Habitat Relationships of Bird Communities in Wisconsin Peatlands

Abstract

Effective conservation of biological diversity requires understanding of the influences of habitat composition and structure on species and community composition. Although patterns of bird richness and abundance in peatland natural communities have been studied in Canada, little is known about these relationships in the United States, especially within the Great Lakes region. Furthermore, environmental changes associated with global climate change could significantly impact many natural communities, including peatlands. Although potential impacts of climate change on birds are poorly understood, species composition and habitat use by peatland bird communities may be important to consider when monitoring peatlands for climate change impacts because of the sensitivity of birds to changes in vegetation composition and structure. In this study, I used several analytical methods to explain the distribution and habitat relationships of peatland birds in relation to a suite of habitat variables measured at a diverse array of peatland sites across Wisconsin. In both 2006 and 2007, I surveyed peatland bird communities using standard unlimited-radius point counts at 14 intensive sites and 74 extensive sites previously selected by the WDNR. Intensive sites were non-randomly selected and surveyed ≥2 times each year of the study. Extensive sites were selected using a stratified random sampling design and surveyed only once during the study. At both intensive and extensive sites, point-count stations were established along a transect bisecting the midsection of each peatland site. Vegetation was sampled at each point-count station and the surrounding area following methods previously established by WDNR for the multi-taxa peatland study. To supplement these vegetation data, additional habitat variables were measured in 2007. Vegetation was surveyed at each point within the intensive sites once per season in 2004, 2006, and 2007. Because extensive sites were surveyed only once during the study, vegetation at these sites was surveyed immediately after bird surveys. The area of each peatland site was determined using a Geographic Information System (GIS). I grouped bird species into 3 habitat-use guilds (forest, open-shrub, peatland-associated) for analyses. Further, I grouped species as residents, short-distance migrants, and neotropical migrants. Bird species present at ≥5% of sampling stations were selected for individual analyses. I used three analytical methods to model bird habitat relationships: logistic and linear regression and canonical correspondence analysis (CCA). Data from both intensive and extensive sites previously collected by WDNR in 2004 and 2005 were included in analyses. I used stepwise logistic and linear regression to analyze habitat relationships of 42 species with sufficient detections. Dependent variables in models included species presence (logistic models) and mean abundance per site (linear models), stratified by habitat association and migratory strategy. I also analyzed habitat relationships of selected species that presently were near the edge of their distribution range within the study area. Because of different sampling intensities (i.e., years, number of point-count surveys/year) within intensive and extensive sites, I modeled data from intensive and extensive sites separately. I also analyzed the 2007 extensive sites separately to examine potential differences resulting from including additional habitat variables measured in 2007. I then examined bird community habitat relationships using CCA, stepwise logistic and linear regression, as well as a priori logistic and linear regression models with Akaike’s Information Criterion (AIC) model selection. Generally, significant variables common among analytical approaches indicated that bird occurrence and relative abundance within peatlands increased in response to greater cover and structural diversity of shrub strata. Among logistic and linear regression models selected using AIC, models containing the variables snags, percent cover of low shrubs and high herbs, peatland area, and percent cover of all conifer trees received the strongest empirical support. Overall, foliage height diversity within peatlands appeared to be the most important structural attribute contributing to bird diversity and abundance within these habitats. In addition to providing a better understanding of peatland bird habitat relationships within the Great Lakes region, the results of this study may provide managers with important baseline data that could be used to monitor potential effects of climate change on peatland flora and fauna.