This study was supported by grants from the National Science Foundation (BSR-8614598, BSR-9006320, DEB-9407587, DEB-9527318), the BBIRD (Breeding Biology Research and Monitoring Database) program under the Global Change Research Program of the Biological Resources Division, and Arizona Game and Fish Agency
Detailed vegetation data were collected mid-July through early August at all stations, using a protocol modified from the BBIRD Program (Martin 1992a).