Habitat relationships of secretive marshbirds in urban wetlands around Washington, DC
Rails and bitterns are secretive and rare species, whose populations are decreasing range-wide. These species depend on wetlands for survival, yet human development in coastal areas often results in remnant and restored wetlands in urban areas. The goals of this study were to determine if secretive marshbird species are present and potentially breeding in small, urbanized marshes around Washington, DC and to examine specific habitat characteristics associated with marshbird presence. Secretive marshbird surveys were performed at 51 freshwater marsh locations using the Standardized North American Marshbird Monitoring protocol. This survey method uses repeated callback surveys around sunrise and sunset during the breeding season. All sites were within the Washington, DC metropolitan area. Random vegetation plots were placed around each bird survey point to identify plant species and percent cover. Random dip-net sweeps were also taken to sample the aquatic invertebrate population. Invertebrates were identified to the family level. Total marsh area, buffer width, and interspersion density (length of water/vegetation edge per hectare of marsh) were obtained from aerial images and visually verified during site visits. Data was preliminarily analyzed by performing pairwise contrasts between sites with and without marshbirds.
Secretive marshbirds were present at 11 of the 51 survey locations. King rails (Rallus elegans) and least bittern (Ixobrychus exilis) were the only secretive marshbird species identified. Preliminary results indicate that secretive marshbird presence is positively correlated with percent cover of tall emergent vegetation (p=0.072197) and invertebrate family richness (p=0.091886) and negatively correlated with interspersion (p=0.06597) and percent cover woody vegetation (p=0.088359). There was no correlation with marsh area, buffer width, or plant species richness. Tall emergent vegetation is a critical source of early spring nesting material, while invertebrate richness may be an indicator of an abundant food source. Woody vegetation may provide perches or cover for avian and mammalian predators. While other studies have found a positive correlation with interspersion, in the case of small wetlands, high interspersion may indicate an underdeveloped plant community that is too patchy to provide adequate cover. These results demonstrate that secretive marshbirds will use small urban wetlands and that adequate habitat for these birds can be found despite fragmentation of historic marshes. The next steps in this research are to account for imperfect detection of these secretive species and incorporate these habitat parameters into a habitat model applicable to management and restoration efforts.