An evaluation of population indices for northern bobwhite

Northern bobwhite (Colinus virginianus) populations are monitored using 2 general approaches-population indices and abundance estimators. Due to their convenient sampling designs, indices are commonly used by land managers and researchers to predict or estimate hunting-season populations. We evaluated the efficacy of 4 bobwhite indices (spring cock [adult male] call-counts, autumn covey call-counts, roadside surveys, and helicopter surveys) to predict hunting-season population size by relating them to abundance estimators (minimum known population [no. unique trapped individuals each year], mark-recapture, helicopter-based distance sampling). Data were collected at the Rolling Plains Quail Research Ranch in Fisher County, Texas, USA, during 2009-2018. Spring cock call-counts and autumn covey call-counts were analyzed at 2 spatial scales: ranch scale (1,781 ha) and point scale (i.e., within the presumed radius of audibility of listening points; 600 m or 113 ha). Roadside and helicopter surveys were conducted at the ranch scale along 31- and 90-km transects, respectively. At the ranch scale, mean spring cock call-counts were a modest predictor of minimum known populations (r(2) = 0.54) and good predictor of mark-recapture estimates (r(2) = 0.80). At the point scale, the mean number of cocks heard was a poor predictor of densities surrounding points (r(2) = 0.0003) as estimated by distance sampling. The mean number of coveys heard during autumn counts was a significant predictor of both minimum known populations (r(2) = 0.88) and mark-recapture estimates (r(2) = 0.85) at the ranch scale but was a weak predictor of densities immediately surrounding listening points (r(2) = 0.22). Helicopter surveys during November (individuals/km) were a significant predictor of minimum known populations (r(2) = 0.98) and mark-recapture (r(2) = 0.93). Overall, roadside surveys (individuals/km) conducted during September was the best index for predicting minimum known populations (r(2) = 0.99) and mark-recapture estimates (r(2) = 0.97). Our results suggest the linear relationship between indices and autumn-winter populations for bobwhite are stronger when conducted closer in time to hunting season and at larger scales. (c) 2019 The Wildlife Society.