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Population declines of many grassland-nesting birds are now widely recognized. Fundamental to understanding these declines is knowing if they are caused by changes in the availability of suitable habitats or changes in the densities of birds within those habitats. We address that issue with information from systematic surveys of breeding birds throughout North Dakota in 1967, 1992, and 1993. We compared the availability of 8 major habitat types, and the densities of 24 species of grassland birds in each habitat type, for 128 randomly selected quarter-sections (64.7 ha or 160 ac) that were surveyed in each of those years. Between 1967 and 1992-1993, the area of cropland, planted cover, woody vegetation, and other habitats increased in the 128 quarter-sections, whereas the area of grassland, hayland, and wetland habitats declined. Our results are mixed concerning patterns of population change within habitats, which primarily reflect the disparate habitat requirements of individual species. Some species increased in density in 1 habitat between the 2 periods (e.g., horned lark [Eremophila alpestris] in grassland), whereas others declined in that same habitat (e.g., western meadowlark [Sturnella neglecta]). Other species (e.g., lark bunting [Calamospiza melanocorys]) declined in densities in 1 habitat but increased in another. Some species declined (e.g., Baird’s sparrow [Ammodramus bairdii]) or increased (e.g., northern harrier [Circus cyaneus]) in 1 or more habitats but their statewide populations were stable between the 2 periods; whereas other species were relatively stable within habitats but their statewide populations increased (e.g., upland sandpiper [Bartramia longicauda]) or declined (e.g., Le Conte’s sparrow [Ammodramus leconteii]). Nonetheless, our results provide evidence that populations of some species have declined on their breeding grounds in North Dakota. The disparate habitat requirements of grassland birds emphasize the importance of large-scale conservation efforts for grassland birds, especially those efforts that can provide a complex mixture of vegetation or habitat types.