Yesterday evening around dusk I watched hundreds and hundreds of birds flying over my house. What were they?

That depends on what they looked like! They could have been blackbirds, grackles, cowbirds, crows, nighthawks, robins, or any other number of species that flock. At the end of summer, when birds finish breeding, many species become more social and join flocks. In the evening, hundreds of them may travel toward roosts and spend the night together.