Supplementary material for "Playback experiments highlight the importance of nearest-neighbor distance and social information for nest site selection in the House Martin (Delichon urbicum)"

Abstract

Understanding nest site selection is crucial for species conservation. Bird conservation often involves the installation of nesting aids to increase nest site availability and to induce colonization of unoccupied sites. However, prospecting individuals must find nesting aids, which may be facilitated with the use of social information. Here, we experimentally investigated the effectiveness of the colonization of artificial nests and playback in the declining, migratory House Martin Delichon urbicum. We selected unoccupied sites with artificial nests along a distance gradient to occupied sites and broadcasted conspecific vocalizations in autumn and/or following spring. Visitation and colonization rates considerably increased in proximity to occupied sites. Playback in autumn and spring enhanced visitation rates, while spring-only and autumn-only playback showed positive but smaller effects. Colonization rate increased exclusively with spring-only playback. Colonized playback and non-playback sites had similar breeding success, indicating that playback did not create ecological traps by attracting House Martins to suboptimal sites. Hence, broadcasting conspecific vocalizations can inform prospecting birds of nest site availability, hereby increasing visitation, and to some degree, colonization of unoccupied House Martin sites. To boost colonization, we recommend installing artificial House Martin nests within approx. 500 m from occupied sites and using playback of conspecific vocalizations.