The 2010 Deepwater Horizon oil spill is often cited as the worst environmental disaster in U.S. history—yet its impacts on the marine life of the Gulf of Mexico have gone largely undetermined. Now, new findings published this month in Marine Ecology Progress Series estimate that the number of seabirds lost as a result of the spill may number well into the hundreds of thousands. Birds are especially vulnerable to oil, which can coat their feathers and cause death by dehydration, starvation, or drowning. Seabird mortalities can easily be underestimated following a spill as bodies are lost at sea or go undiscovered. So researchers turned to two different estimation methods—one whereby total mortalities were estimated from the actual number of dead birds recovered, and another in which information on the geographic extent of the oil slick and seabird densities were used to estimate potential mortalities. The scientists found that although the two approaches were based on different data sets, they returned roughly similar estimates of 600,000 and 800,000 oil-related seabird deaths, respectively. Although the number of seabird mortalities from the spill likely centers around 700,000, sources of uncertainty in the estimates indicate the number of deaths could actually lie anywhere between 300,000 and 2 million. In comparison, an estimated 250,000 seabirds were lost during the Exxon Valdez oil spill, and longline fisheries are estimated to contribute to 160,000 to 320,000 seabird deaths globally each year. For some seabirds, such as the laughing gull (Leucophaeus atricilla), the Deepwater Horizon impact translates into an estimated loss of more than 30% of its Gulf of Mexico population. Energy company BP faces civil penalties based in part on the number of birds and other wildlife lost in the spill, therefore the mortality estimates could influence the amount the company will be required to pay.