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The 2010 BP Deepwater Horizon well blowout vomited more than 210 million gallons of oil into the Gulf of Mexico and onto its shores—the largest accidental, offshore oil spill in history. It killed wildlife, tainted fisheries, and damaged coastal ecosystems from marshes in Louisiana to beaches in Florida. But due to a paucity of data, the true extent of the damage is still not yet known, especially where bird mortality is concerned. What research does exist is confidential property of the U.S. government, and will not see the light of day until the lawsuit against BP has run its course, the next phase of which begins in 2015.

Into this vacuum step J. Christopher Haney, Harold Geiger, and Jeffrey Short, three researchers with extensive experience in environmental monitoring and post-spill mortality assessments. In their recent study, which has been accepted for publication in Marine Ecology Progress Series, the authors estimate that up to 800,000 coastal birds died as a direct result of the Deepwater Horizon spill. That number, as large as it is, is on the conservative side, says ±¬ÁϹ«Éç Director of Bird Conservation for the Gulf Coast and Mississippi Flyway, Melanie Driscoll. Once further studies are conducted, says Driscoll, the number will certainly exceed one million. In comparison, a quarter of a million birds are estimated to have died as a direct result of the Exxon Valdez, a spill that was much smaller than that of Deepwater Horizon.

[05.16.14 Author note: Since this story was written, both studies by Haney et al. were published in Marine Ecology Progress Series. The first paper, which looked at mortality of coastal birds, estimated that 800,000 coastal birds died during the acute phase of the Deepwater Horizon spill. The second paper examined the mortality of offshore birds. In that paper, the authors found that approximately 200,000 offshore birds died during the spill's acute phase. These numbers, coupled with ±¬ÁϹ«Éç scientists' observations of bird colonies and bird mortality well after the acute phase, have led scientists like Driscoll to conclude that more than one million birds ultimately succumbed to the lethal effects of the Gulf oil spill.]

The study itself uses two models to estimate coastal bird mortality. The carcass sampling model attempts to answer a seemingly simple question: for every bird corpse found during clean-up efforts, how many bird bodies were missed, due to factors such as scavenging, or the bird dying at sea, or decomposition? The other model, called the exposure probability model, attempts to quantify how many birds of each species would have encountered the oil, given the size of the slick at any given time and estimated population densities. Despite these being two very different ways to estimate bird mortality, the models agreed very closely with the possible range of bird deaths: between 600,000 and 800,000 over the 95 days of the "acute phase" of the spill. Another way to think about that: 8,000 coastal birds died every day during the acute phase.

While the numbers are sobering on their own, drilling down to individual bird species reveal population-level impacts on their numbers. According to the paper, 36 percent of the entire Laughing Gull population in the northern Gulf of Mexico died within that 95-day period. Fifteen percent of Royal Terns perished, as did 12 percent of Brown Pelicans. On Queen Bess Island, Driscoll saw an entire colony of Royal Tern chicks oiled; they all subsequently died due to oil exposure.

The suffering that Driscoll observed during the actual spill foreshadowed this devastating loss of bird life, and she says she has feared that the toll could exceed a million birds. In the paper by Haney et al., says Driscoll, the researchers went to great lengths to explain how they used data from this and other spills to make their calculations. The authors described sources for overcounting and undercounting. For example, if oiled birds tend to fly toward shore, the researchers may have overestimated the number of birds that died. But sources of undercounting are far more prevalent: During the spill, searchers only collected whole carcasses, and they did not search breeding colonies until months after the initial spill. Further, the counts missed the carcasses that were either burned or skimmed away when rescue workers removed oil from the water's surface. The researchers also chose to not count live oiled birds and they deliberately excluded entire classes of birds—marsh-dwellers such as gallinules, rails, bitterns, and some herons and egrets. More than 2,000 miles of marsh were affected by the spill, representing a large number of bird deaths which are not accounted for in the analysis.

The mortality from acute oil exposure is only a fraction of the damage that Deepwater Horizon wreaked upon the Gulf. Four years after the disaster, some 200 miles of Louisiana beach is still contaminated with oil. Studies on shrimp and dolphins have shown long-term health issues with animals exposed to oil and dispersant during the Deepwater Horizon—lowered reproductive success, chronic health problems, and starvation due to loss of food sources.

The most distressing aspect of this entire situation, says Driscoll, is that, four years later, BP is putting more energy into stonewalling than restoring the Gulf. The third phase of the lawsuit against BP for its violation of the Clean Water Act will not begin until 2015—five years after the disaster. This means that most compensatory funds to help restore the Gulf have not yet materialized. Meanwhile, BP attempts to discredit studies that show harm to Gulf resources and has started refusing to fund research to understand delayed and chronic effects on birds and other wildlife, says Driscoll. While birds and other wildlife in the Gulf struggle to recover, the government and conservation communities use early restoration money to repair damage and steward the birds, doing what they can to make sure the animals get the best chance at long-term survival.

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