The Serengeti and Masai Mara in Africa are home to one of the most spectacular animal migrations on the planet: More than a million wildebeest travel nearly 1,600 kilometers round-trip each year, going from Tanzania to Kenya and back again, following the rain and green grass.
If you’re a crocodile, you might take advantage of this grand spectacle by staking out a spot where wildebeest cross the Mara River. A cheetah or lion might prefer to hunt wildebeest when they’re giving birth in the Serengeti. But if you’re a vulture, following a bunch of fat and happy wildebeest probably means you’re going to go hungry. These carrion-eating birds usually follow the wildebeest herd only during the dry season, when there’s more likely to be dead animals along the migration trail, reports Corinne Kendall of Princeton University and her colleagues in a study published January 8 in PLOS ONE.
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Kendall and her colleagues kitted out 39 vultures (15 Ruppell’s vultures, 12 white-backed vultures and 12 lappet-faced vultures) from the Masai Mara National Reserve in Kenya with GPS transmitters and tracked them for months as the birds flew across East Africa. The three bird species each had slightly different patterns of flight — with similar lifestyles, they have to figure out how to not get in each other’s way — but they all showed a preference for visiting the wildebeest herds in Kenya in the dry season, July to October.
That suggests that the vultures take into consideration not only their potential prey’s location but also how likely it is that the prey will die and provide the vultures with a meal. These birds are specialized for eating dead things, after all. Because low rainfall should result in less food for wildebeest and more wildebeest deaths, the best time to find dead wildebeest is during the dry season.
During other times of the year, the vultures took different paths to finding meals. Some went to areas outside the migratory paths, probably trying to find dead animals among the region’s nonmigratory populations. Some headed for other areas of Kenya, such as the Tsavo National Parks. One Ruppell’s vulture took off for three months to a region in Sudan and Ethiopia where a species of antelope, the white-eared kob, follows its own migratory route.
“Despite the fact that migratory wildebeest herds consistently represent the greatest prey abundance in this landscape, vultures selectively associate with them only during the dry season,” the researchers write. “Our study suggests that prey mortality may be a more important driver of vulture habitat use than prey abundance.”
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The vultures’ dependence on the migratory wildebeest during the dry season could be a problem for the birds, though. It’s during those months that the vultures breed, and a successful breeding season depends on having lots of food. “Given that resident wildlife populations are declining rapidly throughout Kenya, particularly in the Masai Mara National Reserve, food availability is likely to become a major issue for vulture survival in the near future,” the researchers write. Vulture species are already declining in other parts of Africa and the world; East Africa’s birds may soon follow.