Effects of cattle grazing on breeding wetland birds in the Aspen Parkland of Alberta

The cattle industry in central Alberta is experiencing rapid growth. Effects of cattle grazing on birds that breed in wetlands of the Aspen Parkland are poorly understood. To understand some of the ecological consequences of cattle grazing, I evaluated impacts of grazing intensity on species richness, nest density and nest survival of wetland birds across 181 wetlands in the Aspen Parkland of Alberta in 2001 and 2002. Species richness of breeding wetland birds increased in relation to residual cover of upland vegetation surrounding wetlands. Blackbird nest density was highest in wetlands moderately impacted by cattle, however nest survival declined as grazing impacts increased. Nest densities of other songbirds were lowest in wetlands adjacent to idled or lightly grazed uplands, however nest survival increased in relation to residual cover of upland vegetation and growth of emergent vegetation. Waterbirds nested at higher densities in moderately grazed wetlands in native pastures, but nest survival increased in relation to residual cover of upland vegetation. Although light to moderate grazing may benefit species richness and nest density of some wetland birds, reduced nest success in wetlands impacted by cattle suggests that managers should pursue management practices that minimize cattle activity in wetlands.