Distribution and abundance of king rails (Rallus elegans) and clapper rails (Rallus crepitans) on the northern Delmarva Penninsula

Date
2017
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
King (Rallus elegans) and Clapper (Rallus crepitans) rails co-occur in marshes on the Delmarva Peninsula with King Rails restricted to brackish and fresh-tidal marshes and Clapper Rails occurring in marshes with greater salinity. Our understanding of the distribution and abundance for these two species is limited and King Rail populations have declined across their entire range in recent years. The marshes of the Mid-Atlantic have historically supported King Rails, however, surveys conducted in 2002 found a marked decrease in the number of sites occupied and estimated abundance in the Chesapeake Bay. In light of the decreasing population trends for King Rail in the Delmarva Peninsula, our goal in this project was to use standardized protocols to sample historic known King Rail sites and new marshes to evaluate King and Clapper rail populations in the Delmarva Peninsula in 2014 and 2015. We used the same two-stage cluster sampling employed by the Saltmarsh Habitat and Avian Research Program (SHARP 2014) to randomly select 40 km^2 hexagons (primary sampling units) in tidal fresh and brackish marshes of the northern Chesapeake and Delaware Bays and randomly located sampling points within each hexagon (secondary sampling units). These points were a mix of historic points and newly generated points for our project. We surveyed the points within each hexagon using the North American Secretive Marsh Bird Monitoring call-back surveys and added an adaptive sampling component to the design by increasing our sampling effort adjacent to points where King Rails were detected. Our first objectives were to estimate the occupancy and abundance of King and Clapper rails and use a suite of a priori hierarchical models to determine which local and landscape features influenced these parameters. Second, we developed adaptive sampling techniques to determine if we could obtain more precise estimates of King Rail abundance in our study area. We found King Rail occupancy was positively correlated to the amount of Big Cordgrass (Spartina cynosuroides) at the point scale and King Rail abundance was positively correlated with the amount of emergent marsh and negatively correlated with the amount of forest at the local scale. At the point scale, Clapper Rail occupancy was positively correlated to salinity, Smooth Cordgrass (Spartina alterniflora) cover, and Salt Meadow Cordgrass (Spartina patens) cover and negatively correlated to Narrow-leaved Cattail (Typha angustifolia) and Common Reed (Phragmites australis) cover. At the local scale, Clapper Rail occupancy was positively correlated to the amount of emergent vegetation and negatively correlated to the amount of forest and the amount of agriculture within 200 meters of a site. At the point scale, Clapper Rail abundance was positively correlated to the salinity, Smooth Cordgrass, Salt Meadow Cordgrass cover and interspersion and negatively correlated with Narrow-leaved Cattail and Common Reed cover. At the local scale, Clapper Rail abundance was positively correlated to the amount of emergent marsh and negatively correlated to the amount of agriculture and amount of forest within 200 m of a site. The adaptive cluster sampling technique we employed was marginally effective at increasing the precision of our abundance estimates. This technique might be improved by constraining the spatial extent at which it is applied. The adaptive neighborhood transect technique was effective at obtaining high precision estimates of average King Rail abundance at occupied sites, which could be useful for increasing the power to detect trends in King Rail abundance.
Description
Keywords
Biological sciences, Delaware, Delmarva, Marsh, Maryland, Rail, Rallus
Citation