the study site decreased by almost 63% while in contrast the number of house sparrow breeding pairs increased almost sixfold. The number of usurped nests by house sparrows was comparatively higher in 2019 as compared to 2018.