resources is frequently assumed to be the main force structuring bird communities. Consequently, understanding the effects of fluctuating food resources on population sizes as well as on bird behavior has been an important goal in avian community ecology and competition theory (Wiens 1989).
Several authors have reported that granivorous bird densities in arid and semi-arid environ-ments track changes in seed abundance (Grant 1986, Schluter and Repasky 1991, Blendinger and Ojeda 2001; but see Marone 1992, Dean and Milton 2001).