Oxyurin Odyssey

A selection of members of Oxyurini. Clockwise: Black-headed duck (Heteronetta atricapilla) drake and hen in Rio Grande do Sul, Brazil; Masked duck (Nomonyx dominicus) drakes in Tobago; Ruddy duck (Oxyura jamaicensis) drakes in South Dakota; Maccoa duck (O. maccoa) hen at the San Diego Zoo. All images are retrieved from Wikimedia Commons.

So, waterfowl. You might take your neighborhood mallards for granted as I often have, but waterfowl are really very cool. Yes, even the endless hordes of Canada geese blocking your commute, disgruntled American readers. The order Anseriformes contains something like 180 extant species, and just about all of them are outright fascinating. Today we'll zero in on a small clan of anatids known variously as the oxyurins, oxyurines, or the stiff-tailed ducks. Exactly which of these three names is best for this group is a little ambiguous. "Stiff-tailed ducks" may well be reserved for the members of the type genus Oxyura, and the entirely superfluous debate over whether they represent a "tribe" or a "subfamily" informs the rest. I'll be calling them oxyurins throughout this post for convenience's sake, irrespective of long-defunct Linnaean concerns.

Traits, Position and History

Oxyurins are so-named for the stiff, upward-pointing tail feathers just about all of them share. They're small, compact ducks, usually ranging between 30 and 55 cm (11.8 to 21.6 inches) in full body length; compare with the widespread mallard duck (Anas platyrhynchos) and its 65 cm (25.6 inches). Like the distantly related aythyin ducks, oxyurins are divers, and as such are built with back-set feet which force them to move awkwardly on land. In fact, this condition is more pronounced in this tribe than in any other sort of waterfowl. Ungainly though they are on land, they're all highly competent in the water, appearing almost exclusively in freshwater environments. Almost all species share distinctive blue coloration in the bill in males, bar the black-headed duck (Heteronetta atricapilla). This deviation is unsurprising, because Heteronetta is undoubtedly the basalmost member of this clade. Often included in Oyxurini but excluded here is the musk duck (Biziura lobata) of Australia. This is a little controversial, and many would still include this duck and its notably stiff tail with the oxyurins. More infrequently included is the white-headed duck (Thalassornis leuconotus), another weird Australian duck. Increasing genetic evidence has removed these two Australian species from the oxyurin fold, showing Biziura to be just a short distance outside of the oxyurins and Thalassornis at a position basal to almost the entire anatid family. Another surprise modern genetic analyses have uncovered for us is the fact that Oxyurini is part of the anserine subfamily. The fact that these "ducks" are far closer to geese and swans than they are to the more familiar anatines so common worldwide is somewhat underappreciated, I think; it's difficult to see this just based on gross morphology, so it's a small wonder that, for a very long time, oxyurins were thought to group with most other "ducks".

Relationships of anatid subfamilies and tribes, as per jboyd.net.

Today there are eight (ish) oxyurin species in three genera. The aforementioned black-headed duck is the only member of its genus, as is the masked duck (Nomonyx dominicus). The remaining six species are all members of the eponymous genus Oxyura. The black-headed duck is restricted to southern South America while the masked duck ranges out of South America and into Mexico and the Caribbean. Members of Oxyura, however, can be found on every continent but Antarctica. The tribe (unsurprisingly) sports a few more among its number once you go back in time. The most recent losses to this clan are all species of Oxyura, the most notable of these being the New Zealand stiff-tailed duck (O. vantetsi). The extinction of this species some time before 1600 CE marked the extirpation of the oxyurins from New Zealand. Closely related to Australia's blue-billed duck (O. australis), the New Zealand stiff-tail was even smaller than this already quite runty waterfowl, but little else is known for sure about it.

The ranges of the various Oxyura species. Yellow: Ruddy duck (O. jamaicensis). Red: Lake duck (O. vittata). Green: Maccoa duck (O. maccoa). Dark blue: Blue-billed duck (O. australis). Light blue: White-headed duck (O. leucocephala). Retrieved from Wikimedia Commons.

A few other fossil species have been assigned to Oxyura, but only O. zapatanima of the Pleistocene of Jalisco, Mexico appears to unambiguously represent a valid example. Further back, the fossil taxon Lavadytis pyrenae from the Miocene of Nevada seems to pin the New World as a major origin point for many oxyurin species, which isn't surprising when you consider that the Americas have the greatest diversity of them. Other extinct taxa like Mionetta and Tirarinetta, which might be oxyurins, suggest that they had their genesis in the Oligocene and rapidly diversified around the time of the Miocene Thermal Optimum. (Also, the Lavadytis paper seems to recover the lake duck [Oxyura vittata] as sister to a clade including all other Oxyura species, but also Nomonyx. Anatid taxonomy is a mess.)

The Ways of the Cuckoo Duck

With the cladograms safely tucked away, we can talk a little more about the varied lifestyles of the stiff-tailed ducks. One of the most-noted talking points about oxyurins are the egg-laying habits of the black-headed duck (Heteronetta atricapilla). A small bird with a vaguely spatulate bill, the black-headed duck is, as discussed above, the basalmost member of its tribe and in many gross details more closely resembles dabbling ducks than its more specialized relatives. Thanks in part to an excellent David Attenborough documentary segment called "Nature of the cuckoo duck", the black-headed duck's status as the only obligatory brood parasite among the world's waterfowl has made it of significant biological interest. For those not tapped in on the terminology, what this means is that the black-headed duck never makes its own nests, and must seek out those of other birds in which to slip its eggs in order to propagate. This is what has earned this remarkable little duck its nickname.

A black-headed duck photographed near Trelew, Argentina by Gustavo Fernando Durán.

Rather than achieving perfect mimicry of another species in their eggs as many brood parasites do, the black-headed duck appears to have made up for it in sheer breadth. Its eggs have been discovered in the nests of many other species, including other waterfowl, coots, gulls, and even harriers! They must be fearless indeed to approach the nests of birds of prey (or, more likely, they can't tell the difference). Since oxyurin ducklings are highly precocial and are capable of wandering off on their own shortly after birth, they don't belabor their adoptive parents too long and strike out into the world in very short order. Interestingly, the literature reports some other alleged cases of brood parasitism in other oxyurins, including the blue-billed duck (Oxyura australis), but no serious research seems to have been committed to this matter. Whether the "cuckoo duck" is really unique among the oxyurins in this respect remains to be seen. In either case, it's no less charming.

The Amazing Whatsit of the Lake Duck

And now to the awkward bit. Like it or not, along with feeding, reproduction is one of the most essential biological functions in nature. If you're not keen on this special brand of scientific enlightenment, then you're welcome to skip forward to the next section. I won't judge you (but the ducks might).

A lake duck, displaying the awkward gait typical of oxyurins, at the Wuppertal Zoo in Wuppertal, North Rhine-Westphalia, Germany. Photographed by Georg Sander.

The reproductive equipment of oxyurins is remarkable, even among ducks. The "whatsit" in question here is the male reproductive organ, the fair and balanced term for which is, in this context, the "intromittent organ", not "penis". This is because, as far as we can tell, the intromittent organs of birds and reptiles aren't homologous with those of mammals - that is to say, they don't share a common evolutionary origin and apparently were developed separately. It's worth noting that most male birds don't have such organs; something like 97% of all avian species reproduce by way of a "cloacal kiss", where the openings of the birds are pressed together for a transfer of semen. The only birds that have retained their intromittent organs, as their closest living relatives (the crocodilians) do, are the basalmost bird clades, including galloanserans and palaeognaths. This in itself has implications for the reproductive habits of non-avian dinosaurs, but that's a post for another day.

At any rate, other ducks have particularly prominent and oversized intromittent organs, but male oxyurins take it to another level. Most famous of these is the lake duck (Oxyura vittata), the intromittent organs of which have been measured in excess of 40 cm (15.75 inches), or almost the length of its entire body (!), though they usually seem to measure in at a "mere" half of that. This gives the lake duck the honor of having the largest reproductive organ-to-body ratio of any vertebrate. That being said, it does seem to get a disproportionate amount of press for this distinction, because other oxyurins also have rather large intromittent organs. Studies have indicated that the size of these organs varies so widely in response to environmental factors, chief among which appears to be competition between males. When in an environment in which more male ducks than females can be found, their average sizes becomes far greater. Oxyurin phalluses notably sport a set of brush-like bristles at the tip, a bizarre little adaptation which seems to serve some role in removing the material of competing males from prospective mates. They also all share the same shape, that of a counterclockwise corkscrew which unfurls when in use, one which is shared by some other waterfowl.

Interestingly, female ducks of this tribe (and a few others) likewise host their own elaborate corkscrews in their internal anatomy, but these run clockwise, opposite to their male counterparts. This seems to pose a means of thwarting penetration by male ducks, a serious concern in a type of bird which practices forced copulation at an alarmingly high rate. Between these bizarre mechanisms and the seemingly ultraflexible sexual anatomy of male oxyurins, it's clear to see how the highly promiscuous habits of these little ducks are a driving force in their reproductive evolution.

The Magnificent Butterball

So, let's move on to less shocking territory and spend a moment focusing on my personal favorite of the group, the ruddy duck (Oxyura jamaicensis). In some parts of the USA, it's apparently sometimes called the "butterball", which is just so impossibly charming that I feel obliged to mention it at least twice. A handsome little duck of about 38 cm (15 inches), in the North American part of its range it's instantly recognizable when compared to conterminous waterfowl, thanks to its bright blue beak, "bumblebee" wing beat, and diminutive size. It ranges widely across North America (including the author's native Ohio, where I've spotted the funny little birds in local wetlands on a couple of occasions), and also into South America. Its populations there (including the Andean duck O. j. ferruginea) are sometimes regarded as separate, but the popular mood among ornithologists now seems to be to treat them as part of the ruddy duck assemblage rather than distinct species. Interestingly, the ruddy duck has also been introduced to Europe, whether purposefully or accidentally by bird collectors, and has caused significant concern by competing and apparently even interbreeding with the endangered white-headed duck (O. leucocephala). Once hosted in the British Isles in the thousands, recent culling efforts have reduced their number to no more than a hundred individuals, seeming to spell an end to this species' European adventure.

Ruddy duck drake in flight. Photographed at the Benton Lake National Wildlife Refuge, Montana, by Neil Mishler (USFWS).

Despite its unwelcome appearance abroad, the ruddy duck remains popular with birders in North America, thanks to its striking appearance and amusingly bold behavior in the breeding season, as well as the odd "bubbling" displays males put on when displaying. The naturalist John Charles Phillips sounds almost charmed by its idiosyncrasies when he writes of the bird in his 1926 work on waterfowl, “Its intimate habits, its stupidity, its curious nesting customs and ludicrous courtship performance place it in a niche by itself…. Everything about this bird is interesting to the naturalist, but almost nothing about it is interesting to the sportsman.” The name "butterball" apparently derives from the almost round shape of the bird when prepared to cook, which certainly lacks the cuddly connotations implied by the alias. Although it has a bad reputation among some hunters as a "trash duck" on account of its diving habits, it was regarded as a delicacy a century ago, and it still has its fans today. Less amenable to ruddy ducks are grebes, with whom they occasionally compete for living space and food; grebes have even been observed to aggressively attack ruddy ducks at some times of the year, even pursuing and pecking at them from underwater where most other animals can't touch them. It's easy to see a bit of karmic justice in this, when you consider that ruddies themselves chase other animals during the breeding season, including other ruddies, different waterfowl, and even rabbits, apparently out of pure pique (or pure testosterone). But needless to say, it's just the way of the wild. It certainly adds to their long list of idiosyncrasies and makes them all that much more likable.

Onward Oxyurins

And so we come to the end of our odyssey and reflect. The charm and scientific depth of these birds certainly makes them worthy of our attention, and as we've seen, some facts about them have become rather well known to general audiences, and not just the scientifically inclined. Still, in the course of doing my research on the stiff-tailed ducks, I discovered that many of the novelties with which we credit individual species are apparently shared among the other members of the group, but ignored or less well-researched. There's certainly a lot more about these ducks that's interesting, but which I just didn't have the time or space to cover here. I encourage anyone interested in waterfowl in particular, or wildlife in general, to go out and do their own reading. Many of the sources mentioned below were very illustrative and led the way to other resources on the subject. And better still, the stiff-tails are practically everywhere; if you get a chance, go out and see some for yourself.

Sources:

• All About Birds. Ruddy Duck. Retrieved on 12/12/17.
• Alvarez, Rafael. "A Pleistocene avifauna from Jalisco, Mexico". Contributions from the Museum of Paleontology, the University of Michigan 24.19 (1977): 205-220.
• Attiwill, A. R. "Possible nest-parasitism in the Australian stiff-tailed ducks (Anatidae: Oxyurini)." Emu 81.1 (1981): 41-42.
• Birkhead, Tim R., and Patricia Brennan. "Elaborate vaginas and long phalli: post-copulatory sexual selection in birds." Biologist 56.1 (2009): 34-38.
• Boyd, John H. III. "TiF Checklist, Anseriformes". Retrieved on 12/12/2017.
• Brennan, Patricia LR, et al. "Coevolution of male and female genital morphology in waterfowl." PLoS one 2.5 (2007): e418.
• Cabrera, María B., Diego Montalti, and Luciano N. Segura. "Breeding phenology and new host list of the black-headed duck (Heteronetta atricapilla) in argentina." The Wilson Journal of Ornithology 129.2 (2017): 311-316.
• Guthrie, Daniel A. "Avian material from Rancho del Oro, a Pleistocene locality in San Diego County, California." Bulletin, Southern California Academy of Sciences 109.1 (2010): 1-7.
• Johnsgard, Paul A. "Handbook of waterfowl behavior." Handbook of Waterfowl Behavior, by Paul Johnsgard (1965): 7.
• Johnsgard, Paul A., and Montserrat Carbonell. Ruddy Ducks & Other Stifftails: Their Behavior and Biology. University of Oklahoma Press, 2010.
• Johnsgard, Paul A. "Waterfowl of North America: STIFF-TAILED DUCKS Tribe Oxyurini." Waterfowl of North America, Revised Edition (2010) (2010): 14.
• Livezey, Bradley C. "Phylogeny and comparative ecology of stiff-tailed ducks (Anatidae: Oxyurini)." The Wilson Bulletin (1995): 214-234.
• Maxmen, Amy. "Sexual competition among ducks wreaks havoc on penis size." Nature 549.7673 (2017): 443-444.
• McCracken, Kevin G. "The 20-cm spiny penis of the Argentine Lake Duck (Oxyura vittata)." The Auk 117.3 (2000): 820-825.
• Shaw, Hank. "Ruddy Ducks: The Original Butterball Turkey", The Atlantic. Retrieved on 12/12/17.
• Stidham, Thomas A., and Richard P. Hilton. "New data on stiff‐tailed duck evolution and dispersal from a new species of diving duck (Anseriformes: Anatidae: cf. Oxyurinae) from the Miocene High Rock Caldera in north‐west Nevada, USA." Papers in Palaeontology 2.1 (2016): 41-58.
• Worthy, Trevor H., and Michael SY Lee. "Affinities of Miocene waterfowl (Anatidae: Manuherikia, Dunstanetta and Miotadorna) from the St Bathans Fauna, New Zealand." Palaeontology 51.3 (2008): 677-708.
• Worthy, Trevor Henry. Tertiary fossil waterfowl (Aves: anseriformes) of Australia and New Zealand. Diss. 2008.