The most common and most likely answer to this crossword clue is the 10 letter word OVERWINTER
I've seen this crossword clue in The Times
O
V
E
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W
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Buddy explains!
The straight part
We know that "some migratory birds do it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case some migratory birds do it = overwinter.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does OVERWINTER mean?
OVERWINTER verb- To keep or preserve for the winter.
- To spend the winter (in a particular place).
- Occurring over the winter season.
- During the winter.