My yard is full of robins - a flock of them. Yes, a flock of American robins (Turdus migratorius). Their orange feathers show brightly on a gray wiinter day here. Robins don't migrate around here - well, they might move to a different location, but it is not a true migration. In the fall these birds join up in groups that are sometimes fairly large - safety in numbers, I suppose. They spend the winter hanging out, keeping warm and looking for food together. In the spring they'll pair off and start nesting.
That thing about the first robin of spring? It's a holdover from Europe, where the robins they have there are a different species (Erithacus rubecula) that does migrate to warmer places for the winter. To travellers far from a European home, their songs (Er vs. Ar) might seem vaguely similar, and they both have orange breast feathers. The American robin, however, is much larger than the European. Adding to the confusion, the European robin was once considered to be in the thrush family, but has now been re-categorized as an old-world flycatcher.
Nobody ever said classification was easy, and to my mind it is somewhat unnecessary. It might make the world easier to understand - a place for everything and everything in its place - but it doesn't always make sense; there are too many variables. And I highly doubt that any of the critters care what names we give them, as long as we give them space and respect.