Buffalo birds in the Northern Mixed Grasslands Ecoregion

While having our breakfast, at Phil and Connie’s place near Pincher Creek, Alberta, we also feasted on the magnificent view from their balcony. We saw a juvenile Brown-headed Cowbird, Rufous and Calliope Hummingbirds, lots of White-crowned Sparrows, a male Wilson’s Warbler and other delightful birds of the area. France and I were visiting the Calgary area in mid-August 2014 to see daughter Stéphanie and we were also visiting my old friend and former work colleague, Phil. In the last week, we’ve gone through 4 or 5 ecoregions in this corner of Alberta but none have helped us advance toward our goal of visiting all 109 of North America, as this corner of Alberta is an area I visited thoroughly when I was a student at the University of Calgary in the late 1970s.

On our way back to Calgary, we stopped at the amazing Head-Smashed-in Buffalo Jump World Heritage Site, near Fort McLeod, Alberta. This is located in the Northern Mixed Grasslands Ecoregion. At this site, starting about 6000 years ago, Amerindian peoples herded buffalo over a cliff, as a specialized hunting technique. The site was used sporadically, sometimes centuries passing with no activity, until the late 1800s when the buffalo (properly known as bison) were essentially eliminated from North America.

Bison were a keystone species in this ecoregion and I wanted in this post to write about how over time they shaped the very nature of this grassland ecoregion and conversely, how their elimination has fundamentally changed the ecoregion’s dynamics. This will have to wait however until we can come back and spend some more time here and I can actually learn something about this topic! Instead, thinking back to breakfast, I found myself wondering about “buffalo birds” as we went through the site.

Of all the birds characteristic of this ecoregion, the buffalo bird was so called by the early settlers as it was most intimately associated with the bison. It fed on insects flushed up by the huge animals and was adept at scurrying between their feet in search of its prey. They rode on the bisons’ backs and picked ticks off, in a mutualistic relationship, much like the African oxpeckers on the “real” buffalo of Africa, as I’ve admired so often. The early Amerindian plains peoples must surely have also called them “buffalo birds”, in their many different languages, recognizing they were just as dependent on the bison as they themselves were. We call them cowbirds, Brown-headed Cowbirds. See the marvelous photo of a cowbird on a bison from National Geographic’s site.

Following the great bison herds provided the cowbirds access to the food they needed, but there was one potential problem. Bison are extremely mobile and move constantly to find forage, water, and probably to avoid predators, like those manning the Smashed-in-Head cliffs. In the time it takes to stop and build a nest, incubate the eggs, and fledge the chicks, cowbirds would have been left in the dust as the bison moved off over the horizon. But they had a fascinating behavioral adaptation, that has evolved independently in many different bird groups around the world that survive on very scattered or mobile food resources. They are “brood parasites”, which is to say the females lay their eggs in the nests of other bird species who then raise the cowbird chicks as their own young. This allowed the adults to keep moving with the bison; the fledglings would later join passing cowbird flocks.

It has been assumed for a long time that this behavior evolved as the ancestral cowbirds followed the bison around, slowly acquiring the fairly sophisticated brood parasite behaviors they have today. This turns out to be not true. There are five cowbird species in the genus Molothrus and recent DNA-based analyses on their origins establishes that the genus evolved in South America, most likely a brood parasite from its earliest appearance (since all Molothrus share this characteristic) and speciated into its different forms as it extended its range northward. The Brown-headed Cowbird is the youngest of the genus and was the last species to split off from the ancestral lineage. Ancestors of today’s cowbirds probably expanded into the grasslands of North America about a million years ago, already brood parasites, and lucked into the perfect storm for such a bird, as the grasslands were teeming with the many great grazing animals of the North American Pleistocene, now mostly extinct (horses, camels, ground sloths, etc.). The bison however were not there! The cowbirds must have been delighted to welcome their arrival as they lumbered over from Asia about 500,000 years ago.

The bison were practically exterminated from this ecoregion (and all of the similar grassland ecoregions of North America) by 1890. This is a shameful and well-known story _– the cause was not the small off-take from Amerindian peoples, at Smashed-in-Head and elsewhere, but a specicide perpetrated by waves of hunters and settlers from the east. Normally such a  specialized bird species would become either very rare or extinct faced by the loss of its food source. However, two interesting things happened as we were finishing off the bison.

First, cows and horses replaced the bison on the prairies and to a cowbird, a foraging cow kicking up insects as it moved along was a pretty good substitute. Secondly, at the same time the bison were being eliminated, we were cutting down the great forests of eastern North America and opening up vast new areas of pastureland that the cowbirds, and other grassland specialists, could expand into. The Brown-headed Cowbird was well on its way to an encore performance, to become one of the commonest and most widespread North American bird species.

The bird species that were “parasitized” by the cowbirds over the last million years in the central grasslands of North America were themselves co-evolving with the cowbirds and they have their own sophisticated defense strategies. The fact that they are all still there after so many generations, is a proof that the arms race between parasite and parasitized had reached a balance of sorts. But when the cowbirds expanded into eastern North America, some resident songbird species had not co-evolved with the cowbirds and many of them may well be particularly vulnerable to cowbird brood parasitism. Some rare bird populations (e.g., Kirtland’s Warblers or Least Bell’s Vireos) are today conserved in part by annual human trapping of cowbirds — a practice however that may not always be needed or effective (see the very good 2004 article in Birding by Stephen Rothstein). This “destructive” side of the cowbird has also earned it the dubious distinction of being the most reviled bird in North America, as it is widely, and probably unfairly, blamed with being the cause of diminishing native songbird populations, distracting our attention from what is almost always the real cause – human-induced loss of habitat. The cowbird is a brilliantly adapted native bird species itself, and if indeed it is negatively impacting bird populations in some areas, this is only as a consequence of our own actions, massively changing and upsetting entire ecoregions.

We didn’t see any bison at the World Heritage Site, except for piles of bones, and we didn’t see any buffalo birds either, but I’m sure they were around – the females carefully scouting around for an inattentive parent at a late nest, so they could slip in and lay an egg – and play out their destiny in their own way, as they have done in this ecoregion for a million years.