To quantify the abundance and density of secretive marsh birds, field teams conducted survey routes from late March to late July 2010 in Louisiana, Alabama, Mississippi, and Florida. The data were collected by MJ Conroy Wildlife Biometrics (Athens, Georgia) personnel contracted by U.S. Fish and Wildlife Service (USFWS) occasionally accompanied by a BP/CardnoENTRIX observer contracted by the Responsible Party. Field teams conducted surveys in three marsh habitat types, saline Juncus marsh, saline Spartina marsh, and brackish Phragmites marsh. A survey route is a permanent grouping of approximately eight stations at least 500 meters apart that are surveyed during the same morning or evening survey window. At each station, for all target species (clapper rails and seaside sparrows, among others), observers recorded all birds seen or heard in a passive five-minute observation period. Following the passive observation period, observers broadcast callbacks and recorded all birds seen or heard during a respective response period. Observers also recorded the distance and direction of each detected bird relative to the station marker, birds detected between stations, and noted birds detected at a previous point. Data were documented on two data forms, the “National Marsh Bird Monitoring Program Survey” datasheet (herein referred to as “NMBMP form”) and the “Secretive Marsh Bird NRDA Surveys” datasheet (herein referred to as “NRDA form”). Field personnel recorded data on the NMBMP forms, and occasionally later transcribed data onto the NRDA forms. Note that the available datasheet type varies by route (i.e., a route might have data recorded on NMBMP, NRDA, or both forms). As part of the verification and validation process, the DOI Data Verification/Validation Contractor consolidated these data into one comprehensive dataset, Secretive Marsh Bird Callback Survey dataset. Additional details on the methodologies are available in the Standardized North American Marsh Bird Monitoring Protocols Wildlife Research Report #2009-02 and additional details on the surveys, data, and results are provided in the End of Study Report (Data analysis regarding oil spill effects on secretive marsh birds (pre-oiling), Final Report, October 4, 2013 by MJ Conroy Wildlife Biometrics)