If you are a small bird you want to team up with other birds to help each other in finding food. Plus, there are more eyes in a group of birds to help spot larger birds like hawks that may want them for lunch. You increase your odds of surviving the winter by being in a mixed group of birds.
The Chickadees will look up, down and sideways to make sure it is safe before they move most of the time. If they do see a predator, they will sound out alarm calls.
Carolina Chickadee and Tufted Titmouse.