During the colder months, having completed their parental duties, many birds roost in large flocks. Come morning, they scatter across the countryside, usually feeding in small groups, and then return to the roost during the last hours of daylight. This behavior likely evolved as a means of protection; numerous eyes and ears offer better detection of predators and the hunter may be overwhelmed by the huge number of potential targets. Furthermore, such roosts are often established on naturally protected sites such as islands, isolated groves, remote beaches or, in the case of waterfowl, out in the middle of a lake; such locations make it difficult for a predator to attack without early detection.
Though we may not know where the roosts are, we certainly can observe the traffic as birds head toward these sites late in the day. In Missouri, as is typical throughout the eastern U.S., these gathering flocks are usually composed of blackbirds (starlings, grackles, crows, redwings) or robins; at times, a river of birds may stretch across the evening sky as they make their way to the nightly roost. Along southern coasts, this daily rush hour is composed of various waders (herons, egrets, ibis), gulls, terns, cormorants, pelicans and shorebirds, making their way to secluded beaches or mangrove islands.
Here along the Front Range, Canada geese and ring-billed gulls are the primary winter commuters, heading back to favored lakes and reservoirs throughout the area. By spending the night on the open water, or in the middle of a frozen lake, they are well protected from the coyotes and fox that hunt the darkness. When the sun returns, they will head out to feed on grasslands, along the South Platte or, for many of the gulls, at landfills and parking lots.
