Molt and body measurements for Gray Hawks in the Lower Rio Grande Valley of Texas

We captured 235 Gray Hawks (Buteo plagiatus) in the Lower Rio Grande Valley (Hidalgo, Willacy, and Cameron counties) of Texas from 6 February 2003 to 8 April 2023. We identified birds in five molt cycles: 115 birds in their first cycle, 16 in their second cycle, 73 in at least their second cycle, 19 in at least their third cycle, and 12 in at least their fourth cycle. Of these, we documented 15 instances of preformative molt, six instances where birds had an incomplete second prebasic molt, resulting in one to three retained juvenile rump feathers and/or wing coverts, and 18 instances where incomplete prebasic molts resulted in birds with multiple generations of flight feathers. We also present morphometric data from 144 Gray Hawks (nestlings and first-cycle birds captured prior to 1 October excluded). These results from the Lower Rio Grande Valley, on both molting strategies and measurements, differed in some respects from other sources based on the entire Gray Hawk population, most notably that birds from this northern and non-migratory population may show lower incidence of Stafflemauser molting patterns, and that they are heavier, which supports Bergmann’s rule.

Methods
We primarily used bal-chatri traps with 8–10 cm nooses made from 13.6 kg test monofilament fishing line to capture free-flying Gray Hawks (Bub 1991). We also used phai traps, bow net, and mist nets with a mounted Great Horned Owl (Bubo virginianus) lure near nest sites (Bloom et al. 2007). We fit Gray Hawks with individually numbered U.S. Geological Survey (USGS) aluminum bands, and birds banded after November 2019 also received an aluminum color band (Acraft Sign and Nameplate Co. Ltd., Edmonton, Alberta, Canada and Anillas Talismán S. L., Madrid, Spain). We recorded hallux claw length, tail length, mass, wing chord, and exposed culmen length for each bird (Hull and Bloom 2001, Pyle 2008). Measurements were obtained using digital scales and calipers, clear plastic rulers, and metal wing rules with a 90-degree stop following the techniques outlined in Pyle (2008). We did not measure tarsus length because this was too difficult to standardize among multiple banders. Measurements on 196 birds were performed by WSC or MTS, with ten banders performing measurements on the remaining 39 birds under their direct supervision.

We initially categorized birds as in their first year (juvenile or formative plumage) or as adults (basic plumage) based on the criteria of Pyle (2008). We also categorized birds as actively molting flight feathers or not molting. First-year birds were examined for preformative molt, i.e., newer gray formative body feathers contrasting with juvenile feathers. Preformative molt occurs prior to the onset of the second prebasic molt, as documented and discussed for Gray Hawk and other raptors in Pyle (2005a). Adults were examined for retained, worn, brown juvenile or moderately worn, gray, basic flight feathers. We ensured that our findings were based on molt by examining replacement patterns on both wings, and not adventitious replacement, which is not symmetrical between wings. 

We categorized molt and plumage cycles using Humphrey-Parkes-Howell terminology, and age classes according to molt cycle (Howell et al. 2003, Clark and Pyle 2015, Pyle et al. 2021). We separated age classes into local (nestlings, unable to fly), first cycle (between fledging and dropping the first primary during the second prebasic molt, or HY/SY in calendar-based terms), second cycle (between this and dropping the first primary during the third prebasic molt, or SY/TY), minimum second cycle (AHY/ASY), minimum third cycle (ASY/ATY) , and minimum fourth cycle (ATY/A4Y). When examining flight feathers, we looked for “sets” of sequentially replaced feathers between distally oriented waves
of molt (Pyle 2006, 2008). Sets are defined by an older, more worn primary distal to an adjacent newer primary, each set showing a cline in freshness from older inner to newer outer feathers, although a cline may not be even due to differing generations or molt suspensions; the number of these sets was used to determine minimum age for birds beyond third cycle (Pyle 2006, 2008). Open-wing images showing front and back of an extended wing were taken for archival purposes and to study molting patterns.