Avian response to landscape change in fragmented southern Great Plains grasslands

We examined the dynamics of avian communities associated with fragmented gr asslands in Oklahoma, USA, using long-term (1965-1995) raw (stop-level) dat a from the Breeding Bird Survey (BBS). Aerial photography was used to docum ent changes in land cover type and landscape pattern as affected by woody p lant (mostly Juniperus virginiana L.) encroachment and concurrent cropland conversions to agricultural grassland under the Conservation Reserve Progra m (CRP). Rank trend analysis identified species with significant population trends, and canonical correspondence analysis (CCA) was used to identify i mportant environmental gradients from a group of descriptive habitat variab les that included land cover type composition and indices of vegetation cov er, landscape pattern, and grassland patch structure. Avian community structure shifted along gradients of increasing woody plant cover and indicators of continuing landscape fragmentation. Open-habitat g eneralists, woodland, and successional scrub species generally increased, w hereas many grassland species decreased. In some instances, neotropical mig rants responded positively to increasing woody vegetation. Some grassland b irds also showed a positive response to increases in agricultural grassland , but only in areas of severe juniper encroachment. Most grassland species exhibited consistent declines related to the influx of woody vegetation and associated landscape changes. Woody plant encroachment into southern Great Plains grasslands already frag mented by agricultural activity represents a conservation management dilemm a. Although woody vegetation in remnant native prairies may provide habitat for some declining neotropical migrants that require shrubby areas, grassl and structure and suitability is compromised for many declining grassland-e ndemic birds. Cropland conversion to agricultural grassland does appear to provide suitable habitat for some grassland species. However, this benefit appears to be limited to areas where woody plant invasion into grasslands i s relatively advanced, and may have only a temporary effect, as most CRP ar eas are likely to return to agricultural production in the near future. Cha nges are needed in grassland management practices to restrict woody plant e ncroachment and fragmentation; otherwise, continued declines in grassland b ird populations can be expected.