aggression to smaller animals

[gmchamp]

I need some advice. I have a female boxer/bull 2 yrs old. She has been to obedient training and has the basics down. But over the past month she has shown aggression to smaller animals. She has killed several birds and squirrels and wounded a cat. I have not seen her do any of this but she brings them home and lays them on the side walk. We have a smaller jack russell whom she gets along fine with. She is great with the kids and evens sleeps in their room. While we are at work she roams outside and stays in the house at night. Is she just playting too rough or is she meaning to injury theses animals? The animals do not shown signs of great injury when I find them. What can I do to stop this behavior? What has caused this change in behavior? Please help!!!