Yellow-billed cuckoos are fairly common in the eastern half of the US (while steadily declining in the west), but they are rarely seen. Their underparts are stark white, but they tend to “hunch over” to stay hidden as much as possible. Their diet consists primarily of insects, but in particular caterpillars. Cornell Lab of Ornithology says that they have been known to eat as many as 100 tent caterpillars in one sitting!
We were blessed by this guy sailing right past us while we were warbler-watching this fall, or we would never have noticed him.