From southern Canada to Patagonia, from the temperate rainforests of the Northwest coast to the mangrove muck of the Caribbean, across most of the Americas, carrion draws a close-to-ubiquitous diner: the turkey vulture. A fine-focused new study suggests why this bald, boomerang-winged bird soars such a vast kingdom: basically, a champion schnoz.

It's long been known that the turkey vulture – "buzzard," in widespread North American vernacular, not to be confused with the many Old World hawks called by that name – tracks down carcasses by smell, and that the cousin with which it most overlaps, the black vulture, is a more vision-reliant forager. The new research, published this past December in Scientific Reports, confirms through anatomical analysis that the turkey vulture does indeed boast quite the sense of smell – not only compared with the black or other vultures, but birds in general.

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A turkey vulture (Photo: /National Park Service)

Along with a pronounced nasal cavity, turkey vultures possess a large olfactory bulb, a portion of the forebrain in vertebrates dedicated to scent. The Scientific Reports study aimed to take a deeper look at this structure. As Alicia Ault explained in a Smithsonian summary of the work, "Like the post-apocalyptic living dead roving the earth in zombie films, scientists needed fresh brains to determine exactly what was going on inside the turkey vulture's enlarged olfactory bulb."

The source of those fresh brains? A clutch of both turkey and black vultures legally culled by the United States Department of Agriculture in Nashville, Tennessee. After dissection, the researchers measured the relative volume of the olfactory bulb and, within, the number of mitral cells – which relay information collected by olfactory receptors to other parts of the brain – as well as the size of structures involved in the visual system. The volumetric measurements and mitral-cell count were compared with a wide variety of other bird species.

Turkey vultures claimed an olfactory bulb four times the size of black vultures' and twice as many mitral cells, even though the black vulture's brain is 20 percent larger. When judged against 143 other species of birds, the turkey vulture claimed the biggest olfactory bulb relative to brain volume. Compared with more than 30 other birds, meanwhile, turkey vultures also had the most mitral cells, though the researchers note this is likely simply a reflection of the outsized olfactory bulb.

Old World vultures – whose superficial resemblance to the vultures of the Americas is more a case of convergent evolution than taxonomic relation – are thought to locate carrion by sight alone, which likely explains why they're pretty much exclusively birds of open country. Among the New World clan, turkey vultures as well as their close Neotropical relatives, the greater and lesser yellow-headed vultures, are known for their ability to locate carcasses hidden in heavy forest – a reflection, it seems, of a potent sense of smell lacked by the other New World vultures: the black, the king and the two condors (California and Andean).

The authors of the Scientific Reports study note that turkey and black vultures diverged in the mid-Miocene, when a wide variety of large mammals flourished in North American savannahs and thus provided an ample supply of dead meat – and ample opportunity for vulture-on-vulture competition.

"Through the enlargement of its olfactory system, the turkey vulture was able to occupy a new sensory niche among vultures that depended on olfaction," they write. Today, the turkey vulture enjoys the largest range of any New World vulture – in fact, the largest range of any vulture in the world.

Turkey and black vultures continue to vie for (and rub shoulders around) carcasses from the southeastern US to central South America, pursuing different methods of scavenging. Turkey vultures tend to soar lower in the sky, the better to detect fetid wafts drifting up from critter corpses; black vultures cruise higher to boost their sightlines. Turkey vultures often forage alone or in small groups, whereas black vultures usually patrol en masse. Black vultures usually feed on larger carcasses, probably because those are the more visually conspicuous; turkey vultures can track down smaller, more obscure fare such as dead rabbits, snakes, birds and the like via olfactory detection.

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A black vulture (left) and turkey vulture (right). (Photo: Russ/Flickr)

One way black vultures get fed is by finding turkey vultures, which they typically displace from carcasses: generally speaking the black is more pugnacious and also tends to have numbers on its side. Thus the turkey vulture's superior sense of smell gives it a leg up in avoiding direct competition: it can (1) find carrion in thickly timbered country tough for an eyesight-oriented black vulture to forage in; (2) sniff out smaller nibbles that escape a black vulture's notice (and which would fail to feed a big scavenging flock of them); and (3) reach meat faster, giving it a mealtime head-start before black vultures arrive and muscle it off the prize.

(Interestingly, the Scientific Reports study didn't find evidence for black vultures boasting a keener sense of sight than turkey vultures.)

This makes for interesting ornithology on its own, but it's worth connecting to a bigger, more ecological message: vultures as a whole, Old World and New, perform vital ecosystem services as efficient, first-on-the-scene scavengers. Take these raw-headed carrion birds off the job, and there'd be a lot more putrid meat lying around.

As Ault notes in her Smithsonian article, South Asia has lately offered a cautionary tale: a catastrophic collapse in vulture numbers in India in recent decades – ultimately tied to the use of an anti-inflammatory drug in livestock – led to a buildup of carcasses, contamination of drinking water and a flourishing of scavenging feral dogs, along with a resultant spike in rabies transmission to humans.

So perhaps even the more squeamish among us should say a word of thanks to the turkey vulture's supersized olfactory bulb, and the awesome corpse-finding abilities it confers.

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Top header image: Pixabay