Documenting Occurrence and Rates of Spraddle-Leg in American Kestrel Nestlings within a Nest Box Monitoring Network

Winter, migration, and breeding surveys indicate widespread declines of American Kestrels (Falco sparverius; Farmer and Smith 2009, McClure et al. 2017). Suspected drivers of declines include degradation of habitat, direct mortality, and impaired reproduction due to contaminants, including insecticides and anticoagulant rodenticides in the environment, and the loss of natural nest cavities (McClure et al. 2017, Rattner et al. 2020). Documented causes of nestling mortality include the loss of a parent, predation, cannibalism (Bortolotti et al. 1991), and high loads of ectoparasites (Lesko and Smallwood 2012). We report on the frequency of “spraddle-leg,” a mortality source that is well known in the poultry and bird-fancier sectors but minimally documented in wild nestling American Kestrels. Spraddle-leg is a condition that results in malformed legs, which impairs movement and fledging of affected individuals, and is often associated with inadequate nest-site substrate....