When we bought this place there wasn't a bird to be heard. Now it sounds like a giant mosquito ranch in the backyard. Near as we can figure the people before us let the feeder go empty too long and the birds just gave up. Of course the cat isn't even interested in them anymore which also helps.
The male birds seem to be very aggressive/possessive and would prevent the other smaller birds from feeding. One bird seemed to be ruling the roost. With about 3 others he was constantly chasing off, I decided we should add another feeder. My thought here was that it's difficult for him to cover both spots at once.
Before that though I noticed a couple of them would fly in to get chased off. Then another two would come in and eat while the first two were keeping the big guy busy. A great tactical move I must admit.
Not sure exactly why but we have 8 or 9 regulars now that go through at least a feeder-full of stuff a day. Must not be enough nectar producing flowers yet to support thier voracious appetites. Either that or thier sources have dried up.
Not many people know this, but if you stand still enough and put your finger in front of one of the feeding holes, a humming bird will land on your finger to eat.