Scientists thought ravens followed wolves for food. A 2.5-year GPS study told a different story.

For decades, biologists believed ravens found their next meal by shadowing wolves across the landscape until the predators made a kill. A new study in and around Yellowstone National Park suggests the birds are using a far more sophisticated strategy.Instead of constantly trailing wolves, ravens appear to remember the places where wolves are most likely to make a kill and fly straight to those areas, often covering huge distances without following predators at all.The findings are based on two and a half years of GPS trackin and suggest ravens build mental maps of hunting hotspots and use memory to predict where food is most likely to appear."They can fly six hours non-stop, straight to a kill site," said Dr Matthias Loretto, the study's first author.

Tracking ravens from the scavenger's perspective

To understand how ravens locate carcasses, researchers tracked 69 ravens, 20 wolves and 11 cougars in and around Yellowstone over two and a half years.The project generated more than 646,000 GPS locations from ravens, alongside tens of thousands of positions from wolves and cougars. Researchers compared those movements with confirmed predator kills to work out how the birds found food.What they discovered challenged a long-held assumption.

Rather than spending their time following wolves across the landscape, ravens repeatedly returned to areas where kills were more common. The birds often arrived at carcasses with remarkable consistency, even after spending long periods away from wolves."We all assumed that the birds had a very simple rule; just stick close to the wolves," said Yellowstone biologist Dan Stahler."We didn't know what ravens were capable of because nobody had ever put them at the center; nobody had taken the scavenger's point of view," he added.

Following wolves was surprisingly rare

Although ravens can follow wolves, the study found it almost never happens over long distances.Across the entire study, researchers recorded just one clear case of a raven following a wolf for more than one kilometre and for longer than an hour. In that instance, a wandering raven travelled alongside a wolf for around four kilometres over two hours.Instead, most birds relied on memory rather than constantly keeping wolves in sight.Some ravens flew directly to areas where carcasses were likely to appear, travelling as far as 155 kilometres in a single day. Others spent weeks or even months away from wolves before returning to familiar feeding grounds.One GPS-tagged raven followed the wolves on 48 different days, while the gap between visits ranged from an average of 15 days to as long as 363 days across all tracked birds.

A mental map of where food is likely to be

Researchers say the behaviour makes sense because wolf kills are not spread evenly across Yellowstone.Individual kills may be impossible to predict, but over months and years wolves tend to hunt successfully in the same kinds of places, including open valleys, flatter ground and snow-covered areas near streams and roads.Those patterns appear to help ravens build a mental map of the landscape."We already knew that ravens can remember stable food sources, like landfills," Loretto said."What surprised us is that they also seem to learn in which areas wolf kills are more common.

A single kill is unpredictable, but over time some parts of the landscape are more productive than others, and ravens appear to use that pattern to their advantage."Researchers believe ravens still rely on immediate clues once they are nearby. They may notice wolves moving, hear howling, watch other scavengers or respond to the behaviour of other ravens.But... memory seems to guide the birds' first decision about where to begin searching.

Why wolves are better guides than cougars

The study also found that ravens associated much more closely with wolves than with cougars.During the first seven days after an animal died, GPS-tagged ravens visited 48.5 per cent of wolf kills but only 24.8 per cent of cougar kills.Researchers say that difference reflects the hunting habits of the two predators.Wolves hunt in packs, usually in open country, and often leave large carcasses exposed.Cougars hunt alone, frequently in forests or rugged terrain, and commonly cover their kills with leaves, grass or soil, making them much harder for scavengers to locate and access.

Catching clever birds

While wolves in Yellowstone have been fitted with GPS collars for years, ravens presented a different challenge.Researchers first had to catch and tag the birds before they could begin tracking their movements."Ravens are so observant of the landscape that they don't step into traps easily," Loretto said.To avoid raising suspicion, the research team disguised traps with items that blended naturally into the surroundings, including discarded rubbish and fast-food packaging near campsites."Or else the ravens would suspect that something was off and wouldn't come near it," Stahler said.

Changing what scientists know about ravens

Ravens are already recognised as some of the world's most intelligent birds. They store food for later, remember where they have hidden it, steal from other animals and can respond to what other creatures know or can see. Previous studies have even suggested they are capable of forms of future planning once thought to be limited to great apes.The new research adds another skill to that list.Instead of simply remembering individual food caches, the birds appear able to learn the hunting patterns of another species across an enormous landscape and use that knowledge to decide where to forage."Ravens can cover large distances by flying, and they seem to have a good memory, so they don't need to constantly follow wolves in order to profit from the predators," Loretto said.Senior author Professor John M. Marzluff of the University of Washington said the findings reshape scientists' understanding of how scavengers locate food."What our study clearly shows is that ravens are flexible in where they decide to feed. They don't stay tied to a particular wolf pack. With their sharp senses and memory of past feeding locations, they can choose among many foraging opportunities far and wide.

This changes how we think about how scavengers find food, and suggests we may have underestimated some species for a long time," he said.The study, "Ravens anticipate wolf kill sites across broad scales", was published in the journal Science by researchers Matthias-Claudio Loretto, Kristina B. Beck, Douglas W. Smith, Daniel R. Stahler, Lauren E. Walker, Martin Wikelski, Thomas Mueller, Kamran Safi and John M. Marzluff.