Hummingbirds are the smallest birds on the planet with the biggest and baddest attitude, and if you don't see them very often, you may think they are a sweet and gentle bird. Just not the case. They are born with a mean streak that makes Yosemite Sam look like a kind old gentleman. They disrespect their parents, beat up their sibling, and basically look for a fight wherever they can find one. Given their small stature, they makeup for it big time in attitude.
A big question "hummingbirders" often ask is why they fight more in certain areas and feed together in others? It's really quite simple. The further you go north, the smaller the population. In the northernmost regions it's highly unlikely that you'll see 100 hummingbirds in 1 yard. Territory is much bigger and the number of birds is much smaller. This gives each hummingbird a chance to dominate a region without much competition. As you go further south, the population dramatically increases, and as the northern birds migrate south it creates an even bigger population in those regions. They're still just as angry, but it's no longer just one or two birds that they need to chase away from their feeder. Now there's hundreds. Hummingbirds nearly always squeak out of anger, and when you see a number of them gathered, they continue to squeak as much as ever. They just can't afford, nor will they be able to chase 100 hummingbirds away from their feeder. They are still angry, and still as miserable as ever. Simply put, the higher the population the less fighting you'll see. In saying this, circumstances are much different in breeding season. They like to dominate a larger area during this time because it increases the likelihood of finding several mates, and as much as they fight over feeders, they will fight til the death over breeding rights.
Here is an angry battle over one of my feeders that occurred August 2016. It's a young male Ruby-throat that challenged and won territory over this feeder, however, it didn't stop other young birds from stealing nectar and trying to take over the territory themselves.
N.E. of Edmonton, AB.