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As several others mentioned, this is generally very typical of dogs- they just chase and kill.. and lose interest once it dies. This is why dog attacks can be so devastating- high movitation to chase and "kill" a flapping noisy thing..but once it's quiet, the other screeching birds catch their immediate interest, repeat the process.. chickens are killed pretty easily so that often results in a dog killing 10-20 birds in a single uninterrupted event very easily. In fact one of the surest signs of a dog attack is if the yard looks like there was a pillow fight with bodies completely random all over.
It is also because dogs sort of have incomplete predation instincts, one theory is that dogs are essentially "mature" wolf cubs. The same playfulness, adult wolves rarely bark(but cubs can and do bark), etc. I read about an experiment where they exposed a border collie, a LGD breed(think it was great pyr) and a purebred wolf to a live chicken for their very first time. The collie "eyed" and chased/stalked the chicken for a very long time(not sure if they eventually killed the chicken or not before the test was ended) the LGD puppies just either stared at them or tried to play with them like they would another puppy or toy.. but got bored quick.. the wolf was a captive wolf, never hunted or anything before but it immediately set upon the chicken and killed it instantly and started eating it- something none of the dogs did- the eating part(IIRC).
Think about it.. so many dog toys are animated and make noises. And people "shake" the toy to get a dog's interest.. this is pretty much the same thing as flapping/struggling of an prey animal.. And dogs often bite and shake the toy, this is the 'killing' instinct. So a flock of flapping screaming chickens is like a yard full of Very Exciting Toys- powerful stimulus.
It is very typical of dogs that have chased/attacked/killed to become "addicted" to coming back. Chances are pretty high the dog will be back, and possibly try hard to get in the chicken area. I have personally experienced this and heard from many other victims of dog attacks.. dogs that try their darnedest such as climbing fences, digging under, or repeatedly going around the property looking for a way in.
Unfortunately it is also common for a free roaming dog to have owners who either, don't really care.. or make a big show of making how sorry they are, they will try their best to contain the dog.. however it is loose again after a month or two..
Sorry about your loss, it is a sore subject for many(including me) after horrible experiences with both dogs and their owners.. I refused to get chickens at my new place until the 6 foot high chain link went all around the perimeter- and the main reason was to keep dogs OUT.