Money saved, an inducement to a calling bird? (4,3)

I believe the answer is:

nest egg

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'money saved' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for nest egg that I've seen before include "Money set aside for retirement" , "Sum saved for the future" , "Rainy day savings" , "money put away" , "Money kept in reserve" .)

I've seen this clue in The Times.

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