Abundance of northern bobwhite and scaled quail in Texas: influence of weather and land-cover change

Abstract

Despite intensive study, quail populations across the United States have declined since the 1960s. My analysis of a 21-year (1978-98) data set collected by the Texas Parks and Wildlife Department indicated abundance of northern bobwhite (Colinus virginianus) has not declined in central portions of Texas. In eastern Texas ecological regions, however, declines were documented in the Cross Timbers, Gulf Prairies and Marshes, and Pinewoods. Scaled quail (Callipepla squamata) abundance in Texas exhibited significant declines across all Texas ecological regions other then the South Texas Plains. Of particular concern were scaled quail in the Rolling Plains ecological region which declined significantly from 1978 through 1998. Texas Parks and Wildlife Department quail abundance data, from 1978 through 1998, were sufficient to detect doubling in yearly mean abundance for all Texas ecological regions. The Modified Palmer Drought Severity Index (MPDSI), commonly used by meteorologists and climatologists, but previously unexplored in quail literature, was found to more accurately predict annual fluctuations in mean quail abundance per survey route than raw precipitation measurements. The MPDSI predicted changes in quail abundance among years better in dryer ecological regions that it did in wetter ecological regions. Changes in land-cover characteristics were a possible cause of differential population trends in scaled quail in the Rolling Plains and South Texas Plains ecological regions of Texas. A vehicle-based method of classifying land-cover along census routes was found to be effective in detecting land-cover change from 1976 to 1998. In the Rolling Plains, where scaled quail have declined, woody land-cover has generally become denser between 1976 and 1998. In the South Texas Plains, where scaled quail abundance exhibited no trend, woody land-cover generally became less dense between 1976 and 1998. This study indicated weather variability and land-cover change were possible influential factors on quail abundance. It also offered alternative methods for quantifying these variables and applying them to ecological systems.