Some of the most memorable and horrific images of the Gulf oil spill were of birds, struggling and slathered from beak to tail in thick, chocolatey oil.

When oil began spilling into the Gulf on April 20, 2010, it was nesting season in Louisiana, and shorebirds that make their nests on remote barrier islands and mangroves were at their most vulnerable.

Two years later, many of these grounds appear clean and birds have returned to them in large numbers to brood. But scientists say some of the effects of the oil spill on coastal birds may not be visible for years.