An assessment of habitat quality of heavily- and less-browsed Michigan forests for a shrub-nesting songbird.

摘要

Our best opportunity for conserving migratory songbirds in northern forests is to include species of concern in landscape-scale management plans. However, evaluating management effects on species requires knowledge of the relative quality of forests that vary in species composition, management history, and browse pressure from white-tailed deer (Odocoileus virginianus ). I used estimates of relative abundance, demographic data, and spatial analysis of territories to compare the habitat quality of heavily-browsed (HB) and less-browsed (LB) forests for a migratory songbird, the black-throated blue warbler (Dendroica caerulescens). I studied blue warblers because they nest and forage in browse-susceptible understory vegetation, and worked in managed forests within northern Michigan's Hiawatha National Forest (1997–2001). Heavily-browsed (HB) sites had short (0.50–0.75 m) hardwoods in the understory (not completely browsed due to snow protection), and patches of browse-resistant balsam fir (Abies balsamea), while LB-site understory was primarily hardwoods. Blue warblers were most common in high shrub-density areas, regardless of browse level. Within high shrub-density HB sites, high fir-density areas had significantly more warblers. I conducted demographic and spatial distribution studies at densely-vegetated HB and LB sites. At both site-types, over 90% of pairs reproduced successfully, with 22–26% (LB vs. HB) of pairs fledging two broods. This similarity in reproductive success was in spite of higher nest survival on HB sites. HB sites had higher percentages of older males (78% at HB, 63% at LB), especially among new arrivals, but yearly return rates of adults were similar, with approximately 45% of males and 30% of females returning. Territory sizes (estimated using a bootstrapped minimum convex polygon) typically ranged from 1–4 ha, and tended to be larger on HB sites, although values at all sites were highly variable. Geographic boundary analyses suggested that patches of dense hardwood saplings 1–3 m tall were most often occupied at LB sites, while HB-site territories were concentrated in areas with small firs, or with dense patches of both browsed hardwoods and firs. Overall, habitat quality appeared slightly higher in LB forests, primarily as a function of pair density, with firs providing key “refuge” habitat within HB forests.