High Society – The social network of vultures

When you think of vultures, you might just think “feathered carrion-eaters”.  But do you think of their social networks, foraging strategies, and daily lives?  Probably not!  Dr Adam Kane (University College Cork) and Dr Kevin Healy (University of St Andrews) are postdoctoral researchers working on the behaviour, conservation and evolutionary ecology of scavengers.  As International Vulture Awareness Day was on the 3rd September, they have offered to share some insights into vulture social intricacies.

Scavenging is clearly not a lifestyle choice for everyone. Even though many animals will have the occasional meal of carrion, few species can make a living of it. In fact, vultures are the only known obligate vertebrate scavengers. Both the old and new world forms have been picking the bones of carcasses across the globe for millions of years. So how have vultures managed to succeed in a lifestyle no other vertebrate has figured out? One of the keys to success are their social networks.

Immature African White-backed vulture threat display (Photo: Andre Botha)

The social network of vultures, a network based on the flow of information, is a matter of life and death for these birds. To see how, let us consider a day in the life of an African White-backed Vulture Gyps africanus and meet the animals who make up her social world.

As she wakes and looks around the colony she can see many others of her kind tending to their nests, preening and waiting for the heat of the sun to create pockets of hot air necessary for flight. White-backs hitch a ride on these thermals as they soar over the Savannah, a much cheaper form of flight than flapping your wings for a bird of her size.

Along with a mate, our vulture has selected an Acacia tree out of reach of the others for her nest to reduce the number of fights with noisy neighbours, allowing her to focus her attention on her chick. Two pairs who chose the same nesting tree squabble at a distance, their young noticeably thin.

Her mate was unsuccessful in his five-hour foraging trip yesterday which means finding food for their chick today is vital. Luckily, yesterday she noticed the engorged crops of returning birds and sets out following them in the hope that they will lead her to a carcass. A single vulture can eat just over a kilo of carrion, thus the carcasses of especially large herbivores like elephants or hippos can persist for many days.

Vultures on a rhino carcus (Photo: Andre Botha)

Her vision is extremely well developed and she is able to take in her surroundings from at least 8 km away. When coupled with a cruising speed of 50 km per hour at an altitude of 500 m, a White-back can sweep over huge areas where the relief of the land proves no obstacle to the view.  She notices a bird making a sharp descent to the ground below but it’s not a vulture, it’s an eagle. She abandons her pursuit of her fellow vultures and instead flies down to the eagle, a smaller bird who also scavengers and is a lot easier to bully than one of her own. This pays off, the eagle has found the carcass of a recently drowned young wildebeest washed up on a riverbank.

Like the eagle, her actions don’t go unnoticed, and nearby vultures cue to this behaviour, a signal that food has been discovered. Because the birds are concentrated around the roost they end up being concentrated in the air which allows many of them to react to a find. Still, there is a prize for arriving first, the finder’s fee, a portion of food you can eat unmolested. Her preference, like all White-backed Vultures, is for the viscera. But time is of the essence here, so our bird has to gorge while she can, before the rest of the wake arrives.

Vultures feeding (Photo: Andre Botha)

Together, the whole group encircles the carcass and, in between the bickering, make short work of the wildebeest. Yet, the information flow hasn’t ended with the White-backs. Fifty vultures with wingspans in excess of two metres plummeting to the Earth against a blue sky represents an eye-catching, visual dinner bell to the ground-dwelling carnivores of the surrounding area.

A pair of Black-backed jackals are among the more inhospitable guests next to arrive. These opportunistic scavengers take what they can get from the carcass and are more than a match for a 5 kilo bird who cannot afford an injury in a one-on-one contest. However, a group of 10 unsated White-backs are able to hold the jackals at bay through weight of numbers, another advantage of dining together.

Immature vultures posturing (Photo: Andre Botha)

The pack of Spotted Hyenas is a different matter. These powerfully built animals are more than capable of cracking open bones and are happy to be last to the party, even if nothing but the skeleton remains. The sight of the hyenas causes the rest of the scavengers to scatter, aside from one luckless vulture, who, too heavy to fly with its engorged crop, is caught and killed by the pack. The dispersed birds make a bee-line for home. Again soaring between the thermal pockets before the heat of the sun is lost in the evening. Once home, the female feeds her chick and rests, her foraging duty is done and it is now the turn of the male. A pattern that will repeat until the chick fledges.

A remarkable fact about the information flow in this account is that practically all of the signals are inadvertent. Returning vultures don’t appear to actively recruit followers for the next day’s foraging trip and birds who do discover carrion don’t make their descent any more conspicuous than they have to. The scavenging community has instead evolved to keep an eye on their neighbours, be they related or otherwise.

Adult vulture intimidating juvenile (Photo: Andre Botha)

Of this community, it is solely the vultures who have managed to couple social cues with an ability to efficiently cover vast areas. This high ratio of information gain to energy spent has allowed vultures to live as full-time scavengers, while other species can only manage it part time. And though, hyenas, lions and other large predators over geological time, like T.rex and other theropod dinosaurs, may often have/had a competitive advantage at a carcass, it is/was simply too costly for them to cover the ground required to find enough carrion.

The dependence of vultures on a well-connected network can be their undoing. if their populations drop below density thresholds, individuals can no longer see fellow vultures dive towards a carcass, and so their ability to find food drops dramatically. A likely scenario for many persecuted vulture species. It looks like staying plugged into the scavenging social network is key to the vultures’ unique lifestyle.

References

Duriez et al. (2014). How cheap is soaring flight in raptors? A preliminary investigation in freely-flying vultures. PLoS One, 9(1), e84887.

Dermody et al. (2011). The evolutionary pathway to obligate scavenging in Gyps vultures. PLoS One, 6(9), e24635.

Fernández‐Bellon et al. (2016). Density‐dependent productivity in a colonial vulture at two spatial scales. Ecology, 97(2), 406-416.

Harel et al. (2017). Social foraging and individual consistency in following behaviour: testing the information centre hypothesis in free-ranging vultures. In Proc. R. Soc. B (Vol. 284, No. 1852, p. 20162654). The Royal Society.

Jackson et al. (2008). The effect of social facilitation on foraging success in vultures: a modelling study. Biology Letters, 4(3), 311-313.

Kane et al. (2014). Vultures acquire information on carcass location from scavenging eagles. In Proc. R. Soc. B (Vol. 281, No. 1793, p. 20141072). The Royal Society.

Kane et al. (2017). A recipe for scavenging in vertebrates–the natural history of a behaviour. Ecography, 40(2).

Kane et al. (2016). Body size as a driver of scavenging in theropod dinosaurs. The American Naturalist, 187(6), 706-716

Kane et al. (2017). Understanding how mammalian scavengers use information from avian scavengers: cue from above. Journal of Animal Ecology, 86(4), 837-846.

Subalusky et al. (2017). Annual mass drownings of the Serengeti wildebeest migration influence nutrient cycling and storage in the Mara River. Proceedings of the National Academy of Sciences, 114(29), 7647-7652.

Spiegel et al. (2013). Factors influencing foraging search efficiency: why do scarce lappet-faced vultures outperform ubiquitous white-backed vultures? The American Naturalist, 181(5), E102-E115.

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