Nest predation is the primary cause of nest failure among birds. As such, it has large consequences on avian populations and is believed to be an important force in the evolution of avian life-history traits. Therefore, using a combination of laboratory and field research, we investigated the potential of northern flying squirrels (Glaucomys sabrinus) and two species of deer mice (Peromyscus maniculatus and P. keeni) to be predators on eggs and nestlings of birds, particularly the threatened, canopy nesting Marbled Murrelet (Brachyramphus marmoratus). In captivity, both mice and squirrels attempted to prey on eggs and live nestlings of various sizes. Mice attempted to prey on eggs more than nestlings, were more likely to attack nestlings when hungry than when sated, and were more likely to attack small than large nestlings. Individual flying squirrels varied in their propensity to attack but generally were more likely to attempt to open eggs when hungry than when sated. Opening of eggs by both mice and squirrels was limited by egg size. Video monitoring of artificial nests containing live nestlings confirmed that mice prey on large nestlings at canopy nests in Washington State's temperate rainforest. We conclude that mice and flying squirrels are predators of canopy nesting passerines, and their potential as nest predators must be considered to understand the consequences of nest predation. Received 1 April 2003, accepted 12 June 2003.
