Large congregation of birds?
So, we typically think about migrating birds as living somewhere (North) and then traveling South for the winter. This way of thinking is actually not accurate according to a number of ornithologists, including many I used to work with/for. Thy suggest that the birds live in the South and migrate North to breed and to avoid overcrowding of Southern locales. (Though see this article that found support for the more often assumed N-to-S direction of migration). If 'South' is in fact their 'home,' then perhaps they will venture there regardless of weather patterns. However, perhaps neither of these thoughts applies to short-range migrators like robins. 

Also, many birds migrate during the dark hours of the night, so I wonder if the birds you saw were even migrating, per se, or just moving about regionally.