This shouldn’t really come as news, but birds really do communicate with each other, especially when there is an enemy nearby. They signal their friends to warn them of nearby danger, or rally them to come to their aid and attack as a mob.
They can even aim the direction of their calls to the bird in a different area if they so choose. Thanks, science. Now we just need to learn their chirp for “Come here and enjoy these delicious seeds with me” so we can trap them and defeat our areal foes.