What is the name of the nocturnal bird which is a cross between a cuckoo and a kestrel that according to legend steals milk from goats?

nightjar

The family Caprimulgidae, a.k.a Nightjars, which include the whip-poor-will, Chuck-Will's -Widow, and nighthawk. Called Goatsuckers erroneously, they do not suck milk from goats, nor are they hybrid species between any other birds.