Cooperative Fish and Wildlife Research Units Program: Maine
Education, Research and Technical Assistance for Managing Our Natural Resources

Webless marsh birds, such as rails and bitterns, are generally recognized as being among the least studied and understood avian groups because of their secretive nature. Over the last several decades, several species have been in apparent decline, but information on population trends is limited. Previous surveys for marsh birds performed in 73 wetlands during 1989-1990 and 125 wetlands during 1998-2001 provide an excellent opportunity to measure short and long-term changes in wetland occupancy for these species. In the present study I will be re-surveying previous sites to document temporal changes in wetland use by the Least Bittern (Ixobrychus exilis), Sora (Porzana carolina), Virginia Rail (Rallus limicola), American Bittern (Botaurus lentiginosus), and Pied-billed Grebe (Podilymbus podiceps) in Maine. The Least Bittern is of particular concern in Maine because its numbers have declined in Maine recently. PROJECT STATUS Field work for the project was completed in late July of 2005 with a total of 75 sites being surveyed across southern, central and eastern Maine. Of these 75 sites, 37 had originally been surveyed in 1989-1990 and 38 sites had originally been surveyed in 1998-2000. Virginia Rail was the most frequently encountered species. Overall, wetland occupancy was greater for Virginia Rails and American Bitterns, and less for Least Bitterns than in earlier surveys. Wetland occupancy for Soras and Pied-Billed Grebes was similar between periods. Only seven individual Least Bitterns were detected in 2005-2006. These individuals were found in five separate wetlands and breeding pairs were observed at only one site. Detection probabilities were relatively high for each of the species. Rainfall and temperature patterns did not differ substantially among survey periods, and these factors likely did not influence the occupancy trends that we observed. Habitat conditions had not changed substantially in sites previously occupied by Least Bitterns. Preliminary analysis of data on habitat use suggests that Least Bittern presence was strongly associated with wetlands that had large areas of emergent vegetation along with some open water, whereas wetlands dominated by shrub vegetation were avoided. Least bitterns were not sensitive to wetland size. Pied-billed Grebes were strongly associated with large wetlands dominated by open water and having some emergent vegetation. The likelihood of a wetland supporting Pied-billed grebes increased greatly when total wetland area was >400 hectares and there was at least 20% open water. Soras and Virginia Rails were strongly associated with the availability of emergent vegetation, but Soras also nested in wetlands containing a large component of shrub vegetation, whereas Virginia Rails selected against wetlands dominated by shrub vegetation or open water. The presence of American Bitterns was strongly related to the area of shrub vegetation in wetlands. Final analyses will focus on quantifying habitat thresholds values or other probability of occurrence relationships that will provide valuable information for assessing impacts of environmental perturbation on these marsh birds.