Nesting associations and breeding output of Barn Owls Tyto alba and Red-billed Choughs Pyrrhocorax pyrrhocorax sharing nest boxes

ABSTRACT Capsule Barn Owls Tyto alba and Red-billed Choughs Pyrrhocorax pyrrhocorax show differential breeding outputs as a consequence of sharing nest boxes. Aims To describe for the first time a commensal relationship between the Red-billed Chough and the Barn Owl during the breeding period. Methods During systematic monitoring of 90 nest boxes installed for Barn Owls in Navarre, northern Spain, it was found that in some boxes, Barn Owls and Red-billed Choughs nested simultaneously (inside and on top of the box, respectively). The clutch size, breeding success, and the number of fledglings of each species were compared when nesting together versus separately during the breeding period of 2015 and 2017. Results About one-third of the nest boxes observed during the study were simultaneously used by both Barn Owls and Red-billed Choughs. No relationship was found between the composition of the habitat around the boxes and the degree of occupancy for either species. Nest box sharing had no significant effect on the breeding output of Barn Owls, but improved the breeding performance of Red-billed Choughs by increasing the number of fledglings in relation to clutch size. Conclusion Barn Owls can provide benefits to Red-billed Choughs by controlling the populations of rodents, which could act as nest predators. Given that Barn Owls apparently do not obtain benefits nor incur any costs by the association with Red-billed Choughs, this interaction may be defined as commensalism. To our knowledge, this is the first documented nesting association between a corvid and an owl species.