Survey Methods and Habitat Associations of Secretive Marsh Birds in Coastal Wetlands of the Western Lake Erie Basin
James M. Hansen, MS
Advisors: Robert J. Gates and Christopher M. Tonra
Emergent wetland areas in North America have declined over the past century.
Secretive marsh birds, which include rails, bitterns, and gallinules, are marsh-dependent species that have experienced population declines associated with wetland loss and degradation and are of high conservation concern. Standardized marsh bird monitoring surveys have been conducted across North America over the past decade to monitor population trends and investigate habitat relationships for these secretive species. The goals of this study were to (1) evaluate the standardized marsh bird monitoring protocol for northwestern Ohio by determining how survey frequency and survey timing, point count survey location, and intra-seasonal movements of Virginia rail (Rallus limicola) and sora (Porzana carolina) affect estimates of detection probability, occupancy, and abundance, and (2) explore region-specific habitat associations of secretive marsh birds.
I conducted surveys in 2017 and 2018 using the Standardized North American Marsh Bird Monitoring Protocol at 3 managed coastal wetland complexes within the western basin of Lake Erie in northwestern Ohio. Wetland sites were visited 3 times per 2-week survey season during 8 April – 30 June 2017 and 7 May – 30 June 2018. Habitat data including percent cover of wetland vegetation, vegetation height and density, floating substrate density, open water-vegetation interspersion, and water depth were collected within 100 m of survey points. Rails were captured, fitted with radio-transmitters, and tracked using ground-based, aerial, and automated telemetry during March – June 2016 – 2018.
More than 90% of radio-marked Virginia rails and sora that were present during the standardized marsh bird monitoring period apparently emigrated from the study area during the established standardized marsh bird monitoring period in 2016, 2017, and 2018 when their populations were assumed to be closed. The current standardized marsh bird monitoring protocol of 3 visits during 7 May – 19 June provided higher variation in estimates of occupancy and abundance for secretive marsh birds compared to seasons with 6, 9, and 12 visits to survey points during the breeding season. The effect of survey point location and time of day that surveys were conducted varied among all focal secretive marsh bird species, underscoring the importance of including morning and evening surveys and both survey location types to account for variation in detection probability estimates. Previously undocumented emigration of Virginia rails and sora during the breeding season may affect current monitoring efforts for these species and more research is needed to determine if similar intra-seasonal movements are exhibited by other secretive marsh birds across their breeding distributions. The standardized marsh bird monitoring protocol for northwestern Ohio should include surveys conducted during morning and evening periods, survey locations on leveed dikes and within emergent vegetation, increase survey frequency to increase precision in estimates of occupancy and abundance, and avoid conducting surveys during May when there is a high rate of population turnover for Virginia rail and sora.
Wetland plant species composition, vegetation structure and arrangement, and water depth were all important factors that affected estimates of occupancy and abundance for secretive marsh birds. Managers should prioritize increasing the diversity of wetland flora, water depths, vegetation structure, wetland complexity, and increased heterogeneity of open water-vegetation interspersion. The information gained in this study will help inform current strategies used to monitor secretive marsh birds and manage wetlands for their conservation and long-term viability.