||Birds associated with prairie ecosystems are declining and the ecological condition (seral stage) of remaining grassland communities may be a factor. Livestock grazing intensity influences the seral stage of grassland communities and resource managers lack information to assess how grassland birds are affected by these changes. We estimated bird density, species diversity, and species richness on 37 sites in 4 seral stages of western wheatgrass [Pascopyrum smithii (Rydb.) A. Löve]-green needlegrass [Nassella viridula (Trin.) Barkworth] communities of the Fort Pierre National Grassland. Bird species richness did not differ among seral stages (P = 0.57), but bird species diversity was greater (P < 0.10) in early seral stages compared to late-intermediate seral stages. Grasshopper sparrow (Ammodramus savannarum Gmlin), bobolink (Dolichonyx oryzivorus Linnaeus), dickcissel (Spiza americana Gmlin), and brown-headed cowbird (Molothrus ater Boddaert) density increased (P < 0.10) from early to late seral stages. Burrowing owl (Athene cunicularia Molina), upland sandpiper (Bartramia longicauda Bechstein), chestnutcollared longspur (Calcarius ornatus Townsend), and horned lark (Eremophila alpestris Linnaeus) density decreased (P < 0.10) from early to late seral stages. Western meadowlarks (Sturnella neglecta Audubon) were more abundant in early (P = 0.05) and early-intermediate (P = 0.01) seral stages than late seral stages. Birds with habitat requirements including tall vegetation and residual cover were more abundant in later seral stages. Early seral stages were beneficial to birds that prefer short grass and sparse vegetative cover. Seral stage was an effective predictor of density for many bird species. A mosaic that includes all seral stages is necessary to maximize grassland bird species diversity and abundance across the landscape. Managers can assess the effects on grassland birds of management actions that alter the seral stage of the vegetation.