Population ecology and monitoring of secretive marsh-birds in Iowa

Populations of marsh-birds are believed to be declining throughout North America in response to a decrease in wetland habitats. As a result, several species of marshbirds are of heightened conservation status at the local and regional levels and the need to monitor populations of these birds is critical to effective conservation and management. Our objective was to estimate population densities and abundances of secretive marsh-birds in Iowa. We conducted call-broadcast surveys in conjunction with distance sampling for eight species of marsh-birds at wetlands in Iowa during 2009 and 2010. We divided Iowa into three regions based on our observations of microhabitat differences and to improve precision of our density estimates. We used Program Distance to model detection probability and obtain region-specific density estimates for four species of marsh-birds for which we had sufficient detections (pied-billed grebe, least bittern, Virginia rail, and sora). Density estimates ranged from 0.019 birds/ha (95% CI = 0.014-0.024) for least bittern to 0.12 birds/ha (95% CI = 0.11-0.14) for pied-billed grebe. Density estimates were different in all regions for three of the four species (pied-billed grebe, Virginia rail, and sora). Least bittern density was not different between regions 1 and 2, but was lower in region 3. Estimates of density are a reliable metric of population status because they consider the amount of suitable