In a great group photo one bird will stand out as the center of attention. This is probably because the photographer picked that bird and built the shot around it. Choosing a single subject as a focal point works for groups from a few birds to dozens of birds. Once you get to hundreds of birds, you probably really have to luck out to find one that is distinctive enough to focus on, but most of the time picking a subject works.
Being the center of attention doesn’t necessarily mean being in the center of the photo. In a photo where the scene has depth, the subject bird will generally be in the foreground because it is more natural to have blurry birds in the background rather than in the front. The subject bird is often looking at the camera or doing something interesting because you focused on it and waited to capture that moment. The subject bird may be separate from the other birds in the photo.
Below are four photos that, I think, have one bird that is clearly the subject. It might not be equally obvious in each of the photos, but I think most people would agree about which bird it is. What do you think?