Vultures were long thought to make few and fairly simple sounds. Yet no one had investigated in detail how the birds use sound to communicate. For his master's thesis in Jorg Massen's Vulture Cognition Lab, Connor Proudfoot studied the critically endangered Rüppell's vulture. He and his colleagues found that the birds produce a wide range of complex sounds, which they use in a range of social situations. The findings, published today in PLOS One, could potentially help improve the success of captive breeding programmes and shed light on the evolution of communication.
When you think of a vulture, you might picture a solitary bird perched silently on a branch or soaring through the sky. But some vultures are highly sociable. For example, Rüppell's vultures, found in the Sahel and East Africa, live in flexible social groups and breed in colonies of up to a thousand birds.
Complex environment, complex communication
"Within the groups, there's a hierarchy: some birds rank higher than others," says Proudfoot. "In such a large colony, the animals also sometimes form smaller groups to forage together. And when they find a carcass, they often encounter dozens of other vultures and have to compete for a place. You would expect a species living in such a complex social environment to have sophisticated ways of communicating, so we decided to look into that."
Adult birds, chicks and eggs
Proudfoot spent 150 hours recording Rüppell's vultures at two colonies, one at Diergaarde Blijdorp in Rotterdam and the other at Avifauna in Alphen aan den Rijn. Because eggs at Blijdorp are incubated artificially, the researchers were also able to record the eggs and chicks there. In total, they collected 2,500 "vocalisations", sounds produced by the birds' vocal organs.
The researchers analysed each sound individually, measuring its duration and pitch and counting the number of distinct "syllables" or notes it contained. This allowed Proudfoot and his colleagues to identify 16 different call types.
Notably, some calls were clearly distinct, while others blended into one another. "That blending of calls shows there is flexibility in how the birds communicate," says Proudfoot. "For example, they can convey not just that they are excited, but also how excited they are."
Similar to macaques
Sixteen calls, some of them highly variable. Is that a lot? According to Proudfoot, it is. "A study of more than 800 bird species found that birds have an average of six to eight calls. Rüppell's vultures therefore have more calls than the average bird."
Proudfoot continues: "These vultures communicate vocally in ways similar to macaques and other monkey species."
More fertilised eggs
Could this research help conserve Rüppell's vultures, which are listed as Critically Endangered on the IUCN Red List? Proudfoot and his colleagues found that chicks begin vocalising inside the egg two days before they hatch. "In zoos, eggs are often incubated artificially, so the parents can't hear the chicks calling from inside the eggs. As a result, they may be less prepared for the chicks' arrival. Perhaps more chicks could be successfully reared if their parents could hear those sounds."
The researchers also think sounds could help increase the number of fertilised eggs. "The sounds made by a mating pair may encourage other pairs to mate, too. Playing these sounds through speakers could therefore help stimulate breeding."
Evolution of communication
The research also offers insight into how complex communication evolved. "There is a theory that more social species have more complex ways of communicating," says Proudfoot. "That seems to be true for primates, including humans. But to find out whether it applies more broadly, we need to study other animal groups too. Vultures are ideal for this. Even though they often live in similar environments and eat the same food, some species are highly social, while others are much less so. The question is whether this difference is reflected in the complexity of their vocal repertoires."
