Virtual skylarks suffer weed shortfall.

A mathematical simulation suggests that the proliferation of crops genetically modified to resist herbicides might mean the downfall of some seed-eating birds.

That's one scenario to come out of a new farm-field model developed by Andrew R. Watkinson and his colleagues at University of East Anglia in England. In the Sept. 1 SCIENCE, they describe what could happen to a common farmland weed (Chenopodium album)--and to the skylarks that eat its seeds--as farms switch from conventional sugar beets to genetically engineered varieties, such as Roundup Ready Beets produced by Monsanto in St. Louis.

A farmer changing to the herbicide-tolerant beets can use highly potent weed killers and reduce such treatments to one or two per season. In some field trials with these beets, the herbicide has almost wiped out the weeds. Researchers worry that because skylarks feed on weed seeds, such aggressive weed killing could starve the birds.

In the model, the consequences of switching to the genetically modified beets depend on several factors, most notably the farm's weed history and a farmer's willingness to adopt new technology. Like real farms, the researchers' virtual farms rotate crops, growing beets one season out of five.

On one hand, farmers with fields overrun by weeds might be especially willing to adopt the new beets so they can use strong herbicides. In this case, the model calculates severe consequences for birds. Alternatively, farmers who already control weeds well might adopt new technologies most readily and be the ones more inclined to plant a genetically modified crop. Here, the switch wouldn't make much difference to the birds.

The model provides "an elegant way" to use current field data, Les G. Firbank at the Centre for Ecology and Hydrology in Grange-over-Sands, England, and Frank Forcella at the University of Minnesota at Minneapolis St. Paul comment in SCIENCE.

Despite the model's virtues, Firbank and Forcella question the conclusion that switching to the new beets could seriously reduce bird populations. They argue that the herbicide-tolerant crops adopted in the United States in the past 5 years don't seem to be reducing the weediness of farms and so aren't significantly depriving seed-eating birds of food.

In a statement, Monsanto spokeswoman Scarlett Foster emphasizes that the SCIENCE paper represents only a simulation and "does not reflect real farming conditions."