Plight of the Undertakers – the extinction of carrion birds in the Americas

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This is a guest article, kindly contributed by Ilari Pätilä, a bachelors student of Geoscience based at Helsinki university in Finland. Ilari can be found on Deviantart, as well as his twitter profile @IlariPatila.

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The job of an undertaker is likely one of the most stable professions out there. After all, everyone will be dead at some point. And in the natural world, the dead are a free and relatively easy meal ripe for the taking. Of all the scavenging vertebrates, birds are among the most efficient and diverse. In fact, the only group of land vertebrates adapted to feed almost exclusively on carrion are the vultures (1). To achieve this, vultures have multiple special adaptations that make them very skilled at their profession as the flighted undertakers of the biosphere. For example, vultures have enormous wings which let them soar for vast distances and at high altitudes with little effort (1). This, combined with keen eyesight and with some species an excellent sense of smell, means that vultures can easily and efficiently find food (1). Not only that, but vultures are supremely efficient at getting that food inside their bellies. In fact, old world vultures in African national parks consume more flesh than all the big cats combined (2)! That food also needs to be digested, and vultures have just the tool for that. The Ph of a vulture's stomach acid is around 1-2. These highly acidic stomachs can not only digest rotten flesh and viscera, but it also kills off most pathogens' vultures may ingest, like anthrax (3). Considering all this, vultures are doing a very important job as flighted undertakers. In this way, like winged janitors, they keep the bush clean and even reduce the possible number of diseases in a given ecosystem (1). In the old world, there exists a multitude of vultures, each specializing in their own niche to avoid competition (28, 29). Indeed, some old-world vultures, like the Lappet-faced vulture (Torgos tracheliotos), are among the biggest and most imposing of raptors (26). However, in the New World, things are different. Most new-world vultures are relatively small compared to most old-world vultures, and there are only two especially large species, the California condor (Gymnogyps californianus) and the Andean condor (Vultur gryphus), both under threat of extinction (35, 31, 32). And the other major group of carrion birds living in the Americas, the caracaras, are merely generalists who do not specialize in eating carrion, but will gladly take it if available (38). When looking at the fossil record, one will see an astonishing variety of carrion birds (4). But today that diversity is only a portion of what it used to be. Why did so many carrion-eating birds go extinct? The answer to that is the extinction of the megafauna, but how did the ones alive today manage to survive? Why did some species perish while some didn’t? And what exactly was left behind of this past diversity?

Before answering these questions, one must raise a different one. Who exactly were these American flying undertakers? The most important and specialized group were, as they are today, the vultures. However, the word vulture is used for two very different bird groups that are not that closely related. Old world vultures belong to the great family of raptors known as Accipitridae (26). These include species found in Africa, Europe and Asia, and are very familiar to anyone who watches lots of wildlife documentaries. Despite being called the old world vultures, members of this group used to live in North America. During the Pleistocene, that included two species, both predominantly found from the La Brea tar pits (11). New world vultures on the other hand belong to an entirely different raptor family, the Cathartidae, wholly endemic to the Americas, and today includes seven species. They range in size from the moderately sized Turkey vulture (Cathartes aura) to the Andean condor (Vultur gryphus), one of the largest flying birds alive today (35, 32). They are almost exclusive carrion feeders, with the notable exception of the Black vulture (Coragyps atrata), which is an opportunistic feeder of both carrion and live prey (34). The fossil record of new world vultures is quite impressive, ranging from enormous birds like Breagyps, which was even larger than the California condor, to Wingegyps, a condor smaller than a turkey vulture (6, 26). Fossil species are widely known from both of the Americas, and without a doubt these birds were taking advantage of the smorgasbord of megafaunal carrion.

Fig 1. The stormy Patagonian coast got the better of an elephant seal bull. A feast is ready, but the flock of Caracara sp. and the giant petrel are not keen to share. The carcass is attracting more giant petrels, a condor, and even a stork, Ciconia lydekkeri.

Terms of use: Artwork by the author

While vultures are almost exclusive carrion feeders, the caracaras couldn’t be more different in their feeding habit. Despite their eagle-like appearance, they are in fact the largest members of the falcon family Falconidae (36). Carrion is a big part of the diet of most caracaras, but they don’t eat it exclusively. In fact, caracaras are highly opportunistic feeders. Some, besides carrion, eat small animals, like lizards, rodents and large arthropods, and some even eat fruit (30, 43)! Caracaras are also well known for their aggressive behaviour, and are sometimes observed ganging up in order to kill larger prey items. The Striated caracara (Phalcoboenus australis) is especially infamous for this behaviour, killing adult penguins and even sheep in this manner (41). The diversity of caracaras during the Pleistocene is quite astonishing, as it includes both small species, like Milvago brodkorbi, as well as Caracara major and an undescribed species from Argentina, both weighing at over 3 kilograms (20, 21, 22). This diversity of body size indicates different niches, and considering that the large extinct species of caracara were probably as aggressive as modern ones, lots of unpleasant sights.

Teratorns are among the most famous and mysterious of American carrion birds belonging to the family Teratornithidae. The group is entirely extinct today, but are likely closely related to the new world vultures (12). Teratorns were enormous animals, as far as birds are concerned. Teratornis merriami has been estimated to have a wingspan of 3-4 meters, while the enormous Aiolornis incredibilis was estimated to have a wingspan of over 5 meters, making it the biggest flying bird during the Pleistocene (5). While the astonishing size of teratorns is under no doubt, their ecology is. Teratorns were long thought to be analogues of vultures, but that particular hypothesis has been put into question (15). Ecomorphological analyses of raptor skulls have indeed shown that teratorns were not well adapted for scavenging compared to vultures (9).  Another paper concluded, using morphological evidence, that teratorns lived like storks, swallowing small prey items whole and striding across the ground (15). However, nitrogen isotope data found that teratorns were eating the remains of both grazing and browsing megafauna, and one individual was even eating a diet of seafood (8)! All this evidence may point to teratorns as flexible and opportunistic feeders that primarily consumed carrion as well as smaller live prey items that were swallowed whole. Like, gigantic vulturine caracaras.

Fig 2. Megafauna is a great source of food for large carrion birds. In fact, this carcass has attracted California condors, Teratorns, and even a Breagyps clarki from far and wide.

Terms of use: Artwork by the author

Species	Distribution	Family
Gymnogyps californianus	Formerly most of United States and Mexico (31)	Cathartidae
†Gymnogyps amplus	Western United States (31)	Cathartidae
†Gymnogyps howardae	Peru (22)	Cathartidae
†Breagyps clarki	California (6)	Cathartidae
Cathartes aura	Americas from southern Canada to Tierra del Fuego (35)	Cathartidae
Cathartes burrovianus	Tropical Central and South America (36)	Cathartidae
Cathartes melambrotus	Amazon basin (37)	Cathartidae
Coragyps atratus	Southern US to Argentina (34)	Cathartidae
†Coragyps occidentalis	California (34)	Cathartidae
Sarcoramphus papa	Tropical Central and South America (33)	Cathartidae
†Sarcoramphus papa sacra	Florida (25)	Cathartidae
†Sarcoramphus? fisheri	Peru (22)	Cathartidae
†Wingegyps cartellei	Brazil (24)	Cathartidae
Vultur gryphus	Western South America from Argentina to Colombia (32)	Cathartidae
†Geronogyps reliquus	Peru, Argentina (22,19)	Cathartidae
†Pampagyps imperator	Argentina (23)	Cathartidae
†Pleistovultur nevesi	Brazil (16)	Cathartidae
†Teratornis merriami	California to Florida (8)	Teratornithidae
†Teratornis woodburnensis	Oregon (13)	Teratornithidae
†Cathartornis gracilis	California (11)	Teratornithidae
†Aiolornis incredibilis	Nevada, California (5)	Teratornithidae
†Teratornithidae gen. et sp. indet	Argentina (18)	Teratornithidae
†Neogyps errans	California (11)	Accipitridae
†Neophrontops americanus	California (11)	Accipitridae
Caracara plancus	Southern North America to South America (38)	Falconidae
†Caracara prelutosus	North America (38)	Falconidae
†Caracara major	Uruguay (20)	Falconidae
†Caracara sp.	Argentina (21)	Falconidae
†Caracara seymouri	Peru (17)	Falconidae
Milvago chimachima	Tropical South America (39)	Falconidae
Milvago chimango	Southern South America (40)	Falconidae
†Milvago brodkorbi	Peru (22)	Falconidae
Phalcoboenus australis	Tierra del Fuego (41)	Falconidae
Phalcoboenus megalopterus	Peru to Argentina (43)	Falconidae
Phalcoboenus carunculatus	Colombia and Equador (42)	Falconidae
Phalcoboenus albogularis	Southern parts of Chile and Argentina (44)	Falconidae

Table 1: List of carrion-feeding birds found from the Pleistocene of continental North and South America

The Culture of the Vulture

The diet of carrion-eating birds, especially vultures, can be described as offal. And they eat it in vast quantities. However, with vultures, condors, caracaras as well as teratorns eating from the same source of food, competition over carcasses was likely fierce. To avoid injury as well as competition for a comparatively unreliable source of food, American carrion birds were likely filling different ecological roles and eating different things. This happens even today. For example, in Africa, the Lappet faced vulture is dominant, driving away other vultures while mostly eating skin and tendons (26). The griffon vultures, on the other hand, consume flesh and organs, snaking their long necks right into the body cavity (29, 30). These vultures are dominant over the diminutive Hooded vulture (Necrosyrtes monachus), which eats the scraps of other vultures, while the White-headed vulture (Trigonoceps occipitalis) is an opportunist, mostly eating carrion of smaller animals and being a surprisingly competent predator (27, 28). There is even the Bearded vulture (Gypaetus barbatus), which specializes in eating almost exclusively bones (45). A similar phenomenon is observed in the New World, with King vultures being dominant over smaller vultures (33). And among smaller species, aggressive Black vultures and caracaras are dominant over less aggressive Turkey vultures (35, 38). In summary, larger species are more dominant than smaller species, and aggressive species are dominant over less aggressive species. While such interactions can’t be precisely studied in extinct raptors, it can be speculated that large condors and vultures drove smaller species away from carcasses, and were only allowed to eat when the bigger species were finished. Between these larger taxa there may have been different preferences to diet. For example, the extinct Breagyps clarki may have used its bill to eat viscera deep inside the carcass, while the California condor eats soft parts of the carcass (7, 31). Caracaras and teratorns may have had different roles at carrion sites. Being opportunistic, they might have eaten any scraps of meat available (38). It can also be speculated that teratorns may have used their massive size to chase away smaller scavengers to get to their preferred bits, like the Lappet-faced vulture today (26). This may have been how the carrion bird’s banquet played out for hundreds of millennia. But when the megafauna started to go extinct at the end of the ice age, the carrion birds that were relying on them were in big trouble.

Fig 3. Somewhere in the US midwest, a turkey vulture and a flock of black vultures have gathered around the remains of a dead mammoth. There is no food left in the bones, but it's a good place to wait for more meals.

Terms of use: Artwork by the author

The End of the Banquet

After the Quaternary extinction event, only a few carrion birds hung on. When the megafauna, the main source of vulture-food, was taken away, most carrion birds faced severe problems (4). Not surprisingly, the caracaras suffered only minor losses mainly due to their opportunistic feeding habits (42).

However, it was the large, vulturine birds that suffered the most. During the Quaternary extinction event, old world vultures were extirpated from the Americas, and the teratorns had been wiped out entirely (4). The extinction of teratorns in particular is not well understood, because they were likely not specialized scavengers, but opportunistic feeders (8). Speaking of which, new world vultures took a big blow from the extinction event, as their main source of food was mostly unavailable. In fact, the vast majority of extinct new world vultures were quite large birds that probably relied on megafauna for sustenance (4). And when large food sources were gone, large vultures had difficulty keeping themselves and their chicks fed and became extinct. For the ones that were able to survive, it was time to adapt, and fast. And desperate times call for desperate measures.

There are only seven species of new world vultures alive today. Of these, four are generalists and readily feed on the remains of smaller animals, while the king vulture eats remains of small to medium-sized vertebrates. This leaves the California and Andean condors, some of the largest flying birds alive today. Of these, the Andean condor survived by eating the remains of mountain animals like guanacos (Lama guanicoe) (32). The California condor, on the other hand, took a very different approach. The populations of California condors around the Pacific coast gradually switched from a megafauna-based diet to a diet of marine mammal carcasses (8). So, in a way, they didn’t stop eating megafauna, they just started eating a different type of megafauna. In fact, after commercial whaling made seals and whales rarer in California, the condors started eating the remains of livestock as their main food source (31).

Today, vultures are in trouble. Many vultures rely on livestock, like sheep and cattle, as their main source of food. However, there’s a problem. Some medicines commonly used in livestock, especially diclofenac, are deadly poison to vultures (1). Not only that, but vultures face multiple threats, like persecution, destruction of habitat, as well as lead poisoning (1).  Vultures in the new world face similar problems. Lead poisoning and loss of habitat caused the near extinction of the California condor, and the Andean condor faces threats from persecution and the consumption of diclofenac from dead cattle (31, 32). Caracaras, too, were historically heavily persecuted (38, 41). Losing carrion birds wouldn’t just be losing a few majestic raptors, but also the loss of species doing a crucial service. Eating carrion and thus reducing the number of pathogens. In fact, the loss of vultures in some areas increased the number of rabid dogs and the number of bubonic plague cases (1). The job of an undertaker is important. If there is no undertaker, dealing with the dead becomes a lot harder.

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