Reference for Chullin 280:51

היתה יושבת ביניהן פטור מלשלח על גביהן חייב לשלח

For it was taught: If she was sitting among them, one is not bound to let her go, if upon them, one is bound to let her go; if she was hovering over the nest, even though her wings touch the nest, one is not bound to let her go.

Rashi on Deuteronomy

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