One of the mysteries of the English language finally explained.
1historical Any of various birds of prey with bluish-grey plumage; specifically the sparrowhawk (Accipiter nisus), the merlin (Falco columbarius), the peregrine (F. peregrinus), and the male hen harrier (Circus cyaneus). Now historical.
2US rare US. The male hen (or northern) harrier (Circus cyaneus) and the goshawk (Accipiter gentilis). Now rare.
Mid 18th century; earliest use found in The Monthly Review. From blue + hawk.
blue hawk/ˈbluː hɔːk/
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