Cordon Bleu finch head feather loss?

I have a cordon bleu finch and a zebra finch in the same cage - they have lived together happily for almost a year. I just noticed that the cordon bleu is has lost head feathers from behind his eyes, about a 1/4 inch back, straight across the back of his head. He obviously couldn't have done this himself. Is this some strange molting pattern, or has the zebra suddenly become agressive when I'm not around? I haven't noticed any change in behavior for either of them. The rest of his feathers look lovely, as usual. Can anyone help?