How did this happen? Well, one fine day a male duck (the 'victim-duck') flew against the museum's huge window and dropped dead (just like many other birds do apparently). Then another male duck arrived that started ahm "copulating" with the dead duck... Kees Moeliker who worked at the museum happened to pass, video-taped the whole event and described it extensively in a paper (he later even won a Ig Nobel Prize for this).
A funny detail is that when Kees tried to collect the carcass he was faced by an aggressive attack from the 'molester duck' who was defending his 'prey'...!
Well, isn't nature full of surprises?!