Sensitivity of secretive marsh birds to vegetation condition in natural and restored wetlands in Wisconsin

Wetland loss, biological invasions, and ecological restoration are major factors altering wetland resources in the Midwestern United States. Large-scale plant-community change associated with these factors is an under-investigated, potentially strong driver of habitat suitability for wetland-dependent wildlife, such as secretive marsh birds (SMBs), which are of widespread conservation concern. We employed multi-year, hierarchical Bayesian occupancy modeling to investigate sensitivity of 3 SMB species (American bittern, sora, and Virginia rail) to habitat and vegetation characteristics in Wisconsin, USA. We contrasted habitat characteristics and SMB occupancy in natural wetlands with those restored under the federally funded Wetlands Reserve Program (WRP). We also examined the extent to which SMB occupancy was explained by 3 levels of habitat assessment that encompassed different spatial scales and levels of sampling effort (landscape, rapid, and intensive). All species were significantly associated with variables derived from intensive assessment, and showed high sensitivity to differences in plant-community composition and vegetation quality. Both American bittern and Virginia rail were negatively associated with abundance of the invasive wetland grass, Phalaris arundinacea (reed canarygrass), and were positively associated with an indicator of plant-community quality (mean coefficient of conservatism, or C-value). Sora occupancy was positively associated with Typha (cattail) abundance. For all 3 species, occupancy was greater in natural sites than in restored sites, which were characterized by greater Phalaris abundance and lower mean C-values. Our results show broad agreement between botanical and avian indicators of wetland quality, suggesting that enhancing the condition of wetland vegetation can yield ancillary benefits for SMBs. In this region, efforts to control Phalaris and restore diverse, native-dominated plant communities are likely to increase wetlands' capacity to support SMBs.