Nestbox design influences territory occupancy and reproduction in a declining, secondary cavity-breeding bird

Nestboxes are a popular measure to support populations of endangered secondary-cavity breeding birds. Yet, studies of the impact of nestbox abundance and design on bird breeding ecology in intensive farmland remain scarce, We experimentally studied nestbox preferences in a Wryneck Jynx torquilla population in SW Switzerland. Initially designed for the larger Hoopoe Upupa epops, voluminous nestboxes installed at 269 locations within the study area were checked for Wryneck breeding since 2002. In 2008, we installed smaller sized, Wryneck compatible nestboxes at 135 locations (50%) randomly selected from these 269 locations. We recorded the nestbox design preferences of Wrynecks, and measured reproductive output, nestling body mass and tarsus length, while assessing patterns of intra- and interspecific competition. Wrynecks settled in territories equipped with the more suitably designed nestboxes more than expected by chance and preferentially used them for raising their broods. The number of fledged young did not depend on nestbox design, although nestlings were slightly heavier in the smaller nestboxes, probably due to reduced interspecific competition with Hoopoes. This study shows that in particular territory attractiveness can be enhanced when artificial cavities with an appropriate design are provided. Nestbox provisioning is an efficient conservation measure for endangered bird species but attention should be paid to nestbox design which has to be tailored to species-specific requirements.