| Summary: | 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. === Science, Faculty of === Zoology, Department of === Graduate
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