Effects of artificial perches and interspecific interactions on patch use by wintering raptors

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Abstract
We used an experimental approach to investigate the effects of woody vegetation and interspecific interactions on patch use by three wintering raptor species in the coastal prairie in south Texas. We manipulated perch type and density by the addition of artificial perches to patches of coastal prairie grasslands, varying perch height in the first year and canopy density in the second year. American Kestrels (Falco sparverius) used areas with artificial perches more often than they used natural woody vegetation and areas without woody perches. Northern Harrier (Circus cyaneus) patch use did not vary with treatment type. Loggerhead Shrikes (Lanius ludovicianus) used areas of natural woody vegetation more than patches with artificial perches and patches with no perches. Patch use by Loggerhead Shrikes was also influenced by the presence of complex woody vegetation and interspecific interactions, whereas American Kestrel use was limited by landscape matrix and ground cover density. We believe that predation by Northern Harriers limited Loggerhead Shrikes to patches with escape cover, while competition from American Kestrels prevented Loggerhead Shrikes from concurrent patch use.