With its menacing appearance and grisly diet of carrion, the vulture doesn’t seem like a bird that would have a lot of friends. Yet some vulture species, according to the book Carrion Dreams 2.0: A Chronicle of the Human-Vulture Relationship, “are among the most social of all large birds, routinely gathering in the hundreds at roosts and carcasses.”
These vast social networks aren’t about companionship, but survival. Scavenging is a tough way to make a living. Death can take an animal any time and anywhere, so a vulture’s meals are scattered unpredictably in both time and space. While the New World vultures have keen senses of smell and can sniff out a dead rat hidden in a leaf pile, their Old World counterparts in Africa don’t have the sharpest noses. Instead, they rely on their eyes to find food, and the more eyes that are looking, the better. By foraging in groups and watching each other as well as the ground, the vultures get their meals more easily. “Although some fresh carcasses are located directly by searching birds, the majority of individuals find food by following other vultures,” zoologist Andrew Jackson writes. Once one vulture spots a carcass, “a chain reaction of information transfer extends from the carcass as descending birds are followed by other birds, which themselves cannot directly see the carcass, ultimately drawing birds from an extensive area over a short period of time.”
Now, new research suggests that vultures’ social foraging behavior is even more complex than that. They not only look to each other for cues about food location, but will also follow other bird species to carcasses, scavenging information as well as meat.
For their study, Jackson, zoologist Adam Kane, and other researchers from Europe, Africa, and the U.S. set out goat and cow carcasses in a savanna in Kenya and then waited and watched as African white-backed vultures, Rüppell's vultures, and tawny and steppe eagles made their way to them. They found that the eagles arrived at the carcasses first most of the time, but the vultures were never far behind. In fact, the vultures followed the raptors more often and more closely than the scientists expected by chance, indicating that they weren’t just finding the carcasses on their own, but using the eagles as guides.
Following the raptors doesn’t just tip vultures off to the location of a meal; it also makes it easier to eat. Eagles have stronger beaks and can tear into tough hides that vultures can’t. After the eagles get to a carcass, open it up and take a “finder’s fee” of meat, the vultures can swoop in, bully them away, and take what’s left.
That vultures scrounge intel from eagles isn’t just a neat party trick; it also has implications for keeping them around. Gross as their eating habits can be, vultures play a vital role in many ecosystems by consuming carcasses that might otherwise rot and become reservoirs for pathogens. But because of habitat loss, hunting and intentional and accidental poisoning, more than half of the world’s vulture species are declining in number and at risk for extinction. With their social way of finding food, the loss of some vultures puts those that are left in a precarious position. “With every individual lost, the network is less effective at detecting carrion,” the researchers say.
If vultures also find carcasses with help from eagles, the team says, then they’re likewise “vulnerable to declines in raptor populations as their ability to locate food will also decline.” The researchers used a computer simulation that accounted for vulture and eagle flight heights, foraging times and visual acuity to test that idea, and found that the vultures fared better when there were more raptors, and worse when there were fewer.
No animal, not even the vulture, exists in a vacuum. They rely, in part, on eagles to find and prepare their food, and the health of these other species could make the difference between feast or famine. It goes to show that you can’t just save one species at a time, but have to think about the whole ecosystem with all its moving, flying parts.