Nest Box Entrance Hole Size Influences Prey Delivery Success by American Kestrels

Abstract

Nest boxes are a popular tool for research and conservation of American Kestrels (Falco sparverius; hereafter “kestrel”). The size of nest box entrance holes affects occupancy, interspecific competition, and nest predation for a number of bird species, but effects on prey delivery success are unstudied. We used digital video cameras to monitor kestrels using two nest box designs, one with a small circular entrance hole (8.75 cm in diameter; n = 8) and the other with a large U-shaped entrance hole (7.62 cm wide by 12.07 cm high; n = 6), to compare the rates at which kestrels failed to deliver large prey (vertebrates) into the nest box while provisioning nestlings. Across 111 d of monitoring, we observed a significantly higher prey delivery failure rate at the boxes with small holes (8.9%) compared to boxes with large holes (1.6%). Boxes with small holes fledged 4.33 nestlings and boxes with large holes fledged 4.5. These results suggest that kestrels nesting in boxes with small entrance holes may have higher costs while provisioning nestlings, compared to kestrels using boxes with larger entrance holes. We conclude that the ideal entrance hole size for kestrel nest boxes may be a compromise between being small enough to minimize predation, yet large enough to allow kestrels with large prey to easily enter. Future research should investigate whether boxes with higher prey delivery failure rates have lower reproductive success.