Winfried Wisinewski/Corbis

Bill Finch and I are standing knee deep in the salt pan of the Grand Bay marsh of Alabama, staring down at the sheen of blue oil that has worked its way back into the marsh’s deepest recesses. The oil is beautiful in its own way, shining like the backs of the tiny fish -- sailfin mollies, they’re called -- that Bill has shown me. My mind is focused, though, not on the colors, but on the future. In the fall, migrating birds will use this oily marsh as their feeding ground and will prey on those very same mollies. Green herons, yellowlegs, and blue-winged teal will fly through the Gulf region, along with billions of other birds, pouring down this central corridor as they make their arduous journeys from points north to Central and South America, with some stopping in the Gulf for the winter.

Migration is a time of both stress and opportunity, but in this strange and oily year, one worries that the former will overwhelm the latter. That the stress might be of a quieter form than first imagined is only a small consolation. Bill Finch, a naturalist who has known and loved this marsh for most of his life, is glad that what we are seeing on the water is a sheen, not a thick black slick -- but he is still worried.

"People want to see dead animals," he says when I ask him about how the marsh might support, or fail to support, the birds that will stop here this fall. I’m not so sure about this assertion, but since Bill has so far been a knowledgeable and generous guide on my journey, I nod and wait.

"They want an obvious symbol of the devastation to rally around," he explains after a pause. "And when it comes to the fall migration, that would mean dead, oiled birds. But what we are likely to get is going to be a lot more subtle than that."

What we are likely to get, he goes on, are invisible changes that will make it harder to match cause and effect. He points to the millions of periwinkle snails that cling to the tops of marsh grass, creatures that look harmless enough but are at this moment unwittingly injecting a parasite into each blade that will kill them. The periwinkles do not do this because they are some sort of malicious sci-fi creature, but because this is what they do. And what crabs do is prey on the periwinkles. What would happen, Bill speculates, if the off-shore oil affects the breeding of crabs, occurring right now miles to our south out in the Gulf? Well, he continues, the crabs might not eat the periwinkles, and the periwinkles would mow the marsh, slowly killing it, and the marsh would no longer be the bountiful nursery for the sailfin molly and other fish. Which would mean that eventually the green heron, exhausted in its journey and eager to reach what it knows will be a bountiful stopover point, goes hungry.

In other words, the invisible. Over the last three weeks, as I have wandered the Gulf shores and watched birds and talked to bird people, I have gathered a list of the obvious -- and less obvious -- threats to birds.

"Imagine running a marathon in bad air," says Laura Erickson, a bird expert and science writer at the Cornell Lab of Ornithology. "Well, for many species, this is their marathon, and conditions need to be just right."

The comparison to sports seems apt. In this case the marathon is a 600-mile water-crossing after days of travel, and each fall, birds mass nervously before making the journey. But the larger marathon is the central corridor migration itself, the largest in the United States, containing not just a stunning number of birds but a stunning variety.

"It’s hard to think of a species of migrating bird east of the Rockies that doesn’t fly though the Gulf," says Scott Weidensaul, author of Living on the Wind, a powerful compendium of bird migrations. "And these birds, already stressed, are going to be flying into uncertainty."

Migration is always a gambit, a time when everything has to go right, including the building up of body fat from high-quality food sources that birds find at certain stopover points. Imagine if the fish that a tern eats is compromised, or if a certain sandpiper can’t find a certain crustacean, or an abundance of certain crustaceans, where it has reliably found them over the seasons.

"I’m most worried about the shorebirds and diving birds," says John Dindo, the senior marine scientist at the Dauphin Island Sea Lab in Alabama. "The gannets and ospreys and terns and loons that are diving down into it."

Of course, what they will be diving down into is the great unknown.

"There is a whole lot of unaccounted oil out there," Dindo says, "and what happens to that oil is the big question."

Dindo, like every other scientist and birder I talk to, is quick to add how little is certain or obvious about what will happen, and that, right now, the honest answer is that we simply don’t know. In fact, everyone I speak to is understandably quick to use caveats and qualifiers, which leads to other questions. If the damage is to the food chain, we might not see the results immediately. How will we gauge the ultimate effects of the spill? And, more immediately, how will we know what happens during this fall migration?

The simplest answer to the last question is, "By watching."

"People are passionate about birds," says Chris Wood of the Cornell Lab and eBird, the online bird-tracking service that makes use of that passion to create a map of bird populations and bird movement. What Wood and eBird have done is effectively create an army of bird watchers who record movement and species in their particular patches of land, and this fall those birders will focus hard on the shores of the Gulf.  This has also been the strategy of Bethany Kraft, the executive director of the Alabama Coastal Foundation, who has enlisted Alabamans who live along the shore to observe certain sections of beach, in this way transforming homeowners into amateur naturalists.

Another way to determine the number of birds that fly through, and where they land, and how they react to the degraded environment, is through radar. Frank Moore, the chair of the biology department at the University of Southern Mississippi, is open to all methods of charting the migration, including the use of satellite telemetry, but the method that he thinks will have the best chance of really telling us something about the birds’ movements and numbers is weather surveillance radar.

"We have good records for the last 10 years to compare them to," he says. "It can tell us things about general movement, but it can also pick up a skimmer colony of, say, 500 pairs. With the radar you can see the birds in movement. In fact, you can see if they are moving to their traditional sites and how they are behaving in general."

And, since so many of the migrants move at night, the radar has the happy effect of making the invisible visible.