RESUMO
Golden-winged Warblers (Vermivora chrysoptera) have become rare across much of their historic breeding range and response to conservation efforts is variable. Evidence from several recent studies suggests that breeding output is a primary driver explaining responses to conservation and it is hypothesized that differences in food availability may be driving breeding output disparity between two subpopulations of the warbler's Appalachian breeding range. Herein, we studied two subpopulations: central Pennsylvania ("central subpopulation"), where breeding productivity is relatively low, and eastern Pennsylvania ("eastern subpopulation"), where breeding productivity is relatively high. To test the food-availability hypothesis in this system, we measured density of caterpillars, plasma lipid metabolites (triglycerides [TRIG; fat deposition] and glycerol [GLYC; fat breakdown]), body mass of adults males, and acquired body mass data for fledglings at 38 sites managed for nesting habitat. Consistent with our prediction, leaf-roller caterpillar density, the group upon which Golden-winged Warblers specialize, was 45× lower in the central subpopulation than the eastern subpopulation. TRIG concentrations were highest within the eastern subpopulation during breeding grounds arrival. The change in TRIG concentrations from the breeding-grounds-arrival stage to the nestling-rearing stage was subpopulation dependent: TRIG decreased in the eastern subpopulation and was constant in the central subpopulation, resulting in similar concentrations during the nestling-rearing stage. Furthermore, GLYC concentrations were higher in the eastern subpopulation, which suggests greater energy demands in this region. Despite this, adult male warblers in the eastern subpopulation maintained a higher average body mass. Finally, fledgling body mass was 16% greater in the eastern subpopulation than the central subpopulation before and after fledging. Collectively, our results suggest that poor breeding success of Golden-winged Warblers in the central subpopulation could be driven by lower availability of primary prey during the breeding season (leaf-roller caterpillars), and this, in turn, limits their response to conservation efforts.
RESUMO
In the northern Great Lakes region, the creation and maintenance of early-successional woody communities as wildlife habitat have increasingly become a conservation priority. The extent to which insect pollinators use these systems remains largely anecdotal. In summer (June-August) of 2021, we surveyed 49 early-successional sites in the western Great Lakes region treated with either shrub-shearing or silviculture (young forest) for bumble bees, butterflies, and habitat components (i.e., structural vegetation and floral resources). Hierarchical distance models predicted pollinator densities (λ^) to be, on average, λ^â =â 84 bumble bees/ha and λ^â =â 102 butterflies/ha. Although sheared shrubland and young forest communities supported comparable densities of bumble bees and butterflies, density was not equal across all sites. At the microhabitat scale, butterfly density and morphospecies richness were negatively associated with tall shrub cover and butterfly morphospecies richness (but not density) was driven by floral richness. Similarly, bumble bee density was positively associated with metrics of floral resources, underscoring the importance of blooming plants within these woody systems. Landscape covariates explained variation in butterfly density/richness but not bumble bee density. Ultimately, our results demonstrate that blooming plant abundance is an important driver of bumble bee and butterfly densities within these managed early-successional communities. Because early-successional woody communities are dynamic and their herbaceous openings are ephemeral, routine management would ensure that a variety of successional conditions exist on the landscape to meet the needs of bumble bees, butterflies, and potentially other insect pollinators.