When we were on the coast, our feeder was practically owned by a male
Anna's Hummingbird. The first season we saw other hummers (Rufous
and Allen's), but they were migrants, and the Anna's stayed year
round. By the next spring, he had firmly conquered the territory, to
the point of even chasing off female Anna's. He would even hover
over bees, although he wouldn't come closer than a foot or so.

One day we were out in the yard when a Selasphorus managed to get to
the feeder. The Anna's dove in, and a vicious fight followed. They
went round and round the yard, diving at each other and occasionally
locking feet. At one point, they locked feet and fell to the ground
with the Anna's on top, not 5 feet from us. There was a brief pause,
then the Anna's pulled its head back and brought it down with that
bill pointed straight at the other bird's throat. I flinched, but
when I opened my eyes as I heard the Anna's fly away. Neither bird
could be seen, and Steve said he wasn't really sure what happened
either, it happened too fast, but one way or the other, they both
made it out of the yard.

So, yes, I would have to say that Anna's Hummingbirds (at least the
males) are very aggressive at defending feeders.
jw
Jane Westervelt
Steve Kalloger
Moscow, ID