Bald eagle numbers are down at Thurmond Lake, according to an annual survey conducted last week in which only six birds were spotted.

However, dense fog the morning of the survey and low water levels that made boat travel difficult make it likely that some birds were uncounted.

"Conditions weren't ideal," Army Corps of Engineers conservation biologist Ken Boyd said of Thursday's study. "But we already had the survey set up and had a number of people there, so we went ahead with it."

Once the fog lifted at midday, Thurmond Lake spotters recorded -- in addition to six bald eagles -- about 5,000 waterfowl. Of those, 4,100 were coots and the rest included Canada geese, mallards and other species.

By comparison, 15 eagles were spotted in the January 2007 survey, and 14 birds were seen in 2006. In 2005, five eagles were seen.

The study is conducted in many locations each January and is done within a seven-day window to improve accuracy and to prevent migrating birds from being counted more than once.

Two eagle nests believed to be occupied by adult birds also were seen last week, Mr. Boyd said.

Corps employees and others who work along the lake have reported numerous eagle sightings recently, bolstering the likelihood that many birds eluded spotters.

"We've seen more birds than that (six) in the past month-and-a-half, including lots of juveniles and adults," he said. "So we're fairly sure there are more birds than we counted."

Although eagle populations are generally increasing in Georgia, Thurmond Lake is one of a handful of places in the country where a mysterious disorder has killed a number of birds.

The condition, called AVM, or avian vacuolar myelinopathy, is believed to be spread from algae on hydrilla to coots that are eaten by bald eagles.

Since 1995, at least 44 bald eagle deaths have been documented at Thurmond Lake, with the possibility there could be many times that number that were not found by wildlife authorities.

Vic VanSant, a regional biologist with Georgia's Wildlife Resources Division, said one dead eagle had been recovered at Thurmond Lake this season.

It was reported in the Mistletoe State Park area on Christmas Day and was found by corps employees a few days later, he said.

Thurmond Lake is a traditional wintering ground for bald eagles, which nest in late fall through midwinter and rear their young in early spring.