I played a numbers game with Houdi in Vermont, using her four young when they were already partly feathered-out toddlers at near-adult weight. While she was out foraging from a calf carcass I’d left nearby in the woods, I would rush out my bedroom window to her nest and bring two of them back inside. The little birds were still too young to be alarmed. They were at an age when they remained calm as long as they did not hear the parents’ alarm calls. Since Fuzz, her mate, was missing, I could be confident no parent had seen me handling the young. When she returned to the nest with half her brood missing, she fed her young in several seconds as always, then hopped to the water tray for a drink, perched nearby to preen and shake, and then went out of the aviary to the calf carcass for another load of meat. As soon as she was out of sight, I rushed out and took the other two young, replacing the first two. When she came back, she again made no vocalizations and showed no emotion. I offered her a chicken egg. She took it eagerly, cracked it, and fed the two young three times. After she left the nest, I replaced the second pair of young to complete the clutch to four. Houdi again showed no reaction to the sudden change in the number of her brood. For all appearances, Houdi didn’t count, didn’t care, and/or didn’t recognize the difference in appearance of a nest with two versus four young.
As I explained earlier, Houdi’s four young later were orphaned. The responsibility of tending to the ravenous quadruplets that required hourly attention was not on my agenda at that time. I sought to pawn off my responsibility and hoped to do an experiment at the same time. Goliath and Whitefeather in Maine had only two offspring, though much younger and still naked. Would they accept Houdi’s four orphans in spite of the age difference?
I drove to Maine on May 6, and delivered Houdi’s four feathered young into Goliath and Whitefeather’s nest in the middle of the night so that they would not see what was happening, temporarily replacing the two much younger offspring of Goliath and Whitefeather. I took their own young out because I feared the parents might compare the newcomers against their own, and then reject and harm them. I went into the nearby observation hut at dawn to watch what would happen.
By 5:10 A.M., it had been light for at least ten minutes, and the pair perched quietly in the shed by the nest. They could not help seeing the four new strange young in their nest, nor miss that their own two were missing, but they showed no visible reaction.
At 5:29, one young began to beg. Whitefeather stretched. Another young began to beg. Ten minutes later, when the sun first peeked over the ridge and shone directly into the nest shed, the adults suddenly stood immobilized like statues. The young were silent, and their blue eyes blinked as their heads rested on the nest rim. The adults stood as if dumbstruck, intently examining the young. Both moved their heads quizzically from side to side, looking at the young in their nest first with one eye, then with the other from a different angle. Several young soon stood up and begged. Whitefeather’s head then went fuzzy. She was agitated. At first tentatively, then vigorously, she started making kek-kek-kek alarm calls. The young, responding to her noise and movement, begged even more. She then made pecking motions at their heads, vigorously gesticulating her anger without hitting them. With her head still fuzzy, she partially opened her bill, as birds do when frightened or alarmed. This was not going as I had hoped. She was not happy. Goliath, in contrast, showed no reaction.
Five minutes later, both birds finally flew out of the nest shed and made long and rapidly repeated rasping alarm calls, such as when an intruder comes near. Whitefeather started hammering at branches in anger, and she also hung upside down by her feet from the wire screening. I had never seen her do either of these behaviors before. I then came out of hiding from my observation shed and sought to blunt their anger by bringing a roadkill. Both birds greeted me (and the dead beaver) without alarm, and both immediately fed. They then began to carry loads of meat up to the nest. The young begged unabashedly, and Whitefeather now readily fed them as if she had completely forgotten her alarm of a few minutes earlier. I was flabbergasted. The four strange young were apparently going to be adopted after all.
It then seemed safe and appropriate to return Goliath and Whitefeather’s own young back to the nest, making it a clutch of six. Would the ravens now preferentially feed their own? To the contrary, in the first thirteen feeding trips to the nest, twelve by Whitefeather and one by Goliath, all of the food went to Houdi’s young, not their own babies. Whitefeather probably simply fed the loudest and most insistent beggars, apparently finding a big pink open mouth irresistible. After the young stopped begging, she stood at the nest edge looking at them, repeatedly making soft low krr-krr sounds that induce hungry young to gape. She was making sure they had enough. Goliath was soon also feeding all the young. The question, and my dilemma, had been resolved: Whitefeather and Goliath had totally accepted the newcomers. Throughout the next few months, I brought them other treats besides the beaver carcass, but the pair were now on their own, and they successfully reared “their” six young to independence.
The next year, in spring 1997, Goliath and Whitefeather had started to rebuild their nest in the aviary, but they later abandoned the breeding attempt. I brought other young ravens I was then rearing for later observations and experiments to test insight, curious to see if these would also be openly received. At first, I kept these youngsters inside the cabin. Goliath responded quickly to their loud begging when I fed them. He came and perched on the birch tree outside the cabin, making long, high-pitched, trilling, upward-inflected alarm calls. He came several times right next to the cabin, as if wanting to come in, exhibiting the male dominance display with fuzzy head, bill-snapping, and bowing simultaneous with flaring his tail and wings.
On this same day, I had seen him chase a red-tailed hawk, giving the rapid staccato calls normally given to aerial predators. When three turkey vultures had come by, he immediately flew to the nest by the aviary, making sharp alarm calls and fluffing himself out. Other ravens had come, and he had vigorously chased them off as well.
I finally brought two of the young, nearly grown ravens out of the cabin and let them hop around on the picnic table. As the young begged from me, Goliath, perched right next to me in the top of the dead birch, showed anger. He stared at the youngsters and made single long calls, one after another, with a high pitch and an upward-inflection normally given as a territorial advertisement. He flew over the youngsters, who ducked and momentarily stopped their begging. Next, he perched forty yards away in a spruce tree, puffed himself out, and angrily attacked cones and branches, hacking them off. Curiously, he made no attempt to hop onto the table to harm the young.
Ravens and most other birds have not evolved behavior to reject young from their own nests, even when these young may look odd to them, because foreign young would normally almost never be deposited there. It is sometimes a different story with eggs that could become stranger’s young. An egg could be laid into a neighbor’s nest accidentally, and some bird species specialize in parasitizing other parents by dumping their eggs off to surrogate parents. They no longer build any nest of their own. The best-known examples of this are cuckoos in the Old World and cowbirds in the New.
As is usual in evolution, almost every strategy has a counter-strategy aimed at neutralizing it, and so on in a continuing tit-for-tat game that continues until a mutual balance is reached in a messy “real world,” or continuing until one of the contestants becomes extinct. In birds, the arms race between bird nest parasites and their hosts is played out largely with egg color. There is the strategy of color matching to deceive by the parasites, pitted against that of detection of color differences to recognize deception, in what is a classic example of coevolution, where one species evolves in response to another or others. The variety and the beauty of the coloring of birds’ eggs is one of the marvels of nature, and it is likely the outcome of the conflict between egg-dumping parasites and their hosts of those birds with open nests where the eggs are visible (most hole nesters have uncolo
red eggs).
At the population level, color variety between species creates difficulties for the local nest parasites, and such differences could easily evolve. For example, if one parasite species specializes to dump its eggs into victims with blue eggs, say species A, B, C, and D, then an individual bird of one of these victim species C would be protected if it has a mutation that results in the production of eggs with purple speckles. That mutation would then quickly spread, and individuals of that species would eventually all have purple-speckled eggs.
At the present time in evolution, the arms race in egg coloring in songbirds has resulted in a great variety of egg colors. Most familiar to us in North America, for example, are robins with pale blue eggs, phoebes with pure white eggs, and kingbirds with white eggs spotted and blotched with purple, black, and lavender. Similar color variety among species exists in the eggs of European birds. In Europe, the common cuckoo is the main egg parasite. The cuckoo’s eggs closely match their hosts’ eggs. They need to. If cuckoos laid white eggs in Europe, they could not parasitize birds laying blue eggs, because those species, after a long history of parasitism, would instantly recognize a white egg among a clutch of their own that are all blue. The cuckoo would lose that unmatched egg, and that line of cuckoos who persist in laying white eggs into nests with blue eggs would go extinct. In Europe, the outcome has been that each female cuckoo lays eggs of only one color, and lays her eggs in only the “correct” species—the one with those colors that her eggs will match. However, the population of cuckoos has different individuals who lay different-colored eggs, and who dump their eggs in species providing an appropriate match. Of course, as expected, the eggs don’t always match perfectly—and some of the hosts, some of the time, are able to detect an off-color egg and toss it out, because in Europe the chances of an off-color egg being a cuckoo’s is great. The host’s chances of making a correct choice—tossing out the cuckoos’ and not their own egg—increase enormously if it does two things: memorize what its first-laid egg looks like, and have all eggs of its own clutch look nearly alike. That is the case.
Ravens have greenish-blue eggs variously mottled with grays and blacks. They are often, but not always, quite variable within any one clutch, so given the rationale explained above, it seems unlikely that raven egg coloration has been standardized to provide a uniform background against which a strangers’ eggs would stand out. In North America, there are no raven nest parasites, so there is no reason to suppose that common ravens have evolved egg-rejection behavior. They might still recognize a strange egg, but they face no risk of raising a stranger’s young and the cost of making a mistake and ejecting one of their own is great. Evolutionary logic therefore dictates that while they should defend their nest from possible egg-dumping females, a strange-looking egg should not be ejected, because it is most likely one of their own.
I decided to experiment with egg recognition. I began by trying to climb to the raven nest at the Melcher farm, where I knew the birds should be incubating, and to give them a chicken egg. At Melcher’s barn, I stopped to talk with Paul about his raven nest, then crossed his large rolling field, still covered with snow, and went on down to the brook at the other side. The brook was gurgling loudly underneath bridges of ice. Crossing one of these ice bridges, I ascended the steep hill on the other side through woods shaded by tall hemlocks. A pair of ravens had built their nest for tens of years in a grove of tall white pines near the top of the hill. Having seen the pair circle over the hill earlier in the spring, I expected to find the nest again.
The birds’ staccato kek-kek-kek alarm calls commenced as I came close to the pines. To my surprise, the huge stick structure was on the very same spot it had been several years ago.
When I finally made it up to the solid live branches just beneath the nest, I was exhausted, but my spirits soared. The nest contained six eggs. These, like most raven eggs, were in a deep nest cup of shredded cedar bark and tufts of deer and cattle fur, all mixed together with the shredded inner bark of dead ash trees. The eggs were greenish-blue with a variety of irregularly shaped gray and black spots and blotches. Some of the dark spots had an olive tint, others were faintly purple. The spots and blotches varied from a smoky haze of gray to black, and ranged in size from much smaller than a pinhead to larger than a housefly. Little Houdi’s, the six eggs were individually distinct. Some of them showed a background color of light blue-green, and in others most of this background was obscured by dense blotching of dark olive green. I could hardly imagine anything more beautiful, nor could I suppose any more tangible evidence that these birds likely did not have a long history of egg parasitism. These eggs were too variable in color.
After taking pictures of the nest and eggs and of the view, I hung my knapsack on a branch and with blessedly solid footing under me, got out a yogurt container, unwrapped the white chicken egg it held, and placed the egg into the raven nest. I knew that ravens treat round objects as if they might be food, which they usually are. I also knew that chicken and other bird eggs are one of the raven’s favorite food. Would the raven eat this chicken egg, incubate it, throw it out, or desert the nest?
There is nothing like anticipation to flavor a new day, and it was with high hopes that I returned the next day to check on the chicken egg. I can invent reasons to explain almost anything a raven might do with a big white chicken egg miraculously dropped into its nest in the top of a pine tree. In truth, I had not the slightest idea what this pair would actually do. The raven made alarm calls as I neared the nest. When I got to the top of the tree, the eggs were warm. All seven of them.
I had come prepared for the possibility that the chicken egg might be gone. If that had happened, I’d have ended up wondering if the birds might have accepted an egg that looked less strange. Regarding that as a possibility, I had brought up with me a second chicken egg, this one colored greenish-blue and spotted like a raven egg. Since the ravens had already accepted a pure white egg, it seemed remote that they’d now reject a raven-like egg. I assumed they’d accept it. Nevertheless, it had been a hard climb, and I had nothing to lose by exchanging the white chicken egg with the imitation raven egg.
The preceeding day, after I had climbed down and left the pine grove, I had heard a raven make the musical double glug-glug call as the bird returned to the nest containing the white chicken egg. This time after I left the tree and the female returned to her nest, I heard instead the loud, long, deep rasping calls that are associated with anger. Was the raven indifferent the first time or merely puzzled? Was she angry this time because she recognized the new egg as being more like a raven’s, and thought a raven had been up to mischief? There were also plaintive calls, made by both birds. I had never heard either of these calls near the nest before. Strange. The raven’s much different reaction to this egg meant I’d have to climb the tree once more on another day to see if the new egg would be accepted. But where was the limit? Might they even accept a red egg?
On Easter Sunday I painted a chicken egg all red. I had found in previous trials that even robins will throw out red eggs while keeping green- or blue-colored control eggs. I now brought the red egg to the ravens’ nest. I did not expect that it would remain unnoticed for long, and it wasn’t. I hid in the foggy woods within earshot of the anticipated fracas.
When the female flew back to the nest to resume incubation, she erupted in long series of rapid kek-kek-kek calls. These are high-alert alarm calls, normally given in the presence of nest predators. She flew away from the nest, and the alarm calls somewhat subsided. The same calls resumed full vigor on each of her several approaches to the nest. It was only after fifty minutes that the alarm calls eventually stopped. I then heard the soft, conversational gro calls.
The red egg, too, was accepted, as I learned on my climb to the nest the next day. My final nest offering was a black film canister, filled with water to mimic an egg’s weight. This was ejected, even though it was close to the size of a raven’s egg. The canister was not just thr
own out; I could not find it on the ground near the nest tree.
There are limits to what is allowed to remain in the nest. At this raven nest and at two others that I have examined subsequently, if it is an egg, it is accepted, even if it is a very oddly colored egg. In subsequent experiments with captive birds nesting for the first time, I saw no reactions to chicken eggs or the facsimiles in their nest, perhaps because these first breeders had not yet learned to recognize their own eggs. Egg-rejection behavior apparently has not evolved in ravens, even though at least one pair of many years appeared to recognize and be alarmed by strange eggs. The ravens’ behavior might sometimes look dumb to us, but it is not always a reflection of their intelligence. Even little girls accept and cuddle rag dolls. We presume they know the difference between their doll and a real baby. Or do they?
LEFT: Chicken egg in Melcher farm raven nest, for egg-rejection experiment.
RIGHT: One- or two-day-old raven chick. The “egg tooth” on front of bill is used to cut open eggshell. Eyes are still closed. Note ear and nose openings, and sparse fluffy fuzz on head.
THIRTEEN
Sensory Discrimination
Mind of the Raven: Investigations and Adventures With Wolf-Birds Page 18
