Effects of forest edges on ovenbird demography in a managed forest landscape

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We studied the reproductive ecology of Ovenbirds (Seiurus aurocapillus) for 3 years in a primarily forested landscape in northern Wisconsin. We searched for and monitored nests in large, closed-canopy northern hardwood forests adjacent to recent clearcuts ( < 6years old) and measured the effect of proximity to edge on nest success, clutch size, and breeding pair density. Mayfield nest success was lower near the forest edge (0.44, Nnests = 42; < 300 m) than in the forest interior (0.69. Nnests = 47.. > 300 m; x2 = 4.43, df = 1, p ≤ 0.04), and mean clutch size was higher (4.93) near the forest edge than in the forest interior (4.27; t = -3.83, df = 59, p ≤ 0.0003). Edge effects on nest success and clutch size extended farther (300 m) into intact forest than has been documented previously. Using habitat-specific demographic parameters, we found that annual productivity per pair was similar in edge (3.37 fledglings) and interior (3.85 fledglings) habitat. We used our estimates of per capita annual productivity and published estimates of adult and juvenile survival to approximate the finite rate of growth (λ) for birds breeding in edge and interior locations. Based on published estimates of age-specific survival, both edge and interior habitats appear to be source habitats (where λ > 1) for Ovenbirds in the landscape we studied. Our assessment of population status, however, was extremely sensitive to variation in survival estimates. We used geographic information system data from the Nicolet National Forest to estimate population size in northern and mixed-hardwood forests in this region and to quantify annual productivity in this landscape. Ovenbirds near forest edges faced higher predation pressure but laid more eggs on average than birds in the forest interior. Our data suggest that Ovenbirds may make tradeoffs between the probability of nest success and the per-nest productivity as measured by clutch size. Our findings are consistent with the equilibrium state of the ideal free distribution model in that although individual demographic characteristics (nest success and clutch size) varied with distance from edge, an important correlate of fitness, finite rate of increase (λ) remained relatively similar.

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Conservation Biology