Bird cage (3)

I believe the answer is:

pen

I believe this is a double definition.

'bird' is the first definition.
(pen is a kind of bird)

'cage' is the second definition.
(both can mean an enclosure)

(Other definitions for pen that I've seen before include "Animal enclosure" , "Enclosure; a swan" , "International Association of poets, playwrights etc" , "Writing instrument" , "record" .)

I've seen this clue in The Times.

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