Home, dirty home: lesser kestrels prefer to breed in previously used nest sites

Populations of cavity nesting species are typically limited by the availability of suitable nesting sites. However, the factors driving the choice of a suitable nest sites are poorly investigated. For instance, individuals may prefer breeding in previously unused nest sites, that may host less parasites, or in previously used nest sites, which may provide cues of successful breeding attempt. We performed a simple nestbox choice experiment to assess whether breeding individuals prefer dirty and already used nestboxes vs. newly deployed ones. The study was carried out in Matera, which hosts the larger Italian colony of lesser kestrel. All nestboxes used for this experiment were provided within the framework of LIFE project. Before lesser kestrel arrival we crated 40 nestbox dyads, pairing 40 used nest box with 40 new ones with nestbox entrances positioned side by side in order to force the choice. To disentangle the effect of nest substrate from the appearance of the nestbox we randomly exchanged entrance panels (old and new) and nest substrate (organic material from previous nesting attempts vs. sand and gravel). We than assessed which nestbox belonging to each dyads was first occupied. At the end of the egg laying period, 38 of 40 dyads had at least one nestbox occupied. Among these, in 36 cases birds selected the nestboxes with the old substrate. The adjoining nestboxes with new substrate was occupied by a second pair just in 4 case. The experiment suggests a strong preference of lesser kestrel for nest sites already used during previous breeding seasons: despite their possible higher parasitic load, used breeding sites may provide cues of previous breeding attempt. Under a management point of view, this result suggests that conservation actions based on installation of nestboxes can benefit from the translocation of substrates from used nestbox to newly deployed ones.