Nest boxes and tree cavities are different in many respects, but both serve as nesting sites for breeding birds, including Southeastern American Kestrels (Falco sparverius paulus). With these differences in mind, we assessed both types of nesting sites to find out how suggested nest box measurements compare to natural nest cavities within the environment. We monitored breeding F. s. paulus which are cavity-nesters and frequently use nest boxes. They are a state-threatened subspecies of non-migratory falcon in Florida whose population experienced an 82% decline in the previous century. We expected that nest box measurements for cavity height and distance to nearest nest would be significantly different than natural cavities because environmental conditions influence the shape and size of snags (standing dead trees), whereas nest boxes are more uniform in their dimensions. We collected measurements from 85 natural nest cavities in Ocala National Forest (ONF), which contains one of the largest populations of F. s. paulus. We measured cavity height and distance to nearest nest. We used a Welch two sample t-test to compare the natural cavity measurements to the recommendations provided by four nest box programs.
Results/Conclusions
We found that mean cavity height for natural nests in ONF (7.2 m) did not differ from the mean height recommended for nest boxes (6.4 m). However, maximum (9.1 m) and minimum (3.0 m) cavity heights recommended for nest boxes were significantly different from natural cavities in ONF. We found that distance between natural nests was significantly greater than distances recommended for nest boxes. However, that does not necessarily mean that the nest box recommendations cannot produce successful nests. Maximum and minimum cavity height recommendations for next boxes could be compared to other forests with a variety of available natural cavity heights. Future research in ONF comparing natural nest cavities and nearby active nest boxes can indicate which nest sites produce more fledglings.