Somehow, some dogs just know when and what to chase and what belongs to 'your pack'.
My rescue dog, a chocolate lab, chases everything it sees in the yard, woods, or field. It will even "chase' the outside cat we had before Cee Cee was adopted into the family. If only the silly fool of a cat would stop running! Some days she's purring and pawing the dog's side and sleeping spoon-style with her and the next she runs when the dog comes bounding out of the house.
Chickens!
Cee Cee seemed too excited by the chicks to be allowed near them. She was very excitable once the chickens were moved into a pen in the garden shed. By the time their coop was finished and I was able to let them out to free-range, I was worried that I'd have to keep a close rein on the dog.
I let the chickens out of their coop. The dog raced ahead of them and stopped with every appearance of herding them back to the coop. I called her to me, explained that the chickens were okay, and gave her some 'good girl' attention.
Now the only time she minds the chickens is when she hears a strange chicken-noise. Then she runs over just to check and make sure they are not in danger.
Training? Mostly just loving attention.
Love, Linn B (aka Smart Red) Gardening zone 5a - 4b in south-est, central-est Wisconsin