Density and Abundance of Secretive Marsh Birds in Iowa
A decrease in wetland habitats throughout North America has caused a decline in populations of marsh birds. The objective of this study was to estimate population densities and abundances of secretive marsh birds in Iowa. Call-broadcast surveys were conducted in conjunction with distance sampling for eight species of marsh birds at wetlands in three regions of Iowa during 2009 and 2010. Regions were defined by observed microhabitat characteristics which also corresponded to physiographic regions. Region-specific density estimates were obtained using Program Distance for four species of marsh birds for which sufficient detections existed (Pied-billed Grebe [Podilymbus podiceps], Least Bittern [Ixobrychus exilis], Virginia Rail [Rallus limicola] and Sora [Porzana carolina]). The range of density estimates was 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 highest in Region 2 for Pied-billed Grebe, Region 1 for Virginia Rail, and Region 3 for Sora. Least Bittern density was similar between Regions 1 and 2, but was 0.027 birds/ha lower in Region 3. The need to focus conservation efforts on areas of the state where large amounts of suitable habitat exist and marsh bird densities are highest is illustrated by the observed differences in species' densities across regions. Information on the current population status of marsh birds in Iowa and regions where conservation efforts can be directed are provided by these density estimates.