Odd "oily" effect -- caused by the camera? (1 Viewer)

Hi all,
I noticed some of my pictures today had this odd rainbow oil effect going on in the feathers, particularly the rump area and wings. I know for sure this was not present on the bird itself in the field, I have several slightly out of focus images where the effect is completely absent.

Any ideas what this is or what causes it? I've never seen it before. Perhaps its to do with the images being slightly underexposed.

If useful:


f/9.5
1/500
ISO 400
no bias
400 mm focal length

Looks like moiré.

It does doesn't it? Perhaps it was too sharp and the resolution was not high enough to compensate :-O