Data collection for Deepwater Horizon Bird Study #12 occurred in the Spring of 2011. Data were collected primarily by BioDiversity Research Institute (BRI) and R.G. Ford Consulting Company (RGFCC) personnel on behalf of the natural resources trustees as part of the NRDA pre-assessment efforts. The study contained two primary objectives. Objective 1 was to estimate the proportion of target species populations that are visibly oiled within the study area through a) aerial surveys to estimate population abundance, and b) foot-based and boat-based observations of those species to estimate the proportion of birds visibly oiled. Two aerial survey efforts were conducted in mid-January 2011 and mid-to-late-February 2011, by RGFCC. In order to estimate oiling levels within representative areas, BRI personnel conducted field work from February 16, 2011 to April 5, 2011, focusing on the following target species: American White Pelicans, Common Loons, and Northern Gannets. This dataset focuses on the pelican and loon effort. Boat and land-based surveys of American White Pelicans and Common Loons were conducted along transects within an Area Contingency Plan (ACP) grid system (Oiled Bird Observation dataset). Objective 2 was to collect information on target species mortality occurring within the study area using the dead bird survey data collected as part of the Waterfowl study (Bird Study #10) and potential additional dead bird surveys if necessary to adequately address the wintering open-water waterbird target species. This objective, however, was not implemented per the recommendation of the Waterfowl Technical Working Group that additional beached bird surveys after Bird Study #10 was completed were not warranted (see United States Department of the Interior Memorandum: Non-implementation of Objective #2 from the Wintering Open-Water Waterbirds Study). Additional details on Bird Study #12 surveys and results are provided in the End of Study Report (NRDA End of Study Draft Summary Report, Deepwater Horizon Mississippi Canyon 252 Oil Spill: Winter Open-Water Waterbirds, Bird Study #12). The DWH NRDA Oiled Bird Observation dataset includes the information collected during the oiling observation surveys.