Susan J. Hannon

ARE BOREAL BIRDS RESILIENT TO FOREST FRAGMENTATION? AN EXPERIMENTAL STUDY OF SHORT-TERM COMMUNITY RESPONSES

We studied the effect of habitat fragmentation on the richness, diversity, turnover, and abundance of breeding bird communities in old, boreal mixed-wood forest by creating isolated and connected forest fragments of 1, 10, 40, and 100 ha. Connected fragments were linked by 100 m wide riparian buffer strips. Each size class within treatments and controls was replicated three times. We sampled the passerine community using point counts before, and in each of two years after, forest harvesting, accumulating 21340 records representing 59 species. We detected no significant change in species richness as a result of the harvesting, except in the 1-ha connected fragments, where the number of species increased two years after isolation. This increase was accounted for by transient species, suggesting that the adjacent buffer strips were being used as movement corridors. Diversity (log series alpha index) was dependent on area in the isolated fragments only after cutting, having decreased in the smaller areas. Tur...

Population changes of the vertebrate community during a snowshoe hare cycle in Canada’s boreal forest

We measured the density changes of 22 species of vertebrates during a snowshoe cycle in northern Canada. Hares were the dominant herbivore in the system and changes in their numbers were correlated with changes in numbers of arctic ground squirrel, spruce grouse, ptarmigan, lynx, coyote, great horned owl, goshawk, raven and hawk owl. Hare numbers were not correlated with numbers of red-backed vole which showed peaks during the low, increase, and early decline phases of the hare cycle. Hawk owls were the only predator whose numbers correlated with changes in red-backed voles while boreal owls and weasels were correlated with densities of Microtus. Red squirrel, American kestrel, red-tailed hawk, northern harrier, wolverine, magpie, and gray jay showed no correlation with hare or vole numbers. We conclude that species in the boreal forests of Canada do not exhibit the strong synchrony found between voles and other members of the vertebrate community in northern Fennoscandia. We discuss some of the possible reasons for these differences.

Winter Responses of Forest Birds to Habitat Corridors and Gaps

Forest fragmentation and habitat loss may disrupt the movement or dispersal of forest-dwelling birds. Despite much interest in the severity of these effects and ways of mitigating them, little is known about actual movement patterns in different habitat types. We studied the movement of wintering resident birds, lured by playbacks of mobbing calls, to compare the willingness of forest birds to travel various distances in continuous forest, along narrow corridors (fencerows), and across gaps in forest cover. We also quantified the willingness of Black-capped Chickadees ( Poecile atricapillus ) to cross gaps when alternative forested detour routes were available. All species were less likely to respond to the calls as distance increased to 200 m, although White-breasted Nuthatches ( Sitta carolinensis ) and Hairy Woodpeckers ( Picoides villosus ) were generally less likely to respond than chickadees and Downy Woodpeckers (P. pubescens ). Chickadees were as likely to travel in corridors as in continuous forest, but were less likely to cross gaps as the gap distance increased. The other species were less willing to travel in corridors and gaps relative to forest, and the differences among habitats also increased with distance. For chickadees, gap-crossing decisions in the presence of forested detours varied over the range of distances that we tested, and were primarily influenced by detour efficiency (the length of the shortcut relative to the available detour). Over short distances, birds used forested detours, regardless of their efficiency. As absolute distances increased, birds tended to employ larger shortcuts in the open when detour efficiency was low or initial distance in the open was high, but they limited their distance from the nearest forest edge to 25 m. Thus, chickadees were unwilling to cross gaps of > 50 m when they had forested alternatives, yet they sometimes crossed gaps as large as 200 m when no such choice existed. Our results suggest that the presence of corridors enhanced the movement of some, but not all, forest birds, and that even chickadees, which were less sensitive to gap width, preferred not to venture far from forest cover.

A test of the mismatch hypothesis: How is timing of reproduction related to food abundance in an aerial insectivore?

In seasonal environments, vertebrates are generally thought to time their reproduction so offspring are raised during the peak of food abundance. The mismatch hypothesis predicts that reproductive success is maximized when animals synchronize their reproduction with the food supply. Understanding the mechanisms influencing the timing of reproduction has taken on new urgency as climate change is altering environmental conditions during reproduction, and there is concern that species will not be able to synchronize their reproduction with changing food supplies. Using data from five sites over 24 years (37 site-years), we tested the assumptions of the mismatch hypothesis in the Tree Swallow (Tachycineta bicolor), a widespread aerial insectivore, whose timing of egg-laying has shifted earlier by nine days since the 1950s. Contrary to the mismatch hypothesis, the start of egg-laying was strongly related to food abundance (flying insect biomass) during the laying period and not to timing of the seasonal peak in food supply. In fact, food abundance generally continued to increase throughout the breeding season, and there was no evidence of selection based on the mistiming of laying with the seasonal peak of food abundance. In contrast, there was selection for laying earlier, because birds that lay earlier generally have larger clutches and fledge more young. Overall, initial reproductive decisions in this insectivore appear to be based on the food supply during egg formation and not the nestling period. Thus, the mismatch hypothesis may not apply in environments with relatively constant or abundant food throughout the breeding season. Although climate change is often associated with earlier reproduction, our results caution that it is not necessarily driven by selection for synchronized reproduction.

NEST PREDATION IN ASPEN WOODLOTS IN AN AGRICULTURAL AREA IN ALBERTA: THE ENEMY FROM WITHIN

Several studies have documented elevated rates of nest predation for passer- ines and grouse in small forest patches and near anthropogenic edges. We examined patterns of predation on artificial nests in aspen (Populus tremuloides) woodlots and fencerows sur- rounded by agricultural land in Alberta. Nests were intended to mimic those of ground- nesting grouse and shrub-nesting passerines. We evaluated the relative importance of factors at the nest site, the forest patch, and the landscape to risk of nest predation by different predators. Total predation rates were highest in fencerows. Among woodlots, predation rates did not differ with woodlot area except in 1992, when predation on ground nests was higher in large woodlots. Most shrub nests were depredated by birds (corvids and House Wrens (Troglodytes aedon)). Corvid predation on shrub nests was higher in smaller woodlots and was highest on nests closest to the woodlot edge. Predation by small mammals was highest in larger woodlots and woodlots closer to farms and showed no edge effect. House Wren predation of shrub nests did not vary by any woodlot feature, nest cover, or distance to edge. We suggest that corvids forage mainly at the edges of forest patches and can fully penetrate small patches and fencerows. Small mammals are present in all woodlots, but avian pred- ators take the eggs in small woodlots before they are detected by small mammals. Nest pred- ators living within woodlots, such as wrens and small mammals, may be equally or more important than those living outside of woodlots in determining nest-predation risk for birds

LIFE HISTORY STRATEGIES IN EXTREME ENVIRONMENTS: COMPARATIVE DEMOGRAPHY OF ARCTIC AND ALPINE PTARMIGAN

Arctic and alpine habitats are extreme environments characterized by short breeding seasons, cold temperatures, limited food availability, and potentially high predation rates. Stringent ecological conditions are likely to have important consequences for the evolution of life history traits, but direct empirical tests are few. We compare the demog- raphy of three populations of ptarmigan on an environmental gradient spanning alpine, subalpine, and arctic habitats. Female Willow Ptarmigan (Lagopus lagopus) and White- tailed Ptarmigan (L. leucurus) breeding at subalpine and alpine sites had smaller clutches and lower probabilities of nesting success, fledging success, and renesting than Willow Ptarmigan nesting at a low-elevation arctic site. Annual fecundity, measured as female fledglings per breeding female, did not overlap among the three populations and was ranked: alpine (0.40 6 0.08, mean 6 SE, 95% CI 5 0.26-0.58) , subalpine (1.33 6 0.10, 1.13- 1.54) , arctic (2.04 6 0.18, 1.68-2.39). There was a nonsignificant trend for apparent survival rates (f) of breeding females to vary in the opposite direction: alpine (0.46 6 0.04) . subalpine (0.43 6 0.03) . arctic (0.37 6 0.06). Population growth rates predicted significant declines for the alpine population ( l5 0.65 6 0.07, 95% CI 5 0.52-0.79), but not the subalpine ( l5 1.00 6 0.07, 0.86-1.14) or arctic populations ( l5 1.13 6 0.20, 0.78-1.54). The adjusted estimates of survival necessary to sustain a stationary population indicated that actual variation in female survival was more pronounced than the observed rates: alpine (0.71) . subalpine (0.43) . arctic (0.33). Together, the fecundity and survival values provide evidence that even congeneric populations can exhibit a continuum between high reproductive and survivor life history strategies. Variation in ptarmigan life history traits was consistent with population differences in predation rates on eggs and breeding females, and it was not related to duration of the breeding season, climatic conditions, or food availability. Ptarmigan demography also covaried with body size, but not in the predicted pattern. Overall, the life history strategies of ptarmigan are consistent with our current understanding of the impacts of environmental factors upon life history variation in passerine songbirds.

New birds on the block: new neighbors increase defensive costs for territorial male willow ptarmigan

Previous researchers have hypothesized that site-faithful animals may benefit from the presence of familiar neighbors. This study compares the relative costs of territorial defense against new and former neighbors by male willow ptarmigan (Lagopus lagopus). Territorial defense against new neighbors appeared to require a greater expenditure of both time and effort than did defense against former neighbors. Territorial males that had several new neighbors spent a higher proportion of time fighting than did males with fewer new neighbors, and males with both new and former neighbors spent a greater amount of time fighting with their new neighbors, on average, than with their former neighbors. In addition, fights with new neighbors occurred relatively more frequently and were longer than fights with former neighbors. Finally, fights involving new neighbors tended to escalate to higher levels than fights between former neighbors. Reduced defensive costs for site-faithful, territorial males may provide one explanation for the tendency of males to be more site-faithful than females in many species.

Natal philopatry and recruitment of willow ptarmigan in north central and northwestern Canada

SummaryNatal philopatry and recruitment were measured in two populations of willow ptarmigan; one near Churchill, Manitoba and the other in northwestern British Columbia. We examined the return of tagged offspring in subsequent years with respect to geographical area, annual variation, their age when tagged, their sex, their body weight, age and number of their parents, and time of hatch (first nest or renest). Most chicks were tagged before they fledged, but chicks tagged after that had the highest rate of return. We also observed a strong positive relationship between fledging success of broods and offspring return in following years.Patterns of offspring return were similar in both populations except that male offspring in Manitoba settled closer to their natal sites than those in British Columbia and more yearling captured in Manitoba had been tagged as chicks. Return of offspring did not vary with year, their body weights shortly after hatch, or with the age or number of parents raising them. However, a significantly higher proportion of offspring hatched from first nests (first-initiated clutches) returned compared to those hatched from renests (replacement clutches). The low return of chicks hatched from renests may due to low survival, low philopatry, or both. We observed no differences in the mating status (recruitment) of returning offspring with respect to the time they hatched or the number of parents that raised them.

Clutch Predation, Cover, and the Overdispersion of Nests of the Willow Ptarmigan

Clutch predation is generally considered a selective force in nest site selection by gound—nesting birds. We examined characteristics of vegetation and cover around nest sites and inter—nest spacing for Willow Ptarmigan (Lagopus lagopus) to determine whether choice of nest location influenced probability of predation. Types of vegetation around successful and depredated nests and random points were similar. However, hens chose sites that had more lateral and overhead cover than that found at random points. Amount of cover was similar for successful and depredated nests. We Suggest that hens choose cover to deter predation on themselves or to reduce energy costs of incubation. Inter—nest distances were larger and nests were located closer to the centroid of the territory than if nest sites had been selected at random. Hens that renested placed their nests farther from their first nests but not farther from neigboring nests than expected if they had located them at random. Probability of a clutch being preyed upon was not related to its distance from a clutch that had previously been depredated. We conclude that, for the breeding densities that we encountered, nests were spaced far enough apart to deter area—restricted search by predators. Predation may have been a factor causing over—dispersion of nests in the past, but current selection may be masked by other factors such as the hens behavior or stochasticity.

CORRIDORS MAY NOT IMPROVE THE CONSERVATION VALUE OF SMALL RESERVES FOR MOST BOREAL BIRDS

Building or maintaining corridors in fragmented landscapes may be an im- portant method to conserve gap-sensitive species that avoid crossing gaps in forest cover. We tested the effectiveness of corridors by examining the changes in abundance of boreal birds pre- and post-logging in experimental 10-ha and 40-ha reserves that were isolated or connected by corridors, relative to their abundance responses in continuous forest (reference sites). Prior to the analysis, we categorized birds as to their predicted gap sensitivity based on two measures: their use of corridors and gap-crossing behavior in small-scale trials, and their habitat affinities (forest species vs. habitat generalists). The abundance of forest species as a group was consistently higher in reference reserves than in isolated or connected reserves after harvest, except for the first year after harvest, when crowding occurred in isolates. Habitat generalist species showed no differences in abundances across reserve types. As a group, resident species were more abundant in reference and connected reserves than in isolates in three of five years post-harvest, suggesting that corridors might benefit these species. None of the single species analyzed showed consistent evidence of benefiting from corridors. Although four species were most abundant in connected reserves after harvest, their abundances were not significantly lower in isolates than in reference sites. Behavioral classification (gap-crossing propensity) was not useful in classifying single species as to how gap sensitive they would be in response to our experiment: habitat affinity was a better predictor. We suggest that corridors may be useful to retain resident birds on harvested landscapes, but that corridors connecting small reserves of forest are unlikely to offset the impacts of fragmentation for most boreal birds. Assessments of the utility of corridors must, however, be done in the context of the full plant and animal communities that live in the boreal forest.