Bird abundance in cornfield edges in Iowa was documented from mid-April through early August 1992. During this period the barren, sparsely vegetated fields are transformed into fields with dense plant cover; the availability of food resources (corn and arthropods) also changes. Temporal patterns of bird abundance in cornfield edges differed greatly among species—some were present in cornfield edges throughout most, if not all, of the study period (e.g., American robin and eastern kingbird), whereas others restricted their use to brief periods. Some were more abundant early in the season (e.g., killdeer and horned lark); others were more abundant later (e.g., black-capped chickadee and indigo bunting). Much of the seasonal change in bird abundance in cornfield edges was attributed to the habitat affinities of the various species and to seasonal shifts in available food resources. Birds that feed on the ground or in low herbaceous vegetation became less abundant later in the season, whereas species that characteristically feed in shrubs or the lower canopy of trees became more numerous. The availability of waste corn on the soil surface, the phenology of the developing crop and the life history stages of major corn insect pests all contribute to the temporal dynamics of bird abundance in cornfields. Also, seasonal patterns of bird abundance in cornfields influence avian risk of exposure to agricultural pesticides. Effective management and conservation of avian communities associated with cornfields require understanding temporal patterns of bird abundance and their implications.