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 birdfancier 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. Given that kestrel declines have catalyzed the creation and expansion of many nest box monitoring networks that are operated by various groups, including private citizens, research labs in academia, and other conservation groups, we feel it important to spotlight this phenomenon and ensure those working toward the recovery and conservation of kestrels and other cavity-nesting species via management of nest box networks take steps to provide sites that enhance success of these species. We also call for a collaborative approach among kestrel researchers to better document this issue. Since 2014, HawkWatch International and its team of community scientists have monitored an average of 100 nesting kestrel pairs per year within a network of 500þnest boxes and non-box nesting locations in Utah as part of an ongoing demographic study. During this time, we identified six separate broods in which at least one nestling presented with a condition causing both legs to splay laterally from the body, resulting in the nestlings being unable to stand, perch, or jump (Fig. 1A). Between 2017 and 2021, we observed seven spraddle-legged nestlings (four males, two females, and one unknown sex) from six nest boxes (occurring in ,0.35% of 2241 nestlings and in ,1% of 767 nesting attempts from 2014–2021). Siblings of spraddle-legged nestlings fledged from five of the six nest boxes, and one brood included two siblings with spraddleleg. A spraddle-legged nestling was the only individual in a brood in only one case. Unfortunately, we know the breeding history for only one pair of adults that produced spraddle-legged young. This pair successfully raised a healthy brood of three nestlings in 2018 before having a brood in 2019, in which one of two nestlings was spraddlelegged. None of the seven spraddle-legged nestlings fledged, and all presumably died from starvation or other causes 15–30 d after hatch. Our observations fit the description of conditions called ‘‘splay-leg’’ or spraddle leg; as well as angular or rotational limb deformities (Worell 2012). Suspected etiologies for spraddle-leg in captive birds vary from a slippery floor surface during the nestling period to deficiencies in calcium and Vitamin D (Harcourt-Brown 2002). Leg deformities in nestlings may also result from maternal dietary deficiencies and are often seen in the last nestling to hatch in a clutch of eggs (Harcourt-Brown 2002). Such malformations in the legs are typically associated with the tibiotarsus but can occur in the femur or tarsometatarsus (Worell 2012). Spraddle-leg is most often documented in nestling poultry, Psittaciformes, Rock Pigeons (Columba livia), and doves (Pierson and Hester 1982, Worell 2012, Mangus et al. 2021) but can also be found in groups such as ratites, storks, and cranes (Reece et al. 1992). We found only two anecdotal reports of spraddle-leg in wild raptors (Peregrine Falcon [Falco peregrinus] and Harris’s Hawk [Parabuteo unicinctus]). Such abnormalities may go unnoticed if nest visits are infrequent because dead nestlings may be consumed by adults, trampled by their brood mates, or expelled from the nest. Additionally, necropsies may not 1 Email address: jwatson@hawkwatch.org