It’s just that they’re two distinct subspecies of hummingbird with overlapping ranges. It’s not just a colour variation that they made up, the comic accurately portrays the two different types. Which is a nice touch.

Rufous hummingbirds are particularly aggressive and show up in the spring. Anna’s hummingbirds tend to stay (here) through the winter and rely on feeders.

The green one is an Anna’s, and the brown one is a Rufous. So I think part of the joke is that the brown one is being a dick. :)