Growths on a Robin beak

Hi,

Could anyone suggest what might have happened to this Robin? These photos were taken about a week apart - at the base of his beak on both sides, he lost feathers and developed what look like scabs or small black growths. Could this be some kind of disease? He has now disappeared, not seen since the day the second photo was taken (and was a very tame bird) - whether his disappearance is related to what happened to his beak, I'm not sure. Nothing in his behaviour suggested that he was ill, he was still singing, eating and bringing food to his mate.

Thanks in advance

 Mike