Survival of Adult Songbirds in Boreal Forest Landscapes Fragmented by Clearcuts and Natural Openings

Abstract

There exists little information on demographic responses of boreal songbirds to logging. We conducted a 4-yr (2003-2006) songbird mark-recapture study in western Newfoundland, where land cover is a naturally heterogeneous mosaic of productive spruce-fir forest, stunted taiga, and openings such as bogs, fens, and riparian zones. We compared apparent survival and rate of transience for adults of 14 species between areas having forests fragmented primarily by either natural openings or 3-7 yr-old clearcuts. Data were collected on three landscape pairs, with birds being marked on three 4-6 ha netting sites on each landscape (total = 18 netting sites). Survival rates were estimated using multi-strata mark-recapture models with landscape types specified as model strata. Landscape type was retained in the best model for only two species, Ruby-crowned Kinglet and Yellow-rumped Warbler, in both cases indicating lower apparent survival in landscapes having clearcuts. Though parameter estimates suggested lower survival in clearcut landscapes for several species, meta-analysis across all species detected no general difference between landscape types. Further, we did not detect any relation between landscape differences in survival and a species' habitat affinity, migratory strategy, or the proportion of transients in its population. Although sensitivity to logging was limited, we observed high interspecific variation in rates of breeding season apparent survival (48% [Dark-eyed Junco] to 100% [several species]), overwinter apparent survival (0.3% [Ruby-crowned Kinglet] to 86.5% [Gray Jay]), and transience (≈0% [several species] to 61% [Ruby-crowned Kinglet in clearcut landscapes]). For Lincoln's and White-throated Sparrows, over-winter apparent survival was >2× higher for males than females, and rate of transience was > 8× higher for White-throated Sparrow males than females. Moderately male-biased sex ratios suggested that both lower mortality and higher site fidelity contributed to higher apparent survival of males. Overall, variability in our estimates of survival was too great to be explained by mortality alone, suggesting a large influence of landscape-scale movement by adults, e.g., breeding dispersal, extra-territorial forays, and transience, on the dynamics of boreal songbird populations. These movement patterns may also confer resilience to localized disturbance in boreal landscapes.

Keywords

• boreal forest
• clearcutting
• demographics
• forest management
• mark-recapture
• resilience
• songbirds
• apparent survival
• transience.