My dog killed a rabbit

Question
I own a 3 year old Border Collie / Australian Cattle Dog Mix.  The other day I had her in the front yard with me and she was not on a leash.  She never wanders far and we live on a rural road. I heard her bark a few times and then saw her dash off into the bushes.  Then I heard some awful squeaking and saw her trot back out of the bushes with a baby rabbit in her mouth.  I screamed "NO" several times and she put the rabbit down as I ran over there, but I was too late.  I am not sure where to go from here.  She has chased rabbits before, but the adults always get away so I wasn't too worried about it.  When she dashes off into the bushes it seems like nothing I do will call her back immediately, it always takes a few tries before she comes back.  The bushes are too thick for me to go after her so I have to wait.  I know there are other rabbits living in the area and I would hate to think of this happening again.  Is it too late to try and fix this or have I already created a problem by not correcting her the first time she chased a rabbit?  Also, she has a stuffed toy duck that squeaks, could this have influenced her behavior?

Answer
She is doing exactly what her breed is supposed to do - work.  Collies and Cattle Dogs are working breeds and chasing rabbits is her way of exerting the energy she has from not having an official job at home.  I would try to make sure that she is encouraged to do something around the house, similar to a job.  Herding etc.  If you can't teach her some tasks to do, then you may want to think about something like agility or fly ball.  This doesn't mean she won't chase rabbits anymore, but she should be able to come to you on command the first time you ask - this is a difficult task to train and you may need an obedience teacher to help (Please find one that work with positive reinforcement like a clicker).  Also be very vigilant with her off leash when you know baby rabbits are around.