Effects of Cattle Grazing on Birds in Interior Douglas-Fir (Pseudotsuga Menziesii) Forests of British Columbia

Abstract

Livestock grazing is a dominant land use across North America and although the effects of grazing on birds have been studied in grassland, shrubland, and riparian habitats, studies of the effects in forests are rare. We investigated the effects of cattle grazing in forests on vegetation, the relationships between vegetation characteristics and the abundance of foraging and nesting guilds of birds, and the overall effects of grazing on the bird community in the Interior Douglas-fir (Pseudotsuga menziesii) biogeoclimatic zone of British Columbia. Cattle grazing was associated with reduction in ground vegetation height and grass cover, and increases in the number of shrubs and saplings. Bark insectivores, foliage insectivores, cavity nesters, and shrub/tree nesters all responded positively to sapling density. However, this translated into few overall effects of cattle grazing on birds, with only bark insectivores exhibiting greater abundance on grazed areas. Grazed areas also had fewer aerial insectivores but the mechanism driving this remains unclear. Current forest grazing practices at our study sites appear to have few negative effects on bird abundance and diversity, with the possible exception of aerial insectivores. Study of additional sites is required to assess if forest grazing exerts similar effects throughout the Interior Douglas-fir forest. Furthermore, study of the effects of forest grazing on productivity and survival of birds is needed.