The Lost Cowbird of Térapa

“One of the penalties of an ecological education,” the conservationist Aldo Leopold once wrote, “is that one lives alone in a world of wounds.” This is true for the students of prehistory as much as ecologists. Nature has never been in a static balance – change is the overwhelming theme – and the scars of ancient disruptions can still be perceived by those who know what to look for.

Species are disappearing at a rapid rate. The earth’s sixth mass extinction may not be here just yet, but the catastrophe is getting closer every day. Viewed from some future perspective through the lens of Deep Time, though, such a crisis might seem to be a continued winnowing away of species that began during the waning days of the Pleistocene.

Mammoths, sabercats, giant wombats, ground sloths, deep-snouted bears, terror birds – these and other magnificent creatures all disappeared between 80,000 and 4,000 years ago. Why they did so, no one knows for sure. The widespread megafauna of the late Pleistocene did not simply drop dead in unison. The pattern of extinction was complex and does not show the hallmark of a single cause. This an abominable mystery. The extinctions happened virtually yesterday, and the fossil record of the late Pleistocene offers a much higher degree of resolution than we have for earlier eras, yet the confluence of factors that swept away the charismatic beasts remains an especially contentious subject among paleontologists.

There are many hypotheses about what happened. Rapid climate change following the end of the last Ice Age, overkill by hungry humans, a superdisease, and even a swarm of comets have been implicated as potential culprits for various parts of the extinctions. The effects of hunting and climate change on megafauna, in particular, have been tussled over for decades, but it would be a mistake to focus too narrowly on only the large mammals. (Too many discussions center almost exclusively around mammoths in North America to the exclusion of almost anything else.) It was not only large mammals that disappeared during the Pleistocene die offs, and some of the smaller victims may have been intimately connected to the bigger beasts.

Situated in Mexico’s northwestern Sonora region, the fossil site on the outskirts of San Clemente de Térapa preserves echoes of a prehistoric world where ancient creatures co-mingled with species still in existence. Were you to travel back in time 40,000 years ago you would find that the arid thornscrub habitat had been replaced by a wetter environment that supported swaths of savanna, freshwater marsh, and lakeside forest. This mosaic was home to varied assemblage of giant armadillos, camels, ground sloths, dire wolves, tapirs, and mammoths, but the cast of smaller animals would have looked more familiar.

Florida Museum of Natural History paleontologists Jessica Oswald and David Steadman have reported on the numerous birds found at Térapa in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. Many were wetland species – such as ducks and ibis – but the paleontologists also recognized several species of songbirds. The focus of the new report, all the songbirds identified so far represent a single subgroup  – the Icteridae – that contains blackbirds, orioles, grackles, cowbirds, and others. In fact, seven of the bird species found at the site still live in the Sonora region – including red-winged blackbirds and orchard orioles – but one species is entirely extinct.

Named Pandanaris convexa, the single extinct species was a kind of cowbird that had previously been discovered at sites of similar age in California and Florida. Not much of it was found at Térapa. Only nine delicate pieces from two individuals were turned up, but the identification of the bones as those of a cowbird allowed Oswald and Steadman to speculate about the habits of this extinct species.

The life of a cowbird is that of a nomad. Thought to have evolved in conjunction with large herbivorous mammals like bison, cowbirds feed on the insects stirred up by large grazers and the seeds that can be picked from the trampled grass. Their lives are tied to the herds they follow, and so cowbirds also became adapted to being brood parasites – laying their eggs in the nests of other bird species and leaving the unfortunate parents to take care of their fast-growing progeny. (With the recent expansion of grass-lined highways and well-groomed lawns, however, we have created cowbird buffets where passing cars play the role previously held by cattle and bison.)

If Pandanaris convexa was anything like its living cousins, then its ability to forage was probably closely tied to the shuffling of large Pleistocene herbivores. In fact, two species of still-extant cowbirds were found at the same site, so the grasslands of Térapa 40,000 years ago likely held plenty of food that was regularly made available by the grazing of mammoths, ground sloths, and other creatures. This relationship may have also sealed the fate of the extinct bird. “When the mammalian grazers became extinct,” Oswald and Steadman suggest, “P. convexa also died out, perhaps because of a specialized foraging dependency on proboscideans, equids, camelids, or other large mammals.”

But why did the one species of cowbird go extinct while the other two survived? After all, we can observe their behavior today and they probably would have been closely tied to the activities of the large herbivores. Just because a species survives, though, does not mean that it was immune to environmental changes in a particular area. Maybe the two surviving cowbird species had a wider range than the extinct one. The extant species might have been extirpated from Térapa, but surviving populations elsewhere later expanded back into the Sonora. Further sampling of the fossil record in the area might help solve this puzzle.

The details of what happened to the cowbirds is not considered in detail in the new paper. Even the suggestion that Pandanaris convexa was extinguished due to its close association with large mammals is delivered as a brief hypothesis at the end of the report with only inference to back it up. We can’t be sure of what happened to them. Nevertheless, the new paper by Oswald and Steadman raises an significant point about the Pleistocene world. Mammoths, dire wolves, giant sloths, and other strange creatures easily grab our attention, but these animals were not isolated from the other players on the Pleistocene stage. They shaped their environments and, in some cases, created niches for smaller creatures that are often overlooked. If we can better understand the lives of the critters that lived in the shadow of the mammoths and mastodons, perhaps we can better assess the wounds of the Pleistocene and how they were riven.

Top Image: A restoration of some of the mammals that lived during the Pleistocene of Spain by accomplished paleo artist Mauricio Antón. Woolly mammoths, horses, reindeer, and lions were also found in parts of North America during the Pleistocene, although the woolly rhinoceros was unique to Eurasia. Image from Wikipedia.

References:

Leopold, A. 1990. A Sand County Almanac. New York: Ballantine Books. p. 197

Oswald, J., & Steadman, D. (2011). Late pleistocene passerine birds from Sonora, Mexico Palaeogeography, Palaeoclimatology, Palaeoecology, 301 (1-4), 56-63 DOI: 10.1016/j.palaeo.2010.12.020