When wolves take prey, it usually doesn’t take long for ravens to appear. The birds are often already nearby and are waiting to get some of the leftover meat. For a long time the explanation for this seemed simple: ravens follow the wolves and thus find the carcasses. But whether this is really the case has hardly been investigated so far. A study from Yellowstone National Park now provides answers.
Ravens are among the most intelligent birds of all. They can remember places with reliable food sources, solve complex problems and fly to specific locations over long distances. Especially in winter, they often use the prey remains of large predators such as wolves to obtain food in the form of carrion.
Ravens and wolves tracked via GPS
But how exactly do the ravens do this? Matthias-Claudio Loretto from the University of Veterinary Medicine Vienna and his colleagues addressed this question. In their study, they examined the behavior of 69 ravens and 20 wolves over two and a half years in Yellowstone National Park in the USA. The national park was particularly suitable because wolves have been reintroduced there since the mid-1990s. The analysis focused on winter, when ravens come into contact with wolves most often. Additionally, the team considered data about when and where wolves killed prey – particularly elk, bison and mule deer.
The wolves were monitored using GPS collars, which are placed on about a quarter of the wolf population each year. The ravens were also observed. The team equipped them with tiny GPS transmitters. The transmitters recorded the ravens’ positions at intervals of up to 30 minutes and those of the wolves up to once an hour. However, catching the birds was not easy. “Ravens observe the landscape so carefully that they don’t fall into traps so easily,” explains Loretto. For example, traps near campsites had to be disguised with garbage and leftover fast food. “Otherwise the ravens would have noticed something and wouldn’t have come closer,” he says.
How do ravens find the wolf cracks?
The aim of the project was to test a previously untested assumption. “We all assumed that the ravens had a very simple rule: just stay close to the wolves,” says co-author Dan Stahler of Yellowstone National Park. “We didn’t know what ravens were capable of because they had never been the focus of this research; no one had taken the ravens’ perspective,” he says. But contrary to expectations, over the course of the two-and-a-half-year study, the researchers only found one clear case in which a raven followed a wolf for more than a kilometer and for over an hour.
But how did the birds manage to appear so quickly at the wolves’ prey sites? “To be honest, at first we were at a loss,” says Loretto. “When we discovered that ravens don’t follow wolves over long distances, we couldn’t explain for a long time how they still show up at wolf cracks so quickly.”
(Video: Max Planck Institute for Behavioral Biology)
Ravens remember wolf hunting areas
After a detailed analysis of the data, the researchers came to a new conclusion: Instead of following the four-legged predators over long distances, the ravens repeatedly returned to certain areas where wolves had previously frequently preyed. In the study area, wolf kills are more common in areas with certain landscape features, such as shallow valley bottoms, where wolves hunt more successfully, the team reports. Ravens visited these areas significantly more often than those where such cracks were rare.
According to Loretto and his colleagues, this suggests that ravens learn and remember the long-term “resource landscape” created by wolves. To achieve this, some ravens traveled up to 155 kilometers in a single day, flying on straight routes to areas where the chances of finding a carcass were high – although the time and exact location of a wolf tear cannot be predicted in individual cases.
New insights into how scavengers forage
“We already knew that ravens can remember stable food sources like landfills,” says Loretto. “What surprised us is that they also appear to be learning which areas are more likely to have wolf cracks. A single crack is unpredictable, but over time some parts of the landscape are more productive than others – and ravens seem to use this pattern to their advantage.”
However, it cannot be completely ruled out that ravens pursue the wolves over short distances. When they are nearby, they could also use clues such as the behavior of the wolves or their howling to find fresh prey. However, over longer distances, ravens’ memory seems to be particularly important: the birds remember areas where wolves often hunt successfully and search specifically for food there. The results suggest that spatial memory is more important in scavengers than long thought – perhaps also in other animal species that roam large areas and search for food sources.
Source: University of Veterinary Medicine Vienna / Max Planck Institute for Behavioral Biology; Specialist article: Science, doi: 10.1126/science.adz9467