A Red Knot on the water in Pillar Point, Calif.
Millions of birds passed through Virginia last month, and the National Wildlife Federation says many are endangered. Wetlands are drying up, mountaintop cool zones are warming, and often there is less available food.
Bird watchers in North America used to see Red Knots as far north as Cape Cod, but today they're most often seen in Virginia and in states around the Delaware Bay. In the 1980s, experts counted more than 100,000, but over the last 20 years their numbers have fallen to an estimated 25,000.
Virginia Tech Professor Jim Fraser says they might be having trouble finding food.
"Red Knots eat horseshoe crab eggs, and with the overharvest of horseshoe crabs, there may not be enough eggs there," he says.
They also eat tiny surf clams called donax.
"When the waves come in, you can see donax riding in with the wave, and then as the wave recedes they'll bury down in the sand, and in some cases we'd find maybe 300 clams in that little sampling."
But when research specialist Shannon Ritter sifts the sand, she's surprised by what she finds. "I see one. Just one? Pretty disappointing catch. That's maybe 2 percent of what we might expect to see at this time of year in a sample like this."
Fraser says donax populations might have been wiped out during their own breeding season when Hurricane Sandy played havoc with the marine environment.
And he has one other theory on the decline of Red Knots. In the arctic, where lemmings breed in a space between the snow and the ground, warming temperatures and melting snow have taken a toll. Lemming populations have not reached their usual peaks, which mean arctic foxes, owls, hawks, and weasels can't feast on baby lemmings. Instead, they might be eating Red Knot eggs and babies.
"A number of years ago people realized that certain bird species only seem to breed well when the lemming populations were high, and the idea was that then the predators were going after the lemmings and leave the birds alone, so they get a chance to reproduce."
As Fraser continues to probe the mystery of the disappearing Red Knots, he and other conservationists think the U.S. should declare them an endangered species. That would prompt more research and creation of a plan to help the birds recover.