Last evening I noticed that this particular RUBY-THROATED HUMMINGBIRD has declared possession and property of the nectar feeder in our backyard. I think it is competing with only 2 or 3 other hummingbirds for the moment and will give chase to any that come near the feeder. They'll fight when it isn't necessary. Most hummingbird species are extremely territorial and try to chase off intruders, even if there is an abundant food supply for every hummingbird that could possibly come. A solution that might quell the fighting is to place a second nectar feeder, which I may eventually do depending on how long this hummingbird sticks around.
On the chase - the hummingbird is quicker than the lens! After watching this show of territorial behavior for a while I noticed that while the "piggy" hummingbird was busy chasing, a third hummer would come in and quickly take a sip of nectar. Not for long, though. The possessive hummingbird would come back, chase, and then return to perching atop the feeder hanger, darting its head around seemingly looking for the next contender.
Ruby-throated Hummingbird image © 2005 Michael Allen McDowell