There were multiple objectives for data collection as part of the Secretive Marsh Bird assessments. This dataset pertains to Objective 4, referred to in the study workplan as Methodology D, which consists of systematic checks of fiddler crab burrow density to reveal information about secretive marsh bird densities, and specifically the density of the clapper rail. Clapper rail diets depend heavily on fiddler crabs in the northern Gulf Coast ecosystem and consequently clapper rail densities are directly correlated with fiddler crab densities. These relationships make quantifying fiddler crab burrow density before and after an oil-spill valuable to understanding the potential impact of the oil spill on clapper rails and other marsh birds. Transects are oriented perpendicular to the marsh water edge and consist of 12 sample locations of 0.25 m2. The sample locations are along the transect at 1) the intertidal region (i.e., 0 meters from the shore), 2) five meters from shore, 3) 15 meters form shore, and 4) 30 meters from shore (i.e., the interior marsh). At each site, surveyors counted the number of fiddler crab burrows and then took note of GPS coordinates of transects for future (post impact) sampling efforts. These data were collected pre-spill (i.e., before oil from the spill reached the shoreline), and transects were marked with PVC pipes and locations recorded via GPS unit so that data could be collected again post-spill. However, by the time the study was implemented in the field, many of the study sites had been impacted by oil in some manner.