Its spring over there, its seeing its reflection and thinking its another bird, so its defending its territory.
Over here, there is a crazy magpie lark that lives at the train station. Each time a train pulls in, if flys up and down the 6 carriages, eyeballing the pesky intruder it sees in the windows, than sits on the fence to wait 20 minutes for the next one.
You see it thinks hes scaring off the intruding birds as the train leaves every time. These birds often attack the side mirrors of cars waiting at the lights for the same reason.
Dogs do the same thing with the postman. AHA says the dog, I run and bark and he leaves, Im a brave doggie and my strategy works, hes scared of me, I win and when he returns tomorrow, Ill win again!