Yellow-billed Cuckoo in Delaware.

Yellow-billed Cuckoos (Coccyzus americanus) are slender, long-tailed birds that manage to stay well hidden in deciduous woodlands. They usually sit stock still, even hunching their shoulders to conceal their crisp white underparts, as they hunt for large caterpillars. Bold white spots on the tail’s underside are often the most visible feature on a shaded perch. Fortunately, their drawn-out, knocking call is very distinctive. Yellow-billed Cuckoos are fairly common in the East but have become rare in the West in the last half-century. [All About Birds]