The birdfeeder on our deck is popular with chickadees and juncos, and early this spring we began noticing an interesting behavior with the juncos. We set an old, cracked mirror out on the deck and some of the juncos find it fascinating. We never see more than one bird at a time approach the mirror, but a bird will hop around at the base and peck at his reflection, sometimes fluttering against the glass. It's aggressive, but it seems unlikely the bird will hurt himself.
I had a pet parakeet as a kid, and he had a mirror in his cage. He viewed his reflection as more of a companion, and would tap his beak against it, preen his reflection, and sometimes sing to it. That's pretty typical - most animals don't understand that a reflection represents them. A few can pass what's known as the mirror test - Killer whales, bottlenose dolphins, some chimps and orangutans. Among birds, only magpies are known to recognize themselves in a mirror. Biologist put small colored spots on magpies - marks that they could not feel - and when they glimpsed their reflection, they scratched at the mark.
The little junco sees the reflection as another bird. Juncos have a social hierarchy, and it's likely that a dominant male is feeling challenged by his reflection.