On June 5, 1995, a Dutch ornithologist named Kees Moeliker was working quietly in his office in the new wing of the Natural History Museum in Rotterdam, the Netherlands, when there was an unusually loud bang one floor below. The wing's all-glass facade sometimes took on mirror-like qualities, so there was a regular supply of birds colliding with the glass. In this case, the collision was from a drake mallard duck (Anas platyrhynchos) lying dead on its belly in the sand.

Things took an unusual turn when Moeliker spotted a second, living male mallard nearby, which began pecking at the back of the dead duck's head. After a couple of minutes, the living duck "mounted the corpse and started to copulate, with great force," Moeliker recalled, only stopping for a couple of short breaks. The ornithologist managed to snap some photos of this odd behavior before intervening and collecting the dead duck specimen–over the noisy objections of its living "mate." It was the first documented case of homosexual necrophilia in the species.

Moeliker published his findings in a 2001 paper that would eventually snag him the 2003 Ig Nobel Prize in Biology. It also inspired the annual "Dead Duck Day" celebration, held at the very spot the unfortunate duck perished, marked by a memorial plaque. The brief commemorative ceremony—which also acknowledges "the billions of other birds that die(d) from colliding with glass buildings and challenges people to find solutions to this global problem," per Moeliker—is typically followed by a six-course duck dinner at a local Chinese restaurant called Tai Wu. The event is co-organized by the museum and the European Bureau of Improbable Research.

In his paper, Moeliker noted that the museum's park has several water features, like ponds and ditches, favored by a wild population of mallard ducks numbering between 40 and 50 individuals at the time of the incident. His hypothesis is that the two ducks were in the midst of an aerial chase or "pursuit flight"—common mallard behavior—when the doomed duck hit the glass facade. "It is highly unlikely that the [other] drake was just passing by, saw the corpse, and started to rape it," he wrote. One could quibble with the use of the word "rape" to describe the copulation Moeliker observed, but he wrote that given the deceased nature of the penetrated party, "the act was non-consensual anyhow."

Two male mallard ducks copulating would not actually be that surprising. Same-sex pairings have been recorded in some 450 different species, from flamingoes and bison to warthogs, beetles, and guppies. Female koalas sometimes mount other females, while male Amazon river dolphins have been known to penetrate each other’s blowholes. Lepidopterist W.J. Tennent, while diligently tracking Mazarine Blue butterflies in Morocco in 1987, spotted several males of the species mating with each other rather than with females of the species.

Nor is necrophilia limited to mallard ducks. A British naturalist named George Murray Levick traveled to Antarctica with the 1910-1913 Scott expedition and spent several months studying the breeding habits of a colony of Adelie penguins at Cape Adare. Levick was horrified to witness not just male penguins mating with other males but one young male Adelie penguin attempting to copulate with a dead female. Necrophilic behavior has also been observed in ground squirrels, New Zealand sea lions, rock doves, pilot whales, and crows, among other animals. Canadian biologist and linguist Bruce Bagemihl prefers to call this sort of thing “biological exuberance,” and his 2000 book with that title makes for a fascinating read for those curious to learn more.