The true impact of the Deepwater Horizon oil spill on the Gulf of Mexico won't be known for weeks—if not months or years—but already the spreading oil presents a danger to the region's threatened and endangered species.

One of the first species to face risk could be the brown pelican (Pelecanus occidentalis), which has just entered its breeding season on Louisiana's coastal islands, including the Chandeleur Islands, which the oil reached over the weekend. The brown pelican was just removed from the endangered species list last year.

Meanwhile, it's an important season in the Gulf for migratory birds; hundreds of species pass through the region at this time of year. In fact, this coming Sunday, May 9, is International Migratory Bird Day, an event intended to celebrate and call attention to the birds passing through North America every year. But with waters and coastal regions already feeling the impact of the oil spill, all of those birds could be at risk.

According to a report from the Environmental News Service, endangered and threatened bird species immediately at risk include the least tern (Sternula antillarum), piping plover (Charadrius melodus), Wilson's plover (C. wilsonia) and American oystercatcher (Haematopus palliates).

As demonstrated by the Exxon Valdez and other oil spills, crude oil interferes with a bird's ability to keep itself warm and can be fatal when ingested. It is almost impossible for an oil-covered animal to clean itself off. The first oiled bird, a northern gannet, was captured on Friday and is currently being treated.

Meanwhile, one of the world's most endangered species of sea turtles, the Kemp's ridley sea turtle (Lepidochelys kempii), is about to enter its primary nesting season. The spread of oil could interfere with the Kemp's ridley's foraging and migration routes. The species only nests in the western Gulf. Four other species of endangered or threatened sea turtles also live in the area.

The U.S. Fish and Wildlife Service has mobilized several efforts to protect species in the Gulf. Full information is available here.

Photo: Brown pelican, via US Fish and Wildlife Service