The rooster makes the same buk-buk-buk call to his hens that Mama hens make to call their chicks over to find something good to eat. I know it sounds really sweet when the roo does it, and seems really unselfish and altruistic on his part. But it really must serve a more practical purpose for him, it must keep the hens hanging around him so he can more handily mate with them & have a better chance to pass his genes on to future generations. The hens do it for their chicks for the same ultimate reason, so the chicks carrying their genes will survive.