yup that is normal JRT behaviour, harly is doing what he was bred to do and is doing a fine job at it. you could use his instincts to your advantage and do somthing like lure coursing or JRT racing *the lil dogs love it*
try not to worry too much, but you could always keep a close eye on harley when he is outside "hunting"

my rotti X has a fairly high prey drive and will chase rabbits and hares at the park, she has only caught and killed once *i was amazed* the rabbit was sick so clover was realy doing it a favour *it would have had a long painful death*

but then again clover was playing with a pomerainian at the park tonight *i was a lil worried* but she was fine and had fun running around playing chasey.

our chihuahua also loves to hunt bird and he can get pretty close before they fly off then he sprints off after them and gives a good chase lol *very cute to watch*