Category talk:Gulls

Why do many of the species of gull that make their home on the St. Lawrence River (NY) fly solo, in long lines or in groups, upstream at the end of the day? This procession of birds usually starts sometime betweem 8:00 - 9:00 pm. every night. They don't appear to be hunting for food. They just fly upstream (east to west).

I was just wondering if there is a reason for the behavior? It does not seem to be random behavior, and it occurs every night to varying degrees!

Is there an explanation?