Later investigation suggested that the duck was on a "rape flight". Male mallard ducks will often force females to copulate with them, and when several of them converge on the same female she may not survive the experience. In this case, the male may have applied the same instinct to the dead mallard (read Moeliker's paper).
A 1989 review of the human psychiatric literature concluded that the most common cause of necrophilia is not a specific attraction to corpses, but simply "possession of an unresisting and unrejecting partner". Given this, it would seem the duck's behaviour may have been all too human.
Necrophilia has also been observed in ground squirrels and frogs, and seems to be moderately common in mallards. It is often referred to as "Davian behaviour", in reference to a ribald limerick about a necrophiliac miner called Dave.
From a practical point of view, we can define bestiality as the practice of one animal mating an individual from different species, with which it is not genetically compatible. Some instances can, of course, be put down to mistakes, particularly when species appear similar, but there are plenty of examples where the animal has, quite simply, no such excuse.
Similarly, the BBC recently reported an incident in which an Antarctic fur seal sexually assaulted a king penguin. It seems unlikely to have been a simple mistake, and the watching research team were baffled as to why it happened.
As with necrophilia, such behaviour seems to be an evolutionary mistake. By expending sperm without any possibility of producing offspring, the animals are reducing their fitness.
The unfortunate male wasps duped into ejaculating on orchids are one of the most striking examples of this. The orchid benefits by spreading its pollen, but it seems the wasps get nothing out of it.
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