The all glass façade of the extension of the Natuurmuseum Rotterdam, situated in an urban park, acts—under certain light conditions—as a true mirror. Numerous birds, mostly trushes, pigeons and woodcocks, die in collision with the building. Especially during the first months after the new wing was erected in 1995, a 'bang' or a sharp 'tick' on the window meant work for the bird department. Such was the case on 5 June 1995 at 17.55 h. An unusual loud bang, one floor below my office, indicated yet another collision and an addition to the bird collection. I went downstairs immediately to see if the window was damaged, and saw a drake mallard (Anas platyrhynchos LINNAEUS, 1758) lying motionless on its belly in the sand, two metres outside the façade. The unfortunate duck apparently had hit the building in full flight at a height of about three metres from the ground. Next to the obviously dead duck, another male mallard (in full adult plumage without any visible traces of moult) was present. He forcibly picked into the back, the base of the bill and mostly into the back of the head of the dead mallard for about two minutes, then mounted the corpse and started to copulate, with great force, almost continuously picking the side of the head. Rather startled, I watched this scene from close quarters behind the window until 19.10 h during which time (75 minutes!) I made some photographs and the mallard almost continuously copulated his dead congener. He dismounted only twice, stayed near the dead duck and picked the neck and the side of the head before mounting again. The first break (at 18.29 h) lasted three minutes and the second break (at 18.45 h) lasted less than a minute. At 19.12 h, I disturbed this cruel scene.