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Finches need to know if another finch is of the same species. If it is of the same species then it is a potential mate or a potential rival. If it is not of the same species then it is of little interest. So finches - who have similar body shapes - have distinctive plumage to indentify them. A grey-blue cap and reddish cheeks identify a chaffinch. A brown head and grey collar identify a hawfinch. A completely green head means it is a greenfinch. A black cap and red cheeks are those of a bullfinch. And a red face and forehead belong to a goldfinch. All this avoids pointless quarrels.