Crossword Genius

Birds, half of them American (4)

Ross

I believe the answer is:

emus

'birds' is the definition.
(emu is a kind of bird)

'half of them american' is the wordplay.
'half of' means to take half the letters.
'american' becomes 'US' (United States).
'them' cut in half is 'em'.
'em'+'us'='EMUS'

(Other definitions for emus that I've seen before include "Muse about flightless Australian birds" , "Running birds" , "Fast, but not high-fliers" , "laboratory units" , "Large flightless birds of Australia" .)

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