tags: ravens, intelligence, behavior, birds, ornithology



Common Raven, Corvus corax. Image: John James Audubon.

Some of you know that Bernd Heinrich has spent many winters studying ravens and their behavior. This month, Heinrich and his colleague, Thomas Bugnyar, published an article in Scientific American that explores the intelligence of ravens. In this article, they investigate the question; do the birds consciously contemplate alternative behaviors and choose the most appropriate ones, or are they merely relying on instinct or learning to perform specific actions by rote?

They begin by noting that ravens are not the only birds that are reputed to behave intelligently. They state that other relatives of the ravens -- the corvids, such as crows, jays, magpies and nutcrackers -- appear to possess surprising and sophisticated mental abilities. They even mention that these birds' capacities appear to be equivalent to or to even surpass those of the great apes. For example, nutcrackers have the capacity to recall thousands of locations where they have cached food items -- a capacity that exceeds that of humans.

Which leads us to ask; do corvids rely on logic to solve problems or are they relying on instinct? Do corvids distinguish between each other and alter their behaviors accordingly? To more precisely determine the mental capacities of ravens, the largest of the corvids, Heinrich and Bugnyar designed several tests. The first experiment consisted of food hanging from a string below the bottom of the wire cage (pictured right, bigger). To get this treat, the bird had to reach down from a perch and grasp the string in its beak, pull up on the string, place the loop of string on the perch, step on this looped segment of string to prevent it from slipping down, then let go of the string and reach down again and repeat its actions until the morsel of food was within reach.

They found that some adult birds would examine the situation for several minutes and then perform this multistep procedure in as little as 30 seconds without any trial and error -- as if they knew exactly what they were doing. Because there was no opportunity for the birds to be confronted with a similar problem in the wild, the simplest explanation is that they were able to imagine the possibilities and to perform the appropriate behaviors. The authors also found that successfully performing this behavior required maturity: immature birds were unable to do it while year-old birds performed a variety of trials before they were able to succeed.

But was it logic that the birds relied on to solve this problem? The authors assert that, basically, knowing how to do something requires few or no trials, whereas trial-and-error learning requires no logic. In fact, it was possible that the birds were rewarded by having the meat become closer with each looping behavior. So as a result, the authors designed another experiment to find out how the birds were solving the problem by presenting them with a situation that was not immediately rewarding because it was counter-intuitive: a string that must be pulled down to cause the food to move upwards towards the bird (pictured left, bigger).

In this situation, the ravens were still interested in the food but none of them managed to solve the problem of obtaining it even though they would have had to use the same sequence of actions. The authors concluded that the pull-up method of obtaining the meat was mastered quickly because it was logical -- a capacity that is lacking or present only to a limited extent in most animals.

Thinking and logic can be quite unreliable and can cause their own set of problems. For example, paper wasps rely on precise hard-wired behaviors to manufacture paper into a nest with a very precise architecture. No learning is required to create the nest, although the environment can modify some genetically programmed behaviors. So why are corvids different? What is special about their social environment that favored the evolved of intelligence as the source for complex behaviors?

Much of the natural history of ravens suggests that they evolved under circumstances that required them to cope with rapidly changing short-term situations. These birds are opportunists who do some hunting on their own, but are mainly dependent upon food that other animals have killed. The predators that inadvertently provide the birds with food are unpredictable and thus, can also kill ravens. Under these circumstances, trial-and-error is evolutionarily untenable because the first mistake in dealing with an unpredictable predator could cost the birds their lives.

Food bonanzas provided by mammalian carnivores are often quickly consumed by them. As a result, it pays ravens to get an early start in feeding -- often, side-by-side with these carnivores. To do this, the birds must be able to predict the carnivores' behavior, such as when they might attack, how far they can jump and how to distract them, and some of that knowledge needs to be in place before the bird itself is distracted by feeding.

Juvenile birds learn these things early in life by interacting with the predators through testing their reactions. Juvenile ravens often will land nearby and nip them from behind. This so-called risky "play activity" is dangerous but ultimately aids in the birds' survival by providing information about the capabilities of various predators. By deliberately provoking them, ravens learn which animals they can trust and how far away they must stay to remain safe.

Ravens also cache food -- busily hauling it away, burying it in secret locations and eating it later. Because ravens have a nearly nonexistent sense of smell, they must memorize the precise location of this stored food, as is the case for other birds that also engage in caching. However, unlike most other caching birds, ravens observe caching of their competitors and thereby memorize the precise locations of not only their own caches, but also those of their competitors. Because of this, ravens prefer to cache their food in private.

As newly-fledged birds that are still being fed by their parents, young ravens practice caching by hiding inedible items. Not only are the young birds learning which items are edible, but equally important, they were also learning to predict their siblings' behavior -- namely, cache pilfering. To better understand practice caching and pilfering behavior, the authors acted as surrogate parents to several young birds. One person was designated the "thief" and always stole a young bird's cache, whereas the other person consistently examined the young birds' caches but never pilfered them.

When the thief was nearby, the authors found that the young birds significantly delayed the time they waited until they cached their food, and they relocated those caches they previously made. In contrast, the presence of the nonthief did not elicit these behaviors. These experiments reveal that the young birds improved their food-caching skills after others raided them, but they also learned to distinguish individuals, in this case, human thieves from human nonthieves.

Because wild ravens typically feed in groups, it is nearly impossible for them to cache food without another bird catching them in the act, thereby learning the cache's precise location. Thus, it is important that ravens identify individual birds, just as they are able to distinguish between humans. So the authors designed another experiment where they tested this ability in ravens.

A large aviary was designed for caching. Inside a smaller cage within this aviary were two ravens; one was a "knower" bird that was able to observe the test bird's cache locations, and the other was a "nonknower" bird that had not observed the test subject's caching behavior. The cacher was allowed to make three caches and then that bird was removed from the aviary. Within five minutes after the termination of the caching behavior and the removal of the cacher, either the knower and nonknower birds were allowed to go into the caching arena to search for food.

Knowing that caching birds often retrieve their food when robbery seems imminent, the authors tested the caching birds' behavior when they were in the arena privately, when the nonknower bird was present, or when the knower bird was present in the aviary. They found that the caching bird retrieved its food stores more often when the knower bird came within two meters of the cache. Thus, the authors speculated, the caching bird was able to identify which bird had observed it making its caches and was able to discriminate between it and the "nonknower" bird.

The authors also found that the knower birds were careful about their intentions; they did not go to the caches when the caching birds were nearby, but instead, they waited until they were at some distance. This suggests that both birds had the ability to attribute knowledge to specific individuals and to anticipate a particular response.

The authors used another version of this same experiment to determine if the caching birds made subtle cues that the knower birds might be able to decipher. To do this, a human make the caches, and then stood by passively while either two knower birds, or a knower and a nonknower bird were placed in the aviary together. As predicted, the knower birds were quick to pilfer human-made caches when paired with another knower. However, when paired with a nonknower, the knower bird's reactions depended upon social rank. When a knower was paired with a socially dominant nonknower bird, the knower would delay approaching the cache -- instead, waiting until the dominant bird was some distance away before pilfering the stash. Thus, the authors concluded that it was unlikely that the knowers were providing some behavioral cue that cache raiders might use.

In conclusion, the results of the string-pulling experiment indicate that ravens rely on logic to guide their actions. The results of the pilfer anti-pilfer experiments show that ravens react to their competitors based on what they remember them being able to observe, that they can accurately attribute the capacity of knowledge to their competitors, and that they integrate this knowledge together with social status to make strategic decisions when retrieving food caches.