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.