"Rather startled, I watched this scene from close quarters behind the
window until 19.10 hours 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
pecked the neck and the side of the head before mounting again. The
first break (at 18.29 hours) lasted three minutes and the second break
(at 18.45 hours) lasted less than a minute. At 19.12 hours, I
disturbed this cruel scene. The necrophilic mallard only reluctantly
left his 'mate': when I had approached him to about five metres, he
did not fly away but simply walked off a few metres, weakly uttering a
series of two-note 'raeb-raeb' calls (the 'conversation-call' of
Lorentz 1953). I secured the dead duck and left the museum at 19.25
hours. The mallard was still present at the site, calling 'raeb-raeb'
and apparently looking for his victim (who, by then, was in the
freezer)."
https://www.theguardian.com/education/2005/mar/08/highereducation.research