This is actually a common question. My belief is that at this time of the year, when seed crops are ripe, the weather remains warm, and the young have fledged and are on their own, that there is plenty of food out there away from the feeders. I've noticed this at my feeders, particularly with the bluebirds, who have now congregated in a flock that roams the neighborhood. When the weather turns colder, they return to the feeder where they can get sunflower chips without the effort to hunt down insects.
When the weather gets colder, they should return.
--John McKitterick
Columbia, MD