More than one bird rises up, it's said? (5)

I believe the answer is:

rheas

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'more than one bird' is the definition.
(rhea is a kind of bird)

I don't understand how the remainder of the clue works.

(Other definitions for rheas that I've seen before include "Ostrich-like birds" , "these birds won't fly" , "Flightless birds" .)

I've seen this clue in The Guardian.

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