“I foresee some sort of crash,” Dr. van Gils said.

The researchers got their first clue that something was amiss when they studied a subspecies of red knots that winters on the coast of Mauritania in West Africa. Typically, red knots dig into the sand to harvest clams with their long bills. The scientists were intrigued to discover that some of the birds were digging up sea grass roots and eating them instead.

“That puzzled me,” Dr. van Gils said. “So I wondered, ‘Why is this?’”

It turns out that the clams are buried deeper in the sand than the sea grass roots are. Small red knots with short bills could not reach the clams, so they had to make do with the less nutritious sea grass roots. “These smaller birds go for something they don’t like so much,” Dr. van Gils said.

To understand why the birds grew to different sizes, the team began a study of the red knots’ full migratory path. In June, the birds leave Mauritania and fly to the Arctic coast of Russia. At the end of July, they make a return trip. The adults stop off to refuel in the Netherlands, while the juvenile red knots travel through Poland.

The scientists found a disturbing trend: Over the past 30 years, the juveniles arriving in Poland have been shrinking in size. On average, they are about 15 percent smaller today than in 1985.