Blame winter for the vanishing sparrows. (Zoology).

Changes in winter farming practices may help explain a puzzling drop in number of rural house sparrows in southern England, says a University of Oxford research team.

The birds once flocked around farms in such numbers that "sparrow clubs" could earn bounties for delivering heads, explains David G. Hole of Oxford. In recent decades, however, sparrow populations have plunged in enough locales to spark sparrow-sawing crusades.

Theories abound to explain the declines, but Hole and his colleagues report in the Aug. 29 Nature that their experiments point to scarce winter food as a primary cause.

The researchers monitored nests on an Oxfordshire farm that had lost 80 percent of its sparrows since the 1980s. Sparrow pairs still raise the same number of fledglings as they did 2 decades ago. The problem, however, seems to be a low survival rate for sparrows during the winter, the scientists found.

Hole and his colleagues set out extra bird food during the winter at that farm and three others whose sparrow populations were still plentiful. The extra feeding made a difference in survival rates only at the first farm, suggesting that scarce winter food indeed was keeping populations low.

Genetic analysis shows why local disappearances matter. The data reveal that house sparrows stay unusually close to home, says Hole. So if a farm loses its sparrows, recolonization isn't likely.

Hole says it's hard to say exactly what's pinching winter bird food. However, he notes that since World War II, farmers have been largely planting grain in the fall instead of leaving fields as seedy, bird-friendly stubble through the winter. Modern weed control and grain storage don't leave a lot of bird food around either.--S.M.