We had a Brittany Spaniel in the family. If he was running free, he was looking for feathers or fur. Feathers were feathers - he didn’t distinguish between pheasants or chickens & he did kill chickens. The worst massacre was 5 chickens in one spree ... my SIL’s elderly dad let him out, forgetting he’d let the chickens out earlier.
He was a very good hunting dog and a nice family dog so the chicken killing was forgiven & we had to be careful who was out when & supervise his daily runs in the field so he didn’t visit the neighbor’s chickens.
Yeah, seems only natural. Dogs are bred to be almost hard-wired with certain instinctive behaviors, and they don’t understand our rules around those behaviors without proper, patient and thorough training. And even then, don’t tempt a birding dog with some tasty chickens ;)