UBC Theses and Dissertations
Habitat use and nest searching success of red squirrels at a forest edge Pelech, Shawna Anne-Marie
Recent declines in many songbird populations have been attributed to forest fragmentation (Robinson et al. 1995), particularly to elevated rates of nest predation at forest edges. Numerous studies have found an increase in predation rates at forest edges (Paton 1994). However, few studies have tested the processes involved; specifically the habitat use and searching behaviour of individual nest predators. I examined factors influencing the nest searching success of red squirrels (Tamiasciurus hudsonicus), a common predator of songbird nests in North American forests. I tested the hypotheses that (1) red squirrels select forest edge habitat, (2) squirrels initially depredate nests opportunistically, (3) with experience, squirrels search actively for nests, and (4) squirrels use an area-restricted search or use nest microhabitat as a search cue to find nests. I monitored the behaviour and habitat use of individual red squirrels over two summers at a forest-pipeline edge in the south-western Yukon Territory. I tested the influence of nest location, squirrel habitat use and amount of prior nest-finding experience on the survival of 2 artificial nests on each of 40 red squirrel territories. Artificial nests contained Japanese quail and plasticine eggs, and were placed at the base of willow shrubs. I used sandfilled trackboards to record small-scale changes in microhabitat use by squirrels after nests were depredated. Such changes could indicate the use of area-restricted searching or microhabitat-based search cues. Red squirrels selected forest-pipeline edges in late spring and early summer. Selection of edges by squirrels was highly correlated with a greater abundance of white spruce buds at edges. Although squirrels preferred edges for foraging, I found no difference in the survival of artificial nests between forest edge and interior locations. Smaller scale patterns of habitat use by individuals were also not related to nest survival. However, squirrels found second nests on their territory in one-fifth the time required to find the first nest. Further, survival of the first nest was most closely related to whether a squirrel had depredated a nest the previous year. Squirrels returned to willow nest sites after nests had been found, but did not change their use of similar microhabitats at larger spatial scales. Therefore, neither area-restricted search nor microhabitat-based search cues explain how squirrels efficiently located second nests on their territory. My results support the hypotheses that red squirrels select forest edge habitat and search actively for nests. Although nest survival was not related to the location of squirrel activity, I cannot reject the hypothesis that predation by squirrels is initially opportunistic. Squirrels may locate nests by chance during random searches when the abundance of traditional foods is low and caching activities are infrequent. I found that squirrels learned to search for nests, thus squirrels could show a functional response to increasing nest densities. Densities of both squirrels and songbirds can change in fragmented landscapes and I discuss how such changes may influence rates of nest predation. Additional tests are required to determine how squirrels learn to search for natural nests. Olfactory cues may be important for this and other mammalian predators. Future studies should continue to focus on individual predators and relate the survival of natural nests to visual and olfactory cues, and to nest density.
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