Reference for Chullin 280:52

היתה מעופפת אפי' כנפיה נוגעות בקן פטור מלשלח

Now presumably the expression 'upon them' bears the same meaning as 'among them', and just as 'among them' clearly means that she is not touching them from above so 'upon them' also means that she is not touching them from above, and that must be the case where she was upon the branches of a tree! - No, the expression 'upon them' bears the same meaning as 'among them', and just as 'among them' means that she is actually touching them so 'upon them' also means that she is actually touching them, but if she was perched upon the branches of a tree one would not be bound [to let her go].

Rashi on Deuteronomy

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