Nest predation in aspen woodlots in an agricultural area in Alberta : The enemy from within

Abstract

Several studies have documented elevated rates of nest predation for passer- ines and grouse in small forest patches and near anthropogenic edges. We examined patterns of predation on artificial nests in aspen (Populus tremuloides) woodlots and fencerows sur- rounded by agricultural land in Alberta. Nests were intended to mimic those of ground- nesting grouse and shrub-nesting passerines. We evaluated the relative importance of factors at the nest site, the forest patch, and the landscape to risk of nest predation by different predators. Total predation rates were highest in fencerows. Among woodlots, predation rates did not differ with woodlot area except in 1992, when predation on ground nests was higher in large woodlots. Most shrub nests were depredated by birds (corvids and House Wrens (Troglodytes aedon)). Corvid predation on shrub nests was higher in smaller woodlots and was highest on nests closest to the woodlot edge. Predation by small mammals was highest in larger woodlots and woodlots closer to farms and showed no edge effect. House Wren predation of shrub nests did not vary by any woodlot feature, nest cover, or distance to edge. We suggest that corvids forage mainly at the edges of forest patches and can fully penetrate small patches and fencerows. Small mammals are present in all woodlots, but avian pred- ators take the eggs in small woodlots before they are detected by small mammals. Nest pred- ators living within woodlots, such as wrens and small mammals, may be equally or more important than those living outside of woodlots in determining nest-predation risk for birds