I think she is pretending it is her prey. Cats like to show off their prey, and give it to owners (ewwww!) as a gift on the step. Unless you don't notice it and they sneak in with it, then they'll put it by your feet... And she might be giving it to her kittens to show them what Mommy has for them.
Glad to hear that you are having her fixed, though. It is a lot easier to have them around when they are not yowling or fighting.