Many songbirds begin flocking at the end of summer after nesting season is over. This usually goes on until early February when they flocks of about ten to fifty break up. They break up because it means that this is the start of mating season.
So, you will likely see cardinals, Robins, Bluebirds, Blue Jays gathering in flocks during the winter months. When I first heard of this I thought it was rather strange. Strange because these are the very birds that are so aggressive in the spring time when it comes to mating and defending their nests.
So, why then, would these birds flock together? Obviously there are some advantages to it. First, there is safety in numbers. During the winter, there are no leaves on trees, which means that birds have very few places to hide. This leaves them much more vulnerable to predators than in the spring or summer months.
Also, when they travel in these packs, they are more difficult for a predator to be able to snatch one of them. Not only are these flocks better for their safety but with a large group they have more eyes on the lookout for potential predators as well as being able to find food in the harsh winter months.
While it is true that birds such as Cardinals and Robins are made up of only Cardinals and Robins respectively, smaller birds such as Titmice, Kinglets, Downy Woodpeckers, Pine Warblers, and Chickadees often times break species lines and mix together for protection.
Don't be surprised if you are walking along in the woods and all is quiet and then all of a sudden you see and hear a noisy flock of birds eagerly tweeting to each other to point out food or make known the presence of a predator nearby.
But beginning next month when the days are longer. It is an amazing thing that happens to these birds that were just a short while ago so dependent on each other and happy to be around each other. Hormones kick in and the flocks will disband shortly after that. They will scout out their mates and fiercely protect their nesting area and any bird that gets in their way.