Birmingham —- Something is powerfully affecting the birds wintering in northern Alabama, increasing the numbers of many, bringing new species and causing others to dwindle.

Scientists don't know whether it's climate change, a recovery from the banned pesticide DDT or some mystery factor.

Since the 1960s, the numbers of the birds in the reservoirs and refuges in northern Alabama have almost flip-flopped. Almost every species' numbers are either climbing steeply or dwindling.

And other birds are appearing for the first time. Pelicans, terns and gulls by the thousands now winter north of Birmingham.

"When I was a boy growing up in Decatur, a gull, a pelican —- those were all seashore birds," said Keith Hudson, the state's nongame biologist for the northern half of Alabama.

Now, the former beach birds spend the winter in the reservoirs of the Tennessee River or at Decatur's Wheeler National Wildlife Refuge.

And they are joined by nearly every fish-eating bird or duck found in northern Alabama. Almost all are on the increase during the winter.

On the other end of the food chain, the numbers of tiny seed-eating sparrows also are soaring. The exception is the once-common house sparrow, which is in steep decline at Wheeler. Wildlife biologists don't know why.

Nor do they have any idea why robins have multiplied almost by 10. Or why an annual winter bird count at Wheeler finds Eastern bluebirds have increased from an average of seven in the 1970s to 282 in the past decade.

But they have some educated guesses: It's likely that recent warm winters have trained some migrating birds to stop in Alabama instead of flying on to Florida and the Gulf Coast, experts agree.

And there's no question left in birding circles that at least some water birds are rebounding after the United States' 1972 ban of DDT, which entered the food chain through fish.

Nationally, populations of birds known to be most affected by DDT are now fully recovered.

Both the bald eagle and the brown pelican have been removed from the Endangered Species List in the South after they were almost exterminated by DDT.

Birds eating out of reservoirs of the Tennessee River also could be responding to the aquatic plants that have settled in the lakes in recent decades, Hudson said.

That's a change from when the lakes were first dammed and were almost sterile, he said.

Some of the species have their own unique stories.

A flock of Sandhill cranes began appearing at Wheeler in the late 1990s, until it numbered about 500.

The tall red-and-gray cranes once migrated from the upper Midwest to Florida. As the winters warmed in the South, some of the flock began dropping out at a refuge near Chattanooga, The Tennessee flock now numbers more than 10,000, and the overflow crowd moved to Wheeler, said Paul Kittle, chairman of biology at the University of North Alabama.

The white pelican is another bird never found at Wheeler until recently. Now, the National Audubon Society Christmas Bird Count has documented, on average, 98 American white pelicans at the refuge.

They are the freshwater cousin of the brown pelican and are also known to be sensitive to DDT. And they are thriving in their northern, summertime homes, Hudson said.

Warm winters in neighboring states are keeping some birds away from Alabama. Last year, few bald eagles migrated to their traditional spots at Lake Guntersville, and large numbers of eagles were spotted fishing in lakes that had not iced over in Kentucky or Tennessee.

"When you have sudden change in the climate, ... then there's less time for organisms to evolve," Duncan said. "Some individuals are going to suffer." But birds are extremely resilient, and migrants are already accustomed to huge moves, he said.