I have no idea why a rooster is part of the ceiling mosaic in the Aachen Cathedral, but it is. According to the book we have, it is there as a "symbol of vigilance". This part of the cathedral was finished in 800. The mosaics were redone mid-19th century from original plans. This part of the cathedral was Charlemagne's chapel.
And thanks for yesterday's comments. Yes, indeed, shooting through glass is a pain.