Effects of livestock grazing on neotropical migratory landbirds in western North America

Livestock grazing is a widespread and important influence on neotropical migratory birds in four major ecosystems in western North America: grasslands of the Great Plains and Southwest, riparian woodlands, Intermountain shrubsteppe, and open coniferous forests. We have reviewed available literature on avian responses to grazing in these habitats. Among 35 plains species for which data are available, 9 responded positively to grazing, 8 responded negatively, 8 showed a graded response, from generally negative in shorter grasslands to generally positive in taller grasslands, while 8 were unresponsive or inconsistent. A similar comparison for riparian woodlands revealed that 8 of 43 species responded positively to grazing, while 17 were negatively affected, and 18 were unresponsive or showed mixed responses. Data for shrubsteppe habitats are much more limited, but only 3 of 23 species probably have been positively affected, at least by current grazing practices, while 13 probably have been negatively influenced, and at least 7 species showed mixed responses. Virtually nothing is known about effects of grazing on birds of coniferous forests. Most species negatively influenced by grazing have been those dependent on herbaceous ground cover for nesting and/or foraging. Given the ubiquity of livestock in the American West, species dependent upon lush ungrazed ground cover are at risk, and doubtless already are at population levels far below historical levels. Protection and restoration of riparian habitats is of particular importance, because of their limited geographic extent, and the extraordinary abundance and diversity of their neotropical migrants. There is an urgent need for long-term, well-replicated, field studies comparing bird populations in grazed and ungrazed shrubsteppe and montane coniferous forest habitats.