BIRDS LIVE IN A SENSORY WORLD that overlaps ours. Their vision and hearing are acute. They should be able to see and hear roughly what we see and hear, although their attention to specific detail undoubtedly is different, as is the neural processing of that information. They probably pick out details that we are not aware of and miss others that seem obvious to us. In addition, many can detect the earth’s magnetic fields, polarized light, and ultrasound.
From day one out of the nest, Goliath and Whitefeather’s young avidly pursued, pounced on, grabbed, crunched, and swallowed all manner of arthropods. With little hesitation, they ate beetles and maggots, dun-colored moths, brightly colored butterflies, dragonflies, grasshoppers, flies, caddisflies, and all larvae. They even ate carrion beetles, but showed caution in approaching their first mouse. After dismembering and eating the first one, though, they approached and ate others with gusto.
Despite the sometimes disgusting things they eat, ravens do subject potential food to careful taste tests. They delicately crush and palpate new food in their bills for many seconds before either swallowing or rejecting it. Some potential food cannot be ingested, not because it tastes or smells bad, but because it causes such unpleasant reactions as stinging in the mouth. A series of photographs in biology textbooks chronicles a toad’s behavior with a bumblebee: The toad swallows the bee, then spits the bee up. Presumably the bee has stung the toad’s tongue. The toad ducks down in fright of the next bee that comes close. Simple. Ravens should do no less.
My first group of four, Goliath and his nestmates, were avid insect-feeders. Yet I found it strange that they showed little interest in the first bumblebees they ever encountered. It was unusual for them to ignore anything at all. I was shocked to see them only limply pick up a few of the dead bees I had scattered in front of them to test their reactions. They quickly dropped them after shaking their heads violently and puffing out their head feathers, indicating disgust. They could not have been reacting to stings; dead bees don’t sting. They were not afraid. Since there was still a remote possibility that all four had somehow already gained experience with my presumably dead bumblebees’ stingers. I needed to make another test. I did in 1997, with my group of six new ravens.
Luckily for my test, bumblebee drones, which are male bees, don’t have stingers (modified ovipositors), while worker bees are females, and sting. I had learned through several painful lessons that drones and workers look alike, when I tried to show off by popping what I thought were drones into my own mouth. Having as a consequence become alert to the differences between male and female, I collected a handful of drones. All the bees crawled and buzzed. The six ravens gathered around and didn’t hesitate to grab them. Live bees were more to their liking than dead ones, so they obviously had not been conditioned against bumblebees by prior experiences. After grabbing the bees, however, the ravens instantly fluffed out and shook their heads, as the previous group of four had done with the dead bees. Bumblebees apparently tasted disgusting to the ravens! To me, they taste okay.
As with bumblebees, the ravens readily picked up dead honeybees, then dropped them uneaten. My hive had large numbers of drones, and I gave my birds handfuls of live ones that I had rendered flightless by damaging one wing. As with the bumblebees, the ravens showed no fear. They picked up the bees, crushed them, then spit them out. I offered them the hover flies, Eristalis, which mimic honeybees closely enough that few people can distinguish them. The ravens took them, carefully macerated them, drooled saliva, and swallowed. So—flies that looked just like bees were palatable, suitable food.
Wasps? I tried yellow jackets (Vespula) and white-faced hornets (Paravespula). The ravens were not afraid of either. They thoroughly crushed one or two, sometimes ate a little bit, then spit out the rest. As with bee-mimicking Eristalis flies, syrphid flies that mimic these wasps almost perfectly were eaten whole. With each test, I ran a control. I offered an insect other than the test objects. The other insects were always eaten eagerly.
Some other insects are well known to be protected by tasting bad, at least to birds, their potential predators. Their noxiousness is often derived from chemicals they absorb by feeding on chemically defended plants. Usually they advertise their noxiousness so the bird won’t kill the insect before rejecting it because of its bad taste.
The summer of 1997 was a very good one for milkweed in Vermont, and also for the bright, brick-red cerambycid beetles and the flashy, black-white-yellow striped monarch butterfly caterpillars that feed on milkweed. The milkweed-eating beetles and monarch caterpillars are both sufficiently flashy to warn predators against eating them; they should taste noxious from the toxic milkweed, their food.
I raised a dozen monarch caterpillars to adulthood, then released the butterflies in the aviary. The ravens eagerly chased after the big butterflies, caught them, then pulled off their wings. They were not eager to eat them, though. They kept tasting and crushing them, pulling off the abdomen, shaking their heads, then dropping the butterflies piece by piece without eating any part. Other butterflies were eaten enthusiastically, wings and all. These results were classic—“according to the book”—just what I had expected. But these were not the only results. I also offered them the brightly colored monarch caterpillars and the red milkweed-eating beetles. Did they reject them? Not at all! They ate both avidly. I had expected them to relish the deactivated stinging insects, and to reject the insects that advertise themselves with bright colors because they are chemically defended. Clearly, the ravens hadn’t read the texts. They had it mostly backwards.
Some things attract the ravens by their appearance alone. Young ravens that had never before eaten eggs were instantly attracted to them, as though hardwired to recognize this nutritious food. On the other hand, they were also attracted to potato chips, which they vastly preferred over raw beef liver (in agreement with me on that!). The young ravens I raised were, through their periods of rapid growth, fed almost exclusively on chopped whole animals of all kinds. Meat is the mainstay of their diet. It was therefore surprising that although they would gobble down chopped chicken guts, they would turn their beaks up at fresh liver, while treating potato chips like a great delicacy.
What did they “see” in an egg? When I first had offered them a sparrow egg, all six of my young ravens had rushed over to try to be the first to get it. A goose egg was treated similarly. All gathered round and pecked at it. I wondered if a more oblong, smooth object that was new to them would generate the same response. A ripe banana! All looked at it from a distance and ignored it for several minutes. How about a huge egg? I put down an ostrich egg. The dominant male immediately approached it, and the others followed. Then one of them flew up and they all followed suit, making alarm calls. They looked at the ostrich egg for a few more minutes from their perches above, then ignored it.
How about something else roundish but not quite so smooth? I tried hickory nuts. All six immediately approached them, pecked them, and quickly left them. A handful of pistachios? For more than ten minutes they played with them. I say “played,” because as they tried to crush them, the nuts popped out of their bills and shot some five or six feet off to the side. The birds would chase after them, only to have the same thing happen. It was a comical scene. You’d think the nuts were jumping around on their own. Only one bird tried to hold a nut fast with her feet, trying to peck into the crack of the shell. She got a few tiny morsels of the nut inside, but none of the birds succeeded in opening a nut on the first try. I continued to give them the pistachios, and a week later some of them opened the nuts as efficiently as I could, and ate them with the same gusto.
The smooth, round eggs and the moths and butterflies that they all pounced on are all conspicuous items to my eye. I suspected the ravens were attracted to bright, conspicuous objects. Would they pass up bright, colorful flowers? As I walked through the aviary, I dropped as unobtrusively as I could behind me five or six each of red roses, purple phlox, and yellow pea flowers. A couple of the birds hopp
ed down and picked weakly at them, but all lost interest within one minute. An hour later, I repeated the experiment with blue irises, daisies, red clover, and yellow hawkweed. The result was similar, although one bird picked up four individual iris flowers and another took three clover flowers and deposited them into the water pan, then picked them out again. Two more pulled the petals off one daisy each. Three birds took no interest in the flowers whatsoever. Were they just generally not responsive today? A quick test with a handful of pistachio nuts told me no—all sprang into action. I tried a batch of different flowers, wondering if the birds would generalize now and treat all flowers as unworthy of attention. African violets and red cyclamens? They were all ignored. Just to be sure, I tried another batch of flowers, pink and blue lupines, and white clover. One bird hopped down, tore apart a lupine inflorescence, then cached another one, covering it with a leaf. A minute later, he and the rest of the birds were back to playing with rocks, bones, nuts, sticks, and bark. There were still a few flowers left in the garden. I decided to try again. The next batch were red jewelweed and blue petunias. Two birds picked up a couple and flung them aside. Their interest in flowers was nothing like that shown to such toys as a Ping-Pong ball, a lightbulb, two film canisters, and one red and white fishing bobber that I gave them next. Then I tried a skunk.
It was late winter, when skunks emerge for their first walkabouts, and there was plenty of roadkill reeking with typical skunk effluvium. My ravens had never before seen or smelled a skunk, but they didn’t hesitate. They were into a skunk in seconds, yanking fur out and poking into all orifices. They gorged. Skunk is fat, and fat is good. Smell be damned, if they smelled it at all.
My aviary was divided into two sections by an opaque partition, so I could shut them up in one side without them seeing what I did in the other. There was a foot of snow on the ground. Whenever I had buried a squirrel in their view, they invariably had found it and dug it back out. On the other hand, squirrels, peanuts, bread, and chunks of calf meat that they had not seen me bury always remained buried. I concluded that ravens don’t use scent to discover food. But then, I hadn’t yet tested skunk.
Four times I buried the by now partially eaten skunk out of their sight, and when I let them into the approximately 1,500-square-foot aviary section where the skunk was located, they always dug in the right spot within minutes after I let them in. They’d quickly exhume the skunk from the snow and resume their feast. I thought they smelled it, because they had not seen me bury it. But I’m an experimental biologist. I’m slightly obsessive. I wondered if they could possibly be so highly motivated to eat skunk that they’d be observant enough to find it by some other cue.
In the fifth trial, I made four fake burials, plus one with the skunk. As before, when I let the birds back into the arena, they immediately charged around as if looking for the skunk. And as before, they found it. Not only did they dig where it was buried, they also dug holes in the snow at all of the fake burial places. So they had learned a second, even more subtle cue than scent: my disturbance of the snow that marked the spot.
I didn’t immediately follow up on these experiments with the exhausting detail of hundreds of trials, as is proper for scientific protocol. I was satisfied, and the reek of skunk around the aviary and our house after just one week was also by now sufficient. Deciding I’d better curtail the experiment, I hauled the skunk far off into the woods.
A snowstorm came a few days later. When it started to snow, I knew right off that it was too good an opportunity to pass up. The snow would hide my tracks. I immediately reclaimed the discarded skunk, to give the ravens another crack at it, because now my disturbances in the snow could be camouflaged.
This time, after I led the birds out, I made six fake burial sites in the snow in the experimental aviary and one hole where I buried the skunk. I left the ravens in the side aviary until two inches of snow had accumulated, obliterating all tracks. I presumed that when I let the birds in, they would immediately expect food and start looking for it, as they had all the times before, but if I was correct in my hypothesis, they should now not find it.
When I let the birds into the experimental aviary, they did act like a bunch of kids on an Easter egg hunt. They looked everywhere, as though expecting I had hidden something. They didn’t all converge on the spot where the skunk was, as I would have expected if they used scent as a beacon. Nevertheless, they dug into all seven places, despite the fact that the sites were hidden under the fresh snow. The spot where the skunk was buried was the fourth one they went to. Each of the six birds dug in at least three different fake burial places. As one bird started to dig, others would join in, as though thinking that the other, because he or she dug there, must know something. I had no idea what any had known.
After the skunk was partially exhumed, I took it away to try the same experiment again later. As it continued to snow, I buried the skunk in a different place and made six more fake burials, then waited overnight, when nearly eight more inches of snow fell. There was no hint of any mounds in the morning, except a four-foot-high snow pile I had made for them. This time, the birds did not find the skunk in even two days. Instead, they dug only into the huge snow pile. Apparently, they cued into not only the disturbance of snow, but perhaps also to bumps. They had now not noted any bumps in the snow except the ultra-large one. Did they think I’d buried a giant carcass there?
Contrary to what I had presumed from the first tests, the ravens did not provide one shred of evidence that they use scent to locate food. Nor did they appear to smell skunk’s effluvium, much less mind it. What I did learn, though, is that ravens are very alert to subtle visual cues. Reacting to subtle cues is admittedly not intelligence, but it is a prerequisite for many kinds of intelligent behavior.
Mostly I learned that one never really knows what might be relevant to them. One can’t predict what they perceive, what cues they use, or how they will react. There are always surprises. I suspected ever more that what they do has less to do with what they perceive than with how they process information in their minds.
Raven bills tend to be more individually distinct (to our eyes) than other of their facial features.
FOURTEEN
Individual Recognition
IF A NUMBER OF RAVENS WERE LINED up in a row, most of us could not distinguish one from another. I know I can’t, and I’ve had lots of opportunity. I could no more distinguish them as individuals than identify individual peas out of the same pod. I may see a smudge on a wing feather here, a blemish on a tail feather there, but all of these markings are temporary. I see differences in behavior, but I can’t assume behavior is constant, especially if that is what I’m interested in studying.
The birds appear to treat many of their own kind as individuals. One hint of individual recognition can be seen in the evening at communal roosts. Some of the birds’ most vehemently vociferous and sustained squabbles relate to who sleeps with whom. Pairs and preening partners always sit close to each other, and they repeatedly chase off specific individuals while allowing others to perch nearby. Each bird responds to every other bird differently, as if it knows each bird individually. I have observed, for instance, that while the most dominant birds chase any and all other birds to try to induce them to drop the food they are carrying, those lower in the dominance hierarchy chase only those still lower until they drop their food. I have seen many hundreds of such chases, and I have never seen a low-ranking bird take off to try to chase a high-ranking bird. Obviously, it would not be successful in taking its food, but the important point is that it never takes a mistake for them to find out.
Unfortunately, I had no proof of individual recognition, and I was anxious to devise a study to see if my hunch was right. But what might I use for criteria that could be statistically evaluated? What kind of data would a hypothetical raven scientist gather in order to prove whether or not even humans, who we know recognize each other as individuals, actually do? As Paul Sherman, Hudson Reeve, and David Pfenni
s have pointed out in a recent review, the only objective measure of individual recognition is a demonstration of differential treatment. If we discriminate individuals one from another, we must first recognize them. That is probably a conservative approach, because even though we may recognize each other, we might still treat each other democratically.
In January 1998, when my group of six ravens were eight months out of the nest, I weighed them and also determined their dominance hierarchy. The latter is easier to do than the first. When the birds feed together at a carcass, there is a constant shuffling as the birds try to feed next to specific individuals and make aggressive jabs and jumps at other specific individuals. I can score who feeds next to whom and who yields to whom, and in a couple of hours know who is top and who is bottom raven, and who likes to be next to whom. In that case, after only 155 interactions the hierarchy was clear. Blue made seventy-eight challenges, and never backed down from any bird. He was top raven. White, his sister, made no challenges but was challenged a total of fifty-three times by all birds, and she backed off to all of them. She was bottom raven. The other numbers also fell into place to reveal a dominance hierarchy from top to bottom, of: Blue, Orange, Green, Yellow, Red, White. Of this group, only Blue and Orange were males, and they were the largest birds.
In August, eight months later, I reexamined the dominance hierarchy of the same six birds, who had been kept together during the intervening time. One thing had changed; Blue and Red had become friends (they would probably attempt to nest in two years) and regularly fed together, played together, and preened each other. For a week and a half, I brought the birds a frozen calf haunch each day and tabulated 678 dominance interactions. Blue was still the undisputed top bird, dishing out 366 aggressive interactions but never once being challenged. White was still the under-raven, being at the receiving end of 382 of the 678 interactions and never once challenging any other bird. It was with the four ravens in between that relationships got interesting. Orange, the next dominant bird, was still second. He was challenged only by Blue, but he now rarely challenged any bird, although before he had challenged Red. Blue’s friend, Red, had risen one rung in the hierarchy. She registered fifty-six aggressions against Yellow and seventy-five against White, but received none from them in return. Furthermore, Orange and Green, the other two birds above her, hardly touched her—I scored a total of only ten aggressions against her. That is, Red was now dishing it out to two underlings, Yellow and White, and she had become practically immune from the attacks of Orange, Green, and Blue. The reason was easily apparent: She fed under the protective umbrella of her dominant mate, Blue. I couldn’t wait to see what would happen when I removed Blue from the group.
Mind of the Raven: Investigations and Adventures With Wolf-Birds Page 19
