Public information revealed by pellets in nest sites is more important than ecto-parasite avoidance in the settlement decisions of Eurasian kestrels

Animals constantly need to acquire information about the environment for settlement decisions, either by using a trial-and-error strategy or by using public information by monitoring conspecifics. We studied a nest box population of Eurasian kestrels Falco tinnunculus in western Finland to test if pellets and other prey remains accumulated on the bottom of nest boxes are used as public information during settlement. During 2002–2013, nest boxes were randomly cleaned (treatment) or left un-cleaned (control) in each season. It is possible that kestrels reuse nest boxes which include information of successful nesting (i.e. have not been cleaned) because they indicate previous breeding attempt at the site. At the same time, this decision may entail costs because of blood-sucking ecto-parasites like Carnus hemapterus overwintering in the layer of pellets. First, we found that egg-laying date was significantly earlier in un-cleaned control boxes than in cleaned treatment boxes, indicating the use of public information revealed by pellets in the settlement decision. Second, the ecto-parasite burden of young nestlings (age 6–15 days) was significantly higher in un-cleaned control nest boxes. We found higher ecto-parasite infestation in early and lower infestation in late nests, a seasonal trend that is in disagreement with the ecto-parasite avoidance hypothesis. Contrary, in overall lower-infected cleaned boxes, ecto-parasite prevalence remained equal throughout the season. However, the ecto-parasite burden had no obvious effect on breeding success. We conclude that the use of pellets revealing successful breeding attempt of the previous year as public information appeared to be important in the settlement decision of kestrels.