The mystery of seeds' speedy passage.

Birds and fruit-bearing trees work well together. Birds derive nutrients and energy from the fruit, which they eat seeds and all, and the trees benefit from having their seeds, which pass intact through the animals' digestive sys- tems, dispersed widely.

Some researchers have suggested that the plants manipulate birds by lacing the fruit with laxatives. This strategy would make seeds move quickly through the bird's digestive juices, thus avoiding damage, K. Greg Murray and his col- leagues at Hope College in Holland, Mich., reported in 1994.

In their experiments, Murray and his colleagues fed tropical thrush a meal of seeds from a Costa Rican shrub, Witheringia solanacea, mixed into bird food.

When the scientists added juice from the shrub's fruit to the mixture, the birds passed the seeds twice as quickly.

A new study disputes that finding.

The juice's concentration of sugar-not any laxative chemicals-determines the seeds' speed through the digestive tract, suggests biologist Mark C. Witmer of the University of Wyoming in Laramie in the September Ecology.

Sugar, an important nutrient for birds, accounted for 5 percent of W. solanacea juice and 13 percent of the bird food. So the feed with the juice had a lower sugar concentration than the feed without it, says Witmer.

Decreasing the sugar content of other birds' diets consistently speeds up the rate at which they pass food, Witmer's experiments show. Like most other species, birds digest low-nutrient diets faster than more nourishing fare.

Murray responds that seeds move more quickly through birds only when they receive a steady diet of the low-sugar food. In recent experiments, he found no difference in passage time among birds fed single meals of varying sugar concentrations.