stonechat vs Old World chat

stonechat and Old World chat both are nouns.

Word	Noun	Adjective	Verb	Adverb
stonechat	Yes	No	No	No
Old World chat	Yes	No	No	No

As nouns, Old World chat is a hypernym of stonechat; that is, Old World chat is a word with a broader meaning than stonechat:

stonechat: common European chat with black plumage and a reddish-brown breast
Old World chat: songbirds having a chattering call

Other hypernyms of stonechat include chat.

stonechat (noun)	Old World chat (noun)
common European chat with black plumage and a reddish-brown breast	songbirds having a chattering call