A good deal of misinformation has been uttered about the speed of bird's flight. See How Birds Fly - animated Flight speed is dependent upon the bird, the weather, the wind, etc. In general, the flight speed of small passerines varies from 15 to 50 mph. Ducks will cruise around 30 mph but can reach 60. Crows at 25-30, etc.
Many birds fly in flocks.
There can be flight aggregations, whereby birds fly around more or less together but are rarely coordinated - e.g. gull or terns feeding on a school of fish. May be very coordinated - e.g. shore birds, or very loose, e.g. sparrows, robins. There are a number of social factors - reproduction, protection from predators, communication, navigation, etc. But let's look only at aerodynamic factors for the time being. One theory is that in coordinated flight flocks such as those of waterfowl, there's an aerodynamic advantage to flying behind and to the side of another bird to take advantage of its wingtip vortices.