They're terriers!
Mine chases every bird that dares to land in her yard or on her fence. She learned a cute little thing watching my neighbor's bird dog one year. She'd come over here and dance all around a mugo pine and sit up and try to see if there were any birds in the mugo pine. Funny thing was that Kacee started sitting up trying to see in the mugo pine after that and if there were any birds the chase was on - for a little ways. Kacee is always on a leash outside her yard!
Tell me they don't imitate and learn from bigger dogs!
Another time a baby bird landed on the steps outside the patio doors and I had to go save it. Kacee was having a fit at the doors and I didn't dare let her out there until I got that bird outside of her yard. He wasn't very good at the flying thing yet!