Experienced birders know that woodland birds often move about in mixed flocks and that this feeding behavior is especially common during the colder months. One might hike for some distance without seeing (or hearing) any activity and suddenly come upon a large number of birds, usually composed of multiple species. In like manner, the backyard feeder may stand idle for an hour or so and then suddenly become the focus of a roving flock of residential birds.
In the Midwest and eastern U.S., these "feeding groups" usually contain chickadees, titmice, nuthatches, downy woodpeckers, cardinals and a variety of finches. At times, they may also be followed by ground feeders such as juncos, mourning doves and sparrows. This behavior likely reflects both the benefit of group efforts to locate food and improved protection from predators; more eyes offer better detection and the large number of moving targets may confuse the hunter.
Avid birders know that such feeding flocks offer immediate gratification (in terms of number of species) and often attract uncommon birds that are visiting the area. For example, a mixed group of juncos, song sparrows and white-throated sparrows may also harbor a fox or Harris' sparrow. Though most of us enjoy watching our common residents, the possibility of encountering an unexpected guest is always an underlying motivation!