One of the most noticeable bird behaviors in the fall is the formation of large flocks of birds, especially blackbirds.
Today I came across several large flocks of mixed blackbirds. One, at the corner of FM 369 and 277 was a mixed flock of a few hundred starlings and brown-headed cowbirds. Although you may find flocks of blackbirds year around, the flocks become very large after the breeding season ends.
However, other birds also form flocks. It is not uncommon to get fairly significant numbers of cardinals and blue jays in the winter. Not the hundreds seen with blackbirds, but significantly more than the 2 or 3 seen in the summer.
In the summer, most birds tend to disperse in order to reduce the demand on food. Many birds defend territories from others of their own kind in order to ensure offspring belong to them (keep out poachers) and to ensure a good food supply for their young. However, it takes a lot of energy to defend territory. So in the winter, it just isn't worth the effort. In addition, flocks of birds have better defense from predators in a more open environment. So flocking behavior makes more sense outside of the breeding season.
In addition to many blackbirds, I did see two eastern bluebirds on FM367 just outside Iowa Park city limits.