Every spring, scientists tromp through Louisiana's mud and waist-high grass, hunting for the hidden nests of a palm-size bird called the seaside sparrow. Their goal: to see whether the massive oil spill from a broken Gulf of Mexico rig known as Deepwater Horizon has hurt creatures that don't actually inhabit the water.

Five years after the worst oil spill in U.S. history, early reports from this and other research suggest that the ecological damage lingered in unexpected ways. But scientists say cataloging what that means for the Gulf's future grows more complex with time.

Amid the rushes and cordgrass of the Gulf's fragile salt marshes, for example, scientists say they made a surprising discovery: Two years after the spill, in