Chullin 138 - Sending Away the Mother Bird
If one buys a fleece of a flock, then if he left any fleece with the seller, the seller is obligated to give the first fleece, and if he bought it all - he is obligated. Why should it be, it seems to make no difference if he left any or not!? Really, the obligation of the first fleece is always with the original owner. If he is left with some fleece, it is clear that he kept it so that he can give it to a kohen. If he sold all of it, we assume that he simply means that the buyer will give the kohen’s portion, acting as his agent.
If one chances upon a bird’s nest, with a mother bird sitting on the eggs or on the young, he should not take the mother with the young, but he should send her away . The somewhat similar commandment of covering the blood is stricter: it applies to wild animals and birds, whether the birds are prepared or not, while sending away the mother bird applies only to birds, and only if they are "not prepared," but come your way by chance, not like chicken that nested in one's house.
Art: John Anster Fitzgerald - Fairies In A Bird's Nest