American Kestrels Compete with European Starlings over Nest Boxes in Eastern Pennsylvania

MATTHEW J. BOWERS; PAULA MAITEN OROZCO VALOR; REBECCA A. MCCABE; JEAN-FRANÇOIS THERRIEN

Florida

Congreso; Raptor Research Foundation (RRF) Annual Conference; 2022

The American Kestrel (Falco sparverius) has been steadily declining throughout most of its range, the cause of which is still relatively unknown. As a cavity nesting species, the American Kestrel will often compete with other cavity nesters such as the invasive and abundant European Starling (Sturnus vulgaris) over nest boxes. The implications of European Starling presence on American Kestrel occupancy and breeding success are understudied. We analyzed data from nest boxes monitored in eastern Pennsylvania, USA from 1992 to 2021 to identify trends in occupancy of American Kestrels and competitors, and to examine how competition at nest boxes affects American Kestrel breeding success. We found that American Kestrel occupancy decreased while European Starling occupancy increased over the study period, with a negative correlation between the two species. All other species occupying nest boxes (small mammals, passerines, owls, and snakes) showed no significant change in occupancy. On average 21% of nest boxes remained unoccupied annually, and 7% of occupied nest boxes were utilized by both American Kestrels and competitors in the same breeding season. The presence of these competitors had negative effects on American Kestrel occupancy, clutch size, number of fledglings produced, and overall nesting success. Specifically, the rate of nesting success decreased by 26% when European Starlings utilized the same nest box in a given year. Overall, nesting productivity of American Kestrels decreased, with the average number of nestlings, fledglings, and nesting success rate all declining in recent years, while the average clutch size remained constant. This study suggests that American Kestrels in eastern Pennsylvania could be negatively affected by competition for nest boxes during the breeding season. The opposing trends in occupancy for the European Starling and the American Kestrel in this study area coupled with the declining productivity of American Kestrel nests raise concerns over the future of this raptor species.