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TWo biologists on a beach wearing gloves photograph and document a dead sea gull.
Information icon A USFWS biologist documents the GPS coordinates for a dead gull found in Gulfport, Mississippi, during the Deepwater Horizon oil spill. Photo by Bonnie Strawser, USFWS.

Deepwater Horizon oil spill killed as many as 102,000 birds across 93 species

The Deepwater Horizon oil spill occurred more than six years ago, but for many of us the memories of the spill are still fresh in our minds. As the well continued to spew oil for weeks, and given the inevitability of widespread injury and death of wildlife, many people worried whether those responsible for the spill would be held accountable. As proved by the historic $20.8 billion settlement with BP reached in May 2016, responsible parties can be held accountable. The U.S Fish and Wildlife Service played a key role in that outcome.

A plan for assessing injury caused by Deepwater Horizon oil

The Service has a critical role in assessing injuries to federal lands and other natural resources that are caused by an oil spill. We are stewards of wildlife refuges located all along the Gulf coast, and we also serve as a trustee on behalf of the public for natural resources that include migratory birds, threatened and endangered species, and some fish.

Shortly after the Deepwater Horizon spill began, Service biologists started developing plans for documenting and quantifying injury to birds. Service biologist Pete Tuttle, who had 25 years of experience, led the Deepwater Horizon Natural Resource Damage Assessment Trustees (five Gulf States and four federal agencies) in developing an the avian injury assessment. Pete and other Service experts considered the range of bird species that could be affected by the spill and the ways they could be exposed to the oil. The multi-state, multi-year strategy was laid out in 60 detailed work plans that described the data collections necessary for a comprehensive injury assessment for birds.

“The early stages of the spill were certainly interesting times.” Pete notes. “The estimated flow rate of oil seemed to grow each day while prospects of controlling the release diminished. This prompted us to consider worst case scenarios regarding injuries to birds. It was a good thing we did, because no one was thinking the spill would continue for 87 days.”

The largest bird injury assessment ever conducted

The resulting injury assessment strategy included collecting evidence of oil exposure, oil-related mortality, and the effects of the oil on the health of the birds.

Estimating short-term mortality alone is exceedingly complex. For the Deepwater Horizon oil spill, researchers had to examine mortality on sandy beaches, within marshes, island breeding colonies, in offshore open water areas, and everywhere in between. One approach used to estimate mortality was a model that estimates the rate of deposition of spill-affect birds on coastal shorelines. Called the Shoreline Deposition Model, it became a key component of the Trustees’ bird injury assessment.

“It is common knowledge that the number of birds killed by an oil spill is typically much larger than the number of dead birds collected by responders,” Tuttle explained. “For a variety of reasons, many dead birds are never found. They sink or are eaten before washing ashore, or they are buried by sand, washed into inaccessible marshes, or eaten by other animals once they do wash ashore. Even when a carcass does end up in an area being searched, we don’t find some bodies because searcher efficiency is never perfect.”

Several field studies were conducted to inform the Shoreline Deposition Model. One evaluated the probability that a bird that had died in the water would be deposited on a searchable shoreline. This study, the Carcass Drift Study, included placing 248 radio-tagged bird carcasses in numerous locations off-shore across the northern Gulf. This study showed that only a small fraction of birds that died on the water (either in open sea or open water in marsh areas) would ever reach the shore.

A second set of studies evaluated the persistence of bird carcasses on beaches and in marshes. The Carcass Persistence Study involved placing bird carcasses of different sizes along sandy beaches and marsh edges and then revisiting them almost every day for two weeks. The results of this study proved that the rates of disappearance of bird carcasses due to scavengers, rewash, or burial varied by habitat and bird size. This was an important piece of data, considering the diversity of the species killed by the oil and the habitats they used.

A third set of studies evaluated the probability that a person walking along a beach or riding in a boat near a marsh edge would see a dead or dying bird that is present within a search area. Not surprisingly, the Searcher Efficiency Study found efficiency varied by habitat types. For example, it was easier for searchers to find bird carcasses on a sandy beach than along a marsh edge.

Many faceted injury assessment estimate points to enormous loss of birds

Estimating mortality using the Shoreline Deposition Model was only part of the massive bird injury assessment lead by the Service. Other key assessments included looking at the physiological and physical effects of oil exposure on birds.

The Trustees used the Shoreline Deposition Model investigation and many others to estimate the effect of the spill on birds. They had actual counts, but they knew those counts would underestimate the true effect. Accordingly, during the spill more than 8,500 dead and impaired birds were collected, and more than 3,000 live birds were taken to rehabilitation centers (where only half survived). With these numbers serving as the starting point of the injury models, the Trustees final injury assessment estimated that between 65,000 and 102,000 birds across 93 species were killed by the spill. These findings played a key role in determining the amount of damages BP was eventually required to pay when the case was settled in May 2016. As mandated under the Oil Pollution Act, these damages will be used to restore and replace these lost birds.

The mission of the U.S. Fish and Wildlife Service is working with others to conserve, protect, and enhance fish, wildlife, plants, and their habitats for the continuing benefit of the American people. For more information on our work and the people who can make it happen, visit Connect with the Service on Facebook, follow our tweets, watch the YouTube Channel and download photos from Flickr.

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