Hi Tobi
interesting photo, but I would like to point out that some individual of the nominate subspecies citreola can be very dark grey on the upperparts, like this one photographed at Van, Turkey, the same day as the calcarata:http://www.pbase.com/dophoto/image/135160675. I think that a bird like this one could be easily confused with a calcarata at a distance, and your pictures is very distant and fairly blurry. From your picture I can clearly see the the back is not black but dark grey.
Nice picture, but unfortunatly it's not the first, take a look at my picture(s) in the Netherlands back in april 1991 !!! (Under Rare birds in Zeeland 1990 - 2005)
http://www.pbase.com/image/105637333
