Robins come to mind in winter on Bull Creek where they suddenly appear in large flocks. They tend to move from tree to tree in the morning, coming in one at a time until there are thirty plus birds, then moving on, following the crowd with no apparent plan. Later in the day when it warms up they will congregate in the fields, looking for any cold hearty insect foolish enough to come to the surface. In the evening, they congregate in large flocks, especially in our cedar forest where they can escape the wind.
The American Robin (Turdus-migratorius) has probably made accommodation to humans equaling the European Starling. So far they have become successful suburbanites, hang out on golf courses without damaging them, and even had the song Rockin Robin written about them.
They received their name from early European settlers who were reminded of a unrelated smaller European "robin". Actually, it would be better named the American Thrush, but what sane ornithologist would be willing to face the public ire by renaming it after what happened when astronomers downgraded Pluto?
Found throughout North America, they tend to migrate south from Canada to the lower US in winter. Their migration is thought to be driven more by the availability of food than by the weather. We tend to think of robins pulling at worms in our yard, but in winter they become voracious berry eaters as well. Their esophagus is elastic, allowing them to ingest a large amount of berries in a setting, and their intestine is efficient in digesting the waxy coat of winter berries. They consume large amounts of Eastern Red Cedar berries and are implicated in the spread of cedar trees in abandoned fields and fence lines. In the spring, they are occasionally found drunk on the fermented fruit of bushes such as Pyracantha and Deciduous Holly. After surviving the hard winter, who can fault them for going on a little "Spring Break".
Answers to a lot of common questions about robins can be found at RobinNotes. Recordings of Robin's songs and calls and their meaning are at learner.org.
Also read Just A Robin in Missouri Conservationist, written by Carol Davit of the Missouri Prairie Foundation.