L4H has it right. Kittens learn that a special meow from momcat is a "look what I've caught" for you if she's caught a mouse or something. Over the years I have had outdoor cats that would always "call me" to show what they caught....mice, voles, birds, snakes, grasshoppers and one time a weasel. It seems to be instinctive with cats as male cats will do it too. My indoor cats would do the same with socks, etc. I like to think of it as a way they show their love, by bringing you a gift.
