A bird of either sex (5)

I believe the answer is:

robin

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'a bird' is the definition.
(robin is a kind of bird)

I don't understand the remainder of the clue.

(Other definitions for robin that I've seen before include "Christmas songbird" , "Batman's companion" , "Seasonal bird" , "Small songbird" , "Small brown thrush" .)

I've seen this clue in the Evening Standard.

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