There are now thousands and thousands of Robins leaving Toronto. They began over a week ago. Along the bluffs every morning until about 10am,we see huge numbers of Robins,Redwinged Blackbirds and Cowbirds as well as Cedar Waxwings and Chickadees. We are seeing the last of the overnight migration..songbirds fly all night long,while the raptors that eat them are sleeping.
So far as why some birds stay and some go? Its all about food supply. Temperature may effect food supply,but if there was food alot more would stay. There arent the berries,cones,seeds,bugs,small rodents,small birds left for for various birds to feed on their food of choice,so they migrate.