Sglass: You took this dog in? So, it's an adult? If so, you have a lot of intense training to do. And....to be honest..if the dog is over two years old, there's not a lot a hope the animal can be re-trained.
First, you are going to have to make sure she thinks you are the one in control. Put her on a leash and walk her around and around the neighborhood. ANY time she shows interest in something you'd not be in favor of (another dog, stopping at a bush to sniff something, anything.) tug on the leash (just a little snap) and then keep walking.
You, you must be the Alpha dog, If this dog thinks she's the one leading, you are in big trouble. Make sure every time she walks with you and behaves herself for even a few feet, that you say what a good dog she is. Praise and snaps on the lead, praise, praise, praise, snap on the lead.
Once you get her to follow your lead around the neighborhood (It could take weeks of twice a day walks) walk her by the chickens.
Put the dog on the leash and walk her by the chickens. Every time she shows any interest in any chicken, tug on the leash and say "NO" then get moving again watching the dog all the time. She'll alert on those birds until she gets the idea that the Alpha doesn't like it.
I had a girl boxer named Sophie, just like you I got her when she was older. If the back gate was open just a crack, she liked to get out and run (Mostly to the garbage bins behind the grocery store)
First, I walked her around and around the yard, then I walked her with the gate a little open, finally, I walked her with the gate open.
Then......this was after six months, I let her out in the yard. She stayed in. This yard had become her home.
Fencing her in or tying her up won't help. You'll have to keep her in or on a leash until doesn't care about chickens. You have to train her out of the behavior. You have a lot of work ahead of you or you are going to a) buy chickens and roosters until you get fed up or b) your neighbor shoots her ---which he has a right to do in most of these United States, I forget where you are.
Joe(How loud are those roosters in the morning?)Nation