Midrash Deuteronomy Raba, 6:7

Another explanation: What is the meaning of, 'You shall send away (shiluah) the mother bird'[before taking her eggs for food]? If you will fulfill this precept you will hasten thereby the coming of King Messiah, of whom Scripture uses the expression shiluah (‘sending free’), as it is said, “That send forth freely the feet of the ox and the ass” (Isa. 32:20). Another comment: R. Tanhuma said: If you fulfill this precept you will hasten the coming of Elijah the prophet, of blessed memory, of whom Scripture uses the expression, ‘shiluah,’ as it is said, “Behold, I will send (sholeah) you Elijah the prophet” (Malachi 3:23); and he will come and comfort you. Where is this [in Scripture]? For it is said, “And he shall turn the heart of the fathers to the children” (ib., verse 24). [Translation from Judaic Classics Library]