I have been feeding birds for years, they adapt, the birds have survived without us for a long time. If you watch them long enough, you'll notice some birds stay throughout the year, some migrate to warmer climates. There have been times when I ran out of seed for a week or so, which, if they were so dependent, would leave me with piles of dead starving birds all over the yard and clinging to empty feeders. It doesn't happen, they get food in the woods, they eat wild seeds, they scavenge, whatever, they do what they need to survive. Feeding them of course makes it easier on them to find food, and helps build up a little fat to ensure they'll make it through some of the colder weather.