Any size of dog can kill chickens too. Lots of small dogs are VERY high in prey drive!!!! JRT anyone?? (That's Jack Russell Terrier for the non-dog people)
I had a JRx and he was the loveliest dog. I had cats, mice, later rats, parrots and then chickens. He never touched them. He would catch and kill mice and rats, but never touched mine.
When we got the chickens, I didn't even think about 'what if Rex'... in fact, he was afraid of them. If they came to close to him for his comfort, he would jump up and move away from them. Even when the little chirps came, he would rather go into a shivering attack than look at them. If I could have another dog like him, i would any day. I miss him dearly.
When Rex passed away, I got a FT pup. He played with the chicks, but quickly learned that chickens can hurt, so he also kept his distance. Sadly, he got poisoned and also passed away, within 3 months after Rex.
Then I got another JRT (almost grown, but still young. A rescue dog)... within a few weeks, he ripped apart my newest addition to the flock. And after that, we kept on losing chickens and chicks. Regretfully, I had to return him. He was one of those dogs that just waited for you to turn your back. Not everyone can understand the choice I had to make.
I have another fox terrier, Bullet (the name says it all...). He killed our fist rooster, sorry, let me say that correctly... he shredded my poor rooster, then attacked one of the hens. Luckily the workers caught him in the act, before he could kill her. My husband went berserk on him, and we had some self control testers for him. He never tried to kill another chicken. Although he cannot leave the chicks alone. I had to put together a little department for the chicks to be in for their first 10 weeks. Bullet just doesn't know the difference between the squeaky sound chicks make and those mice and rats make.
4 different terriers. 4 different personalities. 4 different experiences.