1. The natural characteristics of bird make the staying together happen, instead of human's effort.
2. Yes, it is true, but birdfeeder provide nutrients to the birds and they are more healthy and strong, the population is increasing comparing to the little decrease.
3. The signal for their migration is the shortening of sunlight instead of food. They will not stay because of food.

Birdfeeder’s negative impact
1. It encourage birds to stay together and transmit diseases. For example, the red eye disease among flinch.
2. Birds are easily be harmed by predators like cats, and they may bump on the windows.
3. It affects their migrating behaviors as they will not move to the warm places if there are enough food, and thus influence their mating productivity.