But a Brigham Young University biologist still thinks giving migrating sparrows and warblers breathalyzer tests isn't a bad idea - though he isn't checking for blood-alcohol content.

Ornithologist Kent Hatch wants to know whether the birds are migrating under the influence of bayberries, gnats, or other foods. It's work he hopes ultimately will lead to efforts that would protect those habitats, which may have been altered or eliminated by urbanization.

''We want to see what's good habitat and what isn't,'' said Hatch, who teamed up with two University of Rhode Island researchers to sample birds on Block Island, off the state's southern coast.

The ornithologists adapted a technique previously used to study where migrating birds have been, but not necessarily what they've been eating.

Hatch and URI's David Podlesak and Scott McWilliams fitted a small face mask over the birds' beaks to capture exhaled carbon dioxide. Then they applied a sophisticated technique called isotope ratio mass-spectroscopy, a mouthful of a method that measures the different kinds of carbon atoms in bird breath for clues about what the birds ate earlier in the day.

That knowledge could help ecologists ensure the availability of appropriate stopover habitats, where songbirds can regain energy and protein exhausted during their migrations, Hatch said.

The scientists' new study appears in the next issue of Oecologia, a leading ecological journal published by Springer-Verlag.

''We still have very limited knowledge,'' Hatch said. ''But we're at the cutting edge of the learning curve.''

The researchers found that three of the species under observation maintained a consistent diet during their migration, but white-throated sparrows have broadened their palate to include a new component - corn from bird feeders. That raises questions about the importance of feeders to migrating birds, he said.

Hatch said his method, which also involves gathering feathers, blood and feces before the birds are released, is ''minimally invasive.'' It also may be more reliable than blood sampling in determining what types of carbohydrates migrating birds nibble on for fuel between long flights.

Hatch is now adapting his method to analyze the breath of bears and snakes.