The importance of red squirrels (Tamiasciurus hudsonicus) as predators of songbird nests in the boreal forest was determined by monitoring survival of artificial nests before and after squirrel removal. In eight conifer-dominated forest fragments surrounded by agricultural fields we placed artificial ground and shrub nests along the edge and interior of each fragment. Nests contained one quail and one clay egg, which were monitored for 12 d. We then removed squirrels from four forest fragments and repeated the experiment. Nests located in reference plots, where squirrel numbers were not manipulated, had similar survival rates among trials (2 ± 7% change between trials). In contrast, the percentage of quail eggs surviving 12 d increased 32 ± 9% after squirrels were removed. The survival of clay eggs increased 22 ± 10% after squirrel removal, although this was not significantly different from changes in clay egg survival between trials in reference plots (−2 ± 6%). The increase in nest survival when squirrels were removed was consistent for nests on edge and interior transects and ground and shrub nests. Unlike previous studies, we did not observe compensatory predation by other predators, although the relative frequency of mice destroying nests increased after squirrel removal.
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Vol. 147 • No. 1