In this very quiet time of summer, most young song sparrows, black-capped chickadees, red-eyed vireos and others have left their nests and are moving about locally, still dependent to some degree on their parents for food and protection.
Little by little, totally independent young of various species are beginning to wander outside their natal areas. For example, in the past couple of days (Aug. 11-12) I have seen yellow warbler and warbling vireo, neither of which nested within earshot of our yard. To date, I haven't seen any migrants from afar, but that will change any day now, probably after the next cold front pushes some of them south.
Once this migration begins, birds finding themselves in unfamiliar territory will seek out the company of other individuals, species and forage with them through the trees in loose flocks. Though they may not see many of their associates, they maintain vocal contact using little chip notes.
Non-migratory black-capped chickadees are often members of these flocks and are believed to provide a level of "comfort" for the migrants because they know the local area and its hazards. When seeking out flocks, which can be few and far between, I listen for the calls of chickadees, track them down, and often find a diverse flock of warblers and other species accompanying them and uttering occasional, quiet chip notes.
There is strong evidence that a primary purpose of flocking is to reduce the chance of predation. If you're a vulnerable bird, having more eyes and ears around you increases the likelihood of a predator being detected, and, once detected, the presence of other birds around you decreases the probability that you will be the prey chosen.
One of my favorite demonstrations of flocking being an anti-predator adaptation was a simple but elegant study from the early 70s. A researcher made a recording of the chipping notes of a foraging flock. Then she broadcast that recording to a bird foraging by itself. Without the recording, the bird spent a lot of time in vigilant mode (looking out for danger). When the recording was played, simulating a flock of other birds around, the bird would spend much less time in vigilance and much more time actually feeding. This is a great example of a speculative hypothesis being confirmed by clever research design.
Birding this time of year can be very challenging because birds are not singing. But by mid-August there are lots of interesting species drifting through quietly on their way south. All it takes is some persistence, a good ear, and a few chickadees to aid you in meeting up with the flockers.