Common Gull with sprayed marking on underwing

[Rainer]

I took some photos of a Common Gull with one partly red-coloured underwing.
(Brombachsee, southern Germany, 21.12.2008). The gull wasn't ringed, but it looks like a signal colour, I can't imagine that this happened without some direct human intervention.
Does someone know what this marking is about?

Rainer

[Mark Grantham]

I think this is probably going to be just a stain of some sort. Having ringed gulls on rubbish tips its amazing what they can pick up, and I remember seeing a bird once covered in ketchup, looking like it had been savaged! The spread of this stain could have come from how the wings close, with the primary patch lying over the patch on the secondaries.

Mark Grantham
BTO

[Rainer]

Thanks for your answer, Mark. I think, you are right, that the gull managed the colourization by its own. Furthermore I couldn't find a reason what a marked underwing should be good for.
Rainer