As winter’s cold settles around us, I often think about how birds cope with the cold. Of course they have many morphological and physiological adaptations such as their thick down coats, an ability to keep their feet cold to minimize heat loss, and an ability to slow their metabolic rates at night to conserve fuel. Obviously, many use their incredible flight ability to migrate to warmer climes. But I’m thinking more about how those that remain use their brains to eek out a living when daylight is short, feeding areas are frozen, and weather inclement.
When it comes to food, birds use two basic strategies during winter. First, many pile into our urban and agricultural areas to eat our waste or the special bird food we offer. Second, a select group of really clever birds exploit natural food bonanzas, often caching surplus for hard times ahead. The chickadees, titmice, crows, and jays are famous for their caching behavior. As a graduate student I had the privilege of seeing extraordinary birds cache and recover their stores of food. The pinyon jays and Clark’s nutcrackers, both members of the crow tribe, my advisor Russ Balda and I studied were setting world records for bird memory.
Russ did all sorts of experiments with captive nutcrackers and jays and found that they could recover caches after weeks or months with uncanny accuracy. Typically around 80-90% of their recovery attempts were on target. The European nutcracker has even been reported to find its caches through a meter or more of snow—digging at the precise angle to intersect a deeply buried cache of hazelnuts or pine seeds. They do this by memory, availing their large hippocampus, not by smell or landmark.
One of my favorite memories of life in the nutcracker lab is recounted in my recent book with Tony Angell, Gifts of the Crow (look for it in paperback next week from Atria Press). Here is how we described it:
A Clark’s nutcracker named Hans, sharply dressed in bold gray, black, and white plumage, peeks through a portal into a small, featureless room in Russ Balda’s lab on the campus of Northern Arizona University, dives to the sand floor, and digs a neat hole. His beak flips sand until he finds a cache of pinyon pine seeds. A month earlier, he alone cached seeds in this barren room. Now, hungry, he is deliberate and exceptionally accurate as he finds his seeds. He digs only where seeds are buried and recovers every one.
Over thousands of years, Hans’s ancestors competed with the Anasazi, Hopi, and Navajo for this nutritious prize. A southwestern tree of life, the pinyon adapted to having people and birds disperse its seeds. The birds and people learned when and where they could harvest seeds, and how best to gather, transport, and store the piney propagules. Indian and nutcracker brains expanded to remember what they learned. The rush for pinyon gold shaped body and mind.
Hans and Russ are pioneers in the study of avian cognition. Together they showed the scientific world that an ordinary bird has an extraordinary brain. Nutcrackers and jays can remember for months the tens of thousands of locations each year where they cache pine seeds. This mental ability enabled the birds to thrive in nature’s boom-and-bust seed economy. Little thwarted the avian mind, which mastered the challenges of deep snow, long distance, time, and competition.
Russ loved to pit bird against graduate student, of which I was one, to make his point. He would ask us to bury nuts in the same room that Hans used. A month later we’d return to the room to recall where we’d stashed each morsel. We found most, but not without a fair number of errors—we’d dig where no seed existed or forget a cache. Hans and Russ’ other birds always kicked our brains. We grad students would assert that our accuracy would improve if we were given beer or money to cache, but when it came to pine seeds, the birds always won.
Another particularly obvious winter strategy for many birds is communal roosting. Perhaps you’ve noticed large flights of blackbirds streaming off into the evening. Here in Seattle, thousands of crows flock together each night, especially during cold winter eves. They fly above our jammed roads during the evening rush-hour as they converge on their sleeping place. In colder climes there may be tens or hundreds of thousands of crows sleeping together; in some places roosts top a million birds. The roosting birds crowd, shoulder-to-shoulder, together. Their combined mass may provide some body heat to stave off the cold. Roosting as a group also helps deter some predators, such as owls. But it seems the real reason to roost together also involves food, in particular the reliability and abundance of food. Superabundant food, such as can be found at some garbage dumps, cattle feedlots, or agricultural fields, enables birds to flock together because one bird’s presence does not appreciably reduce the food available to another bird. In such cases it pays to also roost together (for the benefits of heat and safety), especially if roosts can be located strategically to reduce commuting costs. Smart crows do exactly this. Their roosts are located centrally with respect to a variety of abundant feeding locations so each bird travels the minimum distance to reap the rewards of sleeping with others.
With a close relative of the crow, the raven, the advantages of roosting together are a bit more complex. Ravens thrive during winter in some of the world’s coldest places: Greenland, Alaska, the Himalayan Mountains. Some use human garbage dumps to make it, but others stick with a natural diet of dead moose, deer, and elk. Where large predators, like wolves, are rare ravens use their nightly communal roosts to track the whereabouts of new feeding opportunities. My wife, Colleen, and I studied raven roosts with Bernd Heinrich for three winters in Maine. We quickly discovered that ravens entered roosts from many directions in the evening, but in the morning they headed off together in a single direction. Here is how we put it in our book Dog Days, Raven Nights (Yale University Press):
For ravens, long-distance sharing meant that vagrant, nonbreeding individuals would search widely in many directions for food each day and assemble at a roost to compare notes each night. This habit might explain why each evening birds came from many directions and in parties of various sizes. Larger parties included lucky, successful hunters returning from bonanzas. Small groups and singletons either had been unsuccessful at finding food or had perhaps discovered new foods where defensive adults prevented them from eating. Then, either through mutual understanding of shared knowledge, or perhaps simply by naïve birds following knowledgeable ones, all roostmates would leave together for known feeding locations each morning. It was clear that roosting ravens shared information on the whereabouts of food. But how did they do this? And why?
We were often left with questions, such as those above. As we continued to study ravens in Maine and elsewhere we learned that the soaring flights that often occur around roosts and perhaps some special goose-like morning vocalizations by roosting ravens were important to sharing information. And, we learned that, as with most of animal behavior, ravens roosted together and shared their secrets because it was beneficial to do so. Here is how Colleen and I explained it in Dog Days, Raven Nights:
In mid-December 1989 temperatures stayed below zero. The ravens cared not, but our stints in the uninsulated blinds shortened out of necessity. Throughout this second winter we demonstrated the consequences of status and how this tempered the benefits of flocking for vagrant ravens. At large carcasses, such as the cow, dominant and subordinate ravens benefited from increased feeding opportunity as group sizes increased from 1 to 30 birds. This increase occurred because adult defense and juvenile neophobia, the fear of new situations, both declined. As group sizes swelled beyond 30, foraging rates declined as competition among the many juveniles for limited places at the carcass increased. The increase in group size beyond 30--often to 100 or more--at a large animal like a cow or moose was costly to those ravens already at the feast. Their feeding rates dropped, but group size continued to increase, because from an outsider’s perspective it was still better to join a big group than to venture alone in search of a new, and likely defended, carcass. Selfish ravens joined the feast, and those at the feast suffered competition. The existing flock did not cooperate to keep intruders away and optimize group size because flocks were not of stable membership. A flock member at the cow today might be a latecomer at a bear next week. If she excludes at the cow, she would be excluded at the bear. Ravens at a feast live by the Golden Rule. Those who are not at a feast do what is best for themselves. They control the group dynamic with their decisions. While ravens appear to cooperate--groups of vagrants collectively win access to defended food and soar to attract roost mates to a feast--much of what we observed was the collective sum of competitive, individual, selfish decision-making. Small groups formed at carcasses as searchers orient to the submissive begs of vagrant discoverers. Roost mates were recruited to soaring displays, perhaps actively by vagrants unable to access food, but also as selfish individuals naïve to the location of food parasitized this public information. Groups grew large because of selfish decisions by ravens outside the group.
So, you see thinking about how smart birds survive the cold can really keep your mind warm. There are many costs and benefits to consider, and their importance differs from bird to bird. But the observation of a wild, fragile bird surviving all that old man winter can blow its way does more than just warm the brain it also stirs the heart.