Do diurnal birds of prey recognize each other by their appearance?
Having seen some nature documentaries about Penguins identifying their offspring using sound alone (even in flocks of hundreds of thousands of other birds), my immediate impression was that sound is likely to be more important with regard to recognizing individuals of the same species.

This is corroborated by Melissa Mayntz, a bird enthusiast, who has written a great account on birds use of their 5 senses (Understanding Bird Senses).

To summarize, she explains that whilst sight is a birds most important sense in terms of hunting and avoiding being hunted, their hearing is is their second most important sense and they use this to identify individuals;


  Birds hear a smaller frequency range than humans, but they have much more acute sound recognition skills. Birds are especially sensitive to pitch, tone and rhythm changes and use those variations to recognize other individual birds, even in a noisy flock.