Changes in land‐use patterns that alter habitat may have a delayed negative effect on animal species occupying that habitat, and thus such effects may not be recognized for years. In Water Canyon, located in the Magdalena Mountains of southcentral New Mexico, we studied a relatively stable population of the cooperatively breeding Acorn Woodpecker (Melanerpes formicivorus) from 1975 to 1984. These woodpeckers rely on self‐constructed storage sites, or “granary trees,” to hold the acorns used during the winter and spring. Most granaries were in dead trunks and limbs of the narrow‐leafed cottonwood trees (Populus angustifolia. Storage sites form the primary basis for differential quality among territories. Groups of woodpeckers with large storage facilities (high‐quality territories) have greater annual reproductive success and survival than do pairs or groups with poorly developed storage sites. In the summers of 1994 and 1995 we censused the original study site, which had held 21 territories. Most territories that had contained birds a decade earlier were unoccupied. This drastic decline was correlated with the loss of nearly all large storage facilities because of the collapse of the granary trees. Most neighboring territories with lesser storage facilities also were vacant. The lack of production of new, high‐quality granaries for the period 1975–1995 probably is due inpart to the age structure of the cottonwood trees, which is distinctly bimodal: nearly all trees are either very young or old. There are now fewer old, partly dead trees that could provide granary sites. The scarcity of middle‐aged trees reflects a period of intensive cattle grazing in Water Canyon, during which time production of young cottonwoods was suppressed.