MOBILE, Alabama -- The dead seagull, rotten and stinking, had clearly been in the water for a while by the time it washed up on a Mobile Bay beach last week with a small Styrofoam buoy tethered to its leg.

Sporting an orange-and-white paint job and a small, black antennae protruding from a glop of putty, the cue-ball-sized buoy looked like a Cold War-era listening device.

Instead, the float equipped with a radio-transmitter is part of a $1 million U.S. Fish & Wildlife Service study funded by BP PLC. The study is aimed at estimating how many birds died as a result of exposure to the Deepwater Horizon oil spill last year.

Known as the “Carcass Drift Study,” this particular experiment will help researchers try to learn how many birds carcasses may have disappeared at sea without ever making it to shore.

Federal officials have dropped 248 dead birds into the Gulf since July 16. An additional 66 synthetic dummy carcasses also were dropped. Crews then tracked the carcasses as they drifted around in the Gulf.

R.L. Constantine found one of the birds last week on the beach in front of his house just south of Mullet Point.

“The bird just fell apart when I picked up the buoy. There was a steel wire connected to its leg, and the bird just fell off. It was in pretty bad shape,” Constantine said. “I called the number on the buoy to report it but I never heard back from anyone.”

Federal officials refused to specify when or where the bird that washed up on Constantine’s beach was dropped in the Gulf. All they would offer was that helicopters deposited groups of carcasses in specified locations between Louisiana and Panama City, Fla., over the last several weeks.

Similar drift studies were conducted in Alaskan waters after the Exxon Valdez spill.

Pete Tuttle, who is heading up the Natural Resources Damage Assessment for Fish & Wildlife, said the results of the Gulf drift study will be combined with several others to reach an estimate of the total number of birds killed by the spill.

“The carcass drift study, that’s looking at if a bird is oiled offshore, what are the chances of it coming ashore?” Tuttle said. Scientists believe a good number of carcasses are likely to be eaten by predators such as sharks, and a lot of the dead birds will simply sink into the depths.

One completed study looked at how long a carcass that washes up on a beach will last in the high heat and humidity of the Gulf environment. Another — the searcher efficiency study — examines the likelihood of anyone finding an oiled carcass on the beach before it is either buried by sand, consumed by scavengers or carried away by waves.

“So we’re looking at what is the probability the carcasses will come ashore? If they do come ashore, how long will they stay there? And then, what is the chance the surveyors will find them?” Tuttle said.

He said that by combining the results of the various experiments and working from the actual number of dead birds found after the spill, researchers would be able to extrapolate how many birds were lost but never recovered.

Fish & Wildlife’s Mike Pixley supervised the carcass drops.

“Roughly once a week, we will take 50 total specimens — 40 carcasses, and 10 dummies — and drop one in each grid cell from Louisiana to Florida,” Pixley said. “We have teams following them. We can detect the transmitters from about five miles away.”

Pixley said all of the carcasses were obtained from “nuisance control programs” along the Gulf coast. No birds were killed for the experiment.

Asked what had happened to most of the carcasses so far, Pixley said, “We have no idea at the moment. All the data is still being collected ... when they get waterlogged, they’ll just go down.”