Mr.Sparkle is absolutely correct about them being violently horny. (Warning! No-one wants to know this. Stop reading now!)
On June 5th, 1995, C. W. Moeliker of the Natuurmuseum Rotterdam in the Netherlands had the unprecedented opportunity of witnessing a case of homosexual necrophilia in the mallard Anas platyrhynchos.
Apparently, two male mallards were engaged in a high-speed chase when the one being pursued hit the window of the museum and died. The pursuing male landed, picked the dead one's neck and head in the way described by Mr.Sparkle above, then mounted the corpse and started to copulate.
This went on for over an hour. The live duck only stopped copulating and dismounted twice to pick the neck and head of the dead one before mounting again. Moeliker took some photographs before chasing away the necrophile duck and recovering the corpse. The live one stayed around for a while, making the mallard's "conversation call" and looking for his victim.
Moeliker believes that this incident occurred during what duck ethologists call an 'Attempted Rape Flight', in which a horny drake chases an unwilling female or male. Rape, homosexuality and homosexual rape have all been observed in ducks. However, what Moeliker witnessed was the first recorded case of homosexual necrophilia. Teiresias pointed out that Moeliker received the 2003 Ig Nobel Prize in biology for documenting and publishing this event.
Source (including images!!): http://www.nmr.nl/deins815.htm