During nesting season hummingbirds never intentionally nest, eat, etc. in the same location but sometimes there is a slight overlap in territories. The average hummingbirds usually defend about a ¼ acre, but guarding the borders constantly can expend a lot of energy.
In order to conserve their strength, sometimes they narrow the defense down to a single feeder or flowering bush and allow other hummers to feed nearby. You might also witness air fights between hummingbirds or any birds that get in the way of their meal.
Hummingbirds can starve to death in a matter of hours, so they can be very protective of their food sources. You’ll see hummingbirds feeding actively during the day because they have a very fast metabolism. The nectar they consume at your feeder can be processed through their digestive system in as little as 20 minutes.
You may also be seeing an increase of hummingbirds at this time of year because the babies, which all look similar to females, are starting to visit feeders. From July to late fall, territorial claims can be overlooked, and competition at feeders increases greatly.
Migration is another reason to see different species. San Diego, CA has two hummingbird species year round, Anna's Hummingbirds and Costa'sHummingbirds and several kinds of hummingbirds that are migratory. According to the Calsbad, CA Facebook page: "The Black-Chinned Hummingbirds are in San Diego all summer, however, they head south in September along with the other 3 hummingbirds that you only see at your feeders during migration time early to mid-September. These are: Allen's Hummingbirds, Calliope Hummingbirds and Rufous Hummingbirds."
- Black-chinned Hummingbird photo from: http://en.wikipedia.org/wiki/File:Archilochus-alexandri-003.jpg
- More about hummingbirds online: http://www.allaboutbirds.org/guide/browse_tax.aspx?family=62
- Amazing Hummingbirds by Stan Tekiela (available at Wild Birds Unlimited)