One of the mysteries of the English language finally explained.
A North American warbler, Oporornis tolmiei (family Parulidae), which has a grey head, white eye-ring, and yellow underparts and breeds in south-western Canada and the western United States.
Mid 19th century; earliest use found in John James Audubon (1785–1851), naturalist and artist. From the genitive of the name of William MacGillivray, Scottish naturalist + warbler.
MacGillivray's warbler/mə(k)ˈɡɪlɪvreɪz ˌwɔːblə//mə(k)ˈɡɪlɪvriːz ˌwɔːblə/
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