For decades, a number of birds have been expanding their ranges northward.  Some birds appear to have been aided by bird feeders.  One such bird is the Carolina wren.  A study conducted near Ann Arbor, Michigan seems to corroborate this.

       Here biologists compared the density and survival of Carolina wrens inhabiting residential neighborhoods and parks where they had access to feeders to wren populations living in rural areas devoid of feeders.  Over the course of the winter the wren populations in both study areas dropped, however by the time spring arrived, the survival rates of the birds that had access to feeders was higher than those inhabiting rural sites where no feeders were present.