Temporal variation in detection of marsh birds during broadcast of conspecific calls

The decline in populations of several species of marsh birds in North America has prompted development of a monitoring protocol that involves the broadcast of conspecific calls to enhance detection of these secretive species. However, with a standardized protocol, temporal (seasonal) and geographic variation in responses to the broadcast of calls could lead to inadequate monitoring of migratory species with large ranges. Our objective was to examine temporal variation in the response of marsh birds to playback of conspecific calls in west-central and northern New York to determine if use of the current protocol would permit effective monitoring of their populations. From 11 April to 8 July 2005, we conducted 572 surveys at 143 survey points on 16 marshes and detected 663 individuals of our target species. Our results revealed more detections of American Bitterns (Botaurus lentiginosus) and Pied-billed Grebes (Podilymbus podiceps) early in our survey period, and more detections of Virginia Rails (Rallus limicola) and Least Bitterns (Ixobrychus exilis) later in our survey period. Only 22% of Least Bitterns were detected before 28 May, whereas 76% of American Bitterns and 70% of Pied-billed Grebes were detected before 28 May. With the current recommended monitoring protocol, surveys are to be completed during a 44-day period that includes three 10-day sampling periods separated by 1 week. However, our results indicate that this protocol would lead to inadequate and inaccurate monitoring of marsh birds in New York. Given that the timing of peak detection of different species of marsh birds varies geographically, we recommend flexibility in the timing and duration of surveys so that surveys can be synchronized with location-specific peak-detection periods.