The return of migrant birds from their wintering grounds in the tropics is one of the delights of America's spring, as anyone will testify whose heart has leapt in April or May at the first liquid song of the woodthrush, or the first black-and-orange flash of the Baltimore oriole. But in recent years concern has grown that migrant birds may be declining, perhaps because of deforestation at their winter quarters in the Caribbean and in Central and...