Gulp down bird in flight (7,2)

I believe the answer is:

swallow up

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'flight' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how they can define each other.

'gulp down bird' is the wordplay.
'gulp' becomes 'swallow' (I've seen this before).
'down bird' becomes 'up' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'swallow'+'up'='SWALLOW UP'

'in' acts as a link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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