Reference for Chullin 280:16

זכר על גבי ביצים ונקבה על גבי זכר מהו

Abaye said: Come and hear: IF AN UNCLEAN BIRD WAS SITTING ON THE EGGS OF A CLEAN BIRD, OR A CLEAN BIRD ON THE EGGS OF AN UNCLEAN BIRD, ONE IS NOT BOUND TO LET IT GO.

Jastrow