(2004) Effects of Habitat Fragmentation and Forest Edges on Predators of Marbled Murrelets and Other Forest Birds on Southwest Vancouver Island

By Alan E. Burger et al

We recorded the occurrence and relative abundance of potential predators of the threatened marbled murrelet (Brachyramphus marmoratus) in the Carmanah, Walbran, and Klanawa Valleys on southwest Vancouver Island, British Columbia. Data covering multiple years (1994–2000) came from two series of dawn surveys used to monitor murrelet activities (45 stations in total), and two series of point counts (190 stations). Steller's jays (Cyanocitta stelleri) were consistently the most common potential predator. Common ravens (Corvus corax) and red squirrels (Tamiasciurus hudsonicus) were also frequently encountered, but owls, accipiters, and falcons were rare. Northwestern crows (Corvus caurinus) and bald eagles (Haliaeetus leucocephalus) were rare at our inland stations, but other studies showed that they were common at the coast. The survey and point count data showed that the percent occurrence and relative abundance (individuals per survey) of Steller’s jays, common ravens, and all predators combined were higher at stations bordering clearcuts and roads than at stations within interior forest or bordering streams. Highest counts were usually at sites frequently used by people. Predators were more abundant in the fragmented forests of the Klanawa Valley than in the less disturbed Carmanah-Walbran Valleys. In particular, counts of Steller’s jays at road and clearcut edges were significantly higher in Klanawa than in Carmanah-Walbran. A pilot experiment using 40 artificial nest sites on tree boughs in old-growth patches in the Klanawa Valley revealed that eggs disappeared more rapidly near clearcut edges than in the interior forest. We conclude that predation risk at nests of marbled murrelets is likely to be higher near clearcuts and roads than in interior forest, and higher in fragmented landscapes than in relatively undisturbed old-growth forests.

(2004) Effects of Habitat Fragmentation and Forest Edges on Predators of Marbled Murrelets and Other Forest Birds on Southwest Vancouver Island.pdf