Purple ring around subject, Nikon?

I know there's nothing wrong with my camera, but I'm just curious as to the mechanics of why this happened.

Today a took a photo of my mom's peacock. It was sunny with bright white snow as the only background. His neck is a vibrant, brilliant blue and there's a narrow purple band between his neck and the featureless snow in the background. I was using a 30 year old Spiratone 400mm and my Nikon D40. My photo was overexposed somewhat, especially for the background but I still have good detail of the feathers on his neck. He was in such a hurry to get away I didn't get a chance for a second photo with different exposure. Anyone know why this phenomena occurs? Thanks.