||I examined the effects of successional changes in vegetation on the breeding dispersion of grassland birds in hayfields in New York. I sampled vegetation and counted birds in 90 hayfields of various ages (i.e. number of years since planting) and sizes that were originally planted to a legume-dominated seed mixture. I also resampled a subset of these fields two years later. Over time, these hayfields changed from tall, dense, homogeneous stands of legume-dominated vegetation to short, sparse patchy stands of grass-dominated vegetation. Fields of all ages were dominated by introduced plant species. Red-winged Blackbirds, the most common bird species breeding in these fields, were most common in fields of intermediate ages, whereas numbers of Bobolinks, the second most abundant species, increased logarithmically with field age. Upland Sandpipers, eastern Meadowlarks, Grasshopper Sparrows, and Henslow’s Sparrows were most abundant in the oldest hayfields, whereas a bundances of Savannah Sparrows, third most common species) did not differ in fields of different ages. Bird-species richhness and diversity increased linearly with hayfield age. Three species (Red-winged Blackbird,
Bobolink, Savannah Sparrow) accounted for more than 90% of the breeding individuals. Vegetation structure, composition, and patch size were the most important proximate correlates
of habitat selection for these species. Hayfield size was positively correlated with abundance for five of the seven most common species. Furthermore, those species that nested late in the season (Bobolinks, Grasshopper Sparrows, and Henslow’s Sparrows) had low
breeding densities in fields with early mowing dates the previous year.