Do bird feeders help or harm bird populations?

both.  it's very situational.  they help keep birds alive when food is in short supply, but they help to spread disease.  They are like bait piles hunters use for deer in that predators know where to find their next meal on a regular basis.  So they can help out the cat population, and falcons, hawks, and etc.

Birds do fight with other species of birds, and large birds will attack and kill small birds to keep them from the food. good for the big birds, not so good for the little ones.

Feeding may not always bring a positive outcome for birds. In some cases, attractive feeders can become ecological traps, encouraging birds to settle in an area that cannot support them once supplemental feeding has stopped. Because of this, feeders create a population level that cannot be sustained by natural levels of food. There are also times when feeding can affect the timing of a bird's life in unexpected ways.