Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)
The vegetation in northern New England is at its lush green peak by mid-August. Yet, the azure blue butterflies have not been around since mid-May, and the tiger swallowtails since mid-June. Each species appears and dies in its specific time slot. The pupae of most insects have by now been arrested in their development for a month or two, and they won’t revive from hibernation to develop into adults until their specific times next spring or summer. Meanwhile, the monarchs have finally arrived from the south, and some of their fast-growing caterpillars are already developing into pupae that a week later turn into adults that will begin their journey back to the south when their food, the milkweed plants, begin to dry up. Ants are still tending the aphids that they milk for their honeydew, but soon they will begin to bring them to safe underground quarters for the winter. Bears and woodchucks are fattening themselves up. Cicadas call shrilly during the day and the constant chirping of crickets, grasshoppers, and katydids continues night and day. But in two more months these songsters will be stone-cold dead, perhaps even before the first fall frosts. There are signs all around of profound change about to happen, as the necessary physiological and behavioral adjustments for the coming winter are being made. In chipmunks, certain birds, honeybees, and us, the most important preparation for winter is storage of food.
In our garden the apples, pumpkins, and squash are still ripening, but the onions, garlic, carrots, potatoes, and string beans are ready for harvest. We’re starting to freeze, can, and dry the bounty for winter, and I’ve been sawing wood and stacking it in the cellar. The farmers’ barns have long since been full of hay to tide the cows over through May, and the unmowed fields and orchards are ablaze with the yellow bloom of goldenrod and the blue New England asters that are abuzz with honeybees topping off their own fuel depots in their hives.
For my family these rituals of food and wood storage fulfill some basic urge, but they are certainly not obligatory, thanks to our twenty-first-century transportation and monetary systems. However, just one hundred years ago, the work of food gathering and storage was a necessity for survival for those who did not rely solely on hunting.
Among mammals, food hoarding can be an alternative to hibernating or migrating, but only a small percentage of the total number of species on the planet store food. And while we may be able to do it on the grandest scale, humans are by no means the most spectacular of food hoarders. In late summer, the pikas (Ochotona princeps), a small relative of the hare that live in the high mountains of the West, collect grass, dry it in the sun, and then pack the hay into dry cavities under rocks for winter food. After the leaves turn and fall in the autumn, beavers all over North America begin felling trees and saplings, and dragging them into the water to make huge underwater food caches near their lodges. The icy water keeps the bark fresh throughout the winter, and the beavers live by feeding on it for about six months. Some squirrels and many other rodents, including deer mice, pocket mice, kangaroo rats, and hamsters, stockpile seeds that reduce or eliminate their need for torpor.
Among birds, long-term food storage occurs with generally northern species (Källender and Smith 1990), and in those species that exhibit no or only modest nocturnal torpor of several degrees. Food-caching behavior is found almost prominently within two families; some of the Paridae (chickadees and kin) store food for winter, and most of the Corvidae (crows, jays, magpies, nutcrackers, and ravens) do so. There is much variation within any one group. At one end of the spectrum of behavior are chickadees, which when encountering a food bonanza that they cannot eat all at once will store some of the food, stuffing it into clefts and crevices, and come back later for it. However, there is no evidence that they lay up food stockpiles for long-term use. On the other hand, European marsh tits and nuthatches may depend on stored beechnuts for a significant part of their winter diet.
A frozen apple on a white birch twig cached there by a red squirrel.
Within the crow family there is also a gradation of behavior, ranging from temporary storage of a surplus to long-term storage that sustains the animals through the winter and well into the breeding season. Like pocket mice, kangaroo rats, hamsters, and chipmunks that are adapted to carry off surplus food in their two expandable cheek pouches, corvids that cache have an expandable throat-pouch under their tongue for carrying food to storage.
The super-cachers among the Corvidae are probably the nutcrackers, the Eurasian, and the Clark’s of the mountainous regions of western North America. The first stores mainly hazelnuts. The second lives primarily on the seeds of several species of pines, such as pinyon, limber, and whitebark, which ripen in the fall (Vander Wall and Hutchins 1983). A single nutcracker routinely collects tens of thousands (and as many as 30, 000) pine seeds and stores them in 2, 500 individual caches, commonly in windswept south-facing rock ledges that may be fifteen kilometers from where the seeds were picked. Months later the birds remember the location of about 80 percent of these caches and come back to retrieve the seeds. The nutcrackers’ ability to live off the seasonal seed crop (Vander Wall and Balda 1981) depends on their astounding memory (Vander Wall and Balda 1983) for specific sites that humans would likely find impossible to match. The seed caches that are not retrieved are, in the long term, not wasted because they propogate the food source on which these birds depend.
Common ravens, when feeding on carcasses in winter, cache meat. However, their food stockpiles last only relatively short periods of time, because the caches get buried by frequent snowfalls. Additionally, meat is perishable or is easily found and dug up by carnivores with a keen sense of smell such as shrews, coyotes, and foxes. Thus, their caches are more for immediate, not long-term use.
We have only a glimpse into the general pattern of how ravens maintain an energy balance in winter, but that peek is intriguing. Ravens begin to nest in late winter, and all of the young are kicked out of the parent’s territory by late summer, if they have not left on their own already (Heinrich 1999). The young then follow the food—preferably a constant supply of fresh food such as that provided by a pack of wolves (Stahler, Heinrich, and Smith 2002). In areas of the north woods where there are no longer wolves, the ravens hunt small prey and also scavenge from carcasses opened by other carnivores. Adult pairs stay year-round in their territories and defend carcasses they find there. Wandering juveniles are excluded from the feasts, unless they band together to overpower the resident adults. They “gather the troops” by joining communal roosts at night, and naive hungry birds can get food by tapping into information of food resources, by following those that take the initiative to leave predawn to fly to a food bonanza such as a deer or moose carcass that they had discovered or fed from previously. The problem is, there is a lot of competition at the carcass, and not all of the crowd of dozens to a hundred or more can be assured continuous access to the few available feeding spots. But there is a solution: Those dominant birds that can get at exposed meat haul off as much as they can and hide it, to retrieve later when the gang may have removed most of the meat. And those that couldn’t get through the crowd? They too have a solution: They closely observe where the dominant birds hide “their” meat, wait until these cachers are out of sight, and then recover the other’s cache either to eat the food right there or hide it elsewhere. In turn, a bird trying to hide food avoids potential cache-raiders and positions itself to be out of sight from them. In the intense social interactions among the birds at carcasses in the winter, those that can best anticipate the intentions of competitors (and the potentially dangerous carnivores also at the carcass) are most likely to be reliably fed. Such a scenario, where one or a few individuals may try to control a resource in a crowd, is currently a prime scenario for consciousness or what is known among animal behaviorists and biologists as “theory of mind.” This is a much different sort of mental facility than memory, and ravens, although possibly the most intelligent of birds, do not exhibit impressive long-term memory for cache locations. Cache locations are remembered for two weeks, and a month may be their limit (Heinrich 1999). But given the ravens’ lifestyle and their food, that’s probably sufficient.
Each animal’s lifestyle has its own unique opportunities, and requirements and constraints. That of the gray jay (Perisoreus canadensis) offers intriguing contrasts to that of the raven. These fluffy, diminutive members of the crow family look to me like oversized chickadees. They are everything a crow or a raven is not: tame to the point of seeming friendly, vocally muted, restrained. They are silent fliers who glide on mothlike wings. They never aggregate in big crowds, generally don’t feed on carcasses, and have a soft look and none of the raven’s or the crow’s bold sharpness of eye. Instead of flying from you on rapid wingbeats ripping the air, gray jays more typically silently glide up to you. Gray jays, formerly called Canada jays, are the north woodsman’s endearingly named whiskey jacks, and camp robbers, who are always looking for a handout. That is why they are so intriguing; they seem to have no visible means of support. Yet, they live in what appear to be barren spruce forests, far from human handouts. They live only several miles from my cabin in Maine, and I’ve met them in willow thickets on the Noatak River on the North Slope in Alaska. They are one of the very few birds that survive year-round in the northernmost taiga, breeding as early as March, often two months before the snow has melted. How do they manage it? The answer to this bird’s riddle probably has less to do with either superior memory or intelligence and a lot more to do with their saliva.
William Barnard, an ornithologist from Norwich University who studies a small population of these birds in Vermont’s Victory Bog, tells me that their saliva is “amazing stuff.” Most spit is designed to ease the food down the esophagus. It must be slippery and non-sticky. This birds’ saliva coagulates on contact with air, to become viscous and sticky. In short, once extruded, it becomes glue. It is a very important glue to gray jays. These birds live in environments with deep snows all winter long, where food caches on the ground or in the snow can become unavailable from one day to the next. Gray jays are unique among Corvidae for routinely storing food above ground. And that’s why their spittle is important. It’s the glue they use to cement their food caches to trees, and that capacity allows them to forage in late summer and fall and safely store food for winter. It keeps their food away from numerous ground marauders, and at the same time alleviates the necessity of digging through feet of snow.
Gray jays begin nesting in March, about two months earlier than their cousins, the blue jays. At that time in late winter in the Northern Hemisphere, and on high mountains, they can still expect numerous snowstorms and days of subzero temperatures, if not weeks of subzero nights. So aside from sticky saliva, the next critical component of their energy strategy is nest construction. Unlike the blue jay’s flimsy see-through nests of bare twigs and rootlets, those of the gray jay are bulky, deep, and well-insulated cups lined with fur and feathers that cradle and keep warm the clutch of three or four grayish, olive-brown-spotted eggs.
The early nesting by the jays must have an advantage. We don’t know for sure what it is, but a study by Dan Strickland in Quebec and Ontario provides clues. Gray jays may seem like very friendly birds because they readily approach humans, and Strickland (1991) found their nests by offering the birds prized nesting materials, primarily cotton, facial tissue, and grouse feathers, and then following them. From a study of 470 color-banded nestlings in 179 nests he found a surprising social structure centering on food caching.
In Quebec, food caching for winter by a resident pair of gray jays starts in late summer when food is most abundant, so caching then makes sense. With plenty of food available and little or no competition for food, most corvids are then tolerant of each other, especially toward family members. But not gray jays. Strickland was surprised to see constant aggression and chasing of the juveniles within the family group. The result of that intrafamily strife was almost invariably that only the most dominant of the brood remained in the parents’ territory. (Those that left sometimes joined up with other pairs whose breeding attempts had failed.) It was not clear why the parents should tolerate a freeloading offspring all winter, but some evidence (Waite and Strickland 1997) suggests that the lone stayer eventually pays its due by helping at the nest in the parents’ next nesting attempt. Besides, parenting is always costly. And parents have to do whatever is necessary for the offspring’s survival. But why would a young bird fight with its siblings almost to the death to be able to stay with its parents?
Gray jay “gluing” food onto tree.
To survive the coming winter, the young need to store up food. However, young corvids, like the young of most other birds, require experience to become good foragers and cachers of food; especially those that like gray jays and ravens may learn to forage for rather “exotic” fare, such as the blood-engorged ticks that ingest moose blood in winter (Addison, Strickland, and Fraser 1989). Possibly, young gray jays are too inexperienced and cannot quite find and lay up enough food to survive the winter, and so their only chance of surviving is if they can rely on a partial winter subsidy from their more-skilled parents. If so, then the parents are obliged to provide it, or they lose their genetic investment.
The parents, however, are also limited by food and they can’t shortchange future reproduction. They can perhaps support one freeloading youngster through the winter, but not three to four survivors of a full clutch of eggs. If all of the clutch survived, if all of the young of any one clutch stayed, then the whole family could starve. Thus if brood reduction in the winter is inevitable, then it’s better to force a fight and make it happen in advance of the food crunch. The evicted subordinates’ advantage lies in leaving while they still have a chance of finding a pair of adults whose nesting attempt failed, that would be less predisposed to evict a starving, persistent juvenile. That is, the evicted young in effect parasitize the parental instincts of failed breeders.
I contrast now the behavior of the gray jay with another animal that also survives the winter only because of the energy resources it collects during the summer months. This animal’s social system is also crucial for making the storage of food energy possible. However, there is one large difference between it and the gray jay. In this animal only the mother survives the winter, along with her tens of thousands of daughters. The male offspring, who are unable to feed themselves, get kicked out of the warm nest or are starved in the fall by not being regularly fed. In this case, unlike with the gray jays, all the daughters participate in helping the mother rear subsequent broods of offspring. What I’m talking about, of course, are honeybees (Apis mellifera).
Honeybees manufacture a special product with a high energy content that has a nearly indefinite shelf life in the well-regulated microclimate of their nest. The raw material for honey, nectar from flowers, is carried in a unique distensible stomach that serves as a large bucket. The pollen or “bee bread” used to feed the young is packed into a special hair-structure on each hind leg. The bees make wax, and use it to build finely crafted receptacles to the most precise specifications to store the honey and the pollen, but separately. A colony of honeybees may routinely store a hundred pounds of honey (plus numerous pounds of pollen) for the winter. Special climate-control mechanisms are employed to maintain the wax receptacles holding the honey and/or pollen at precisely the right temperature, to keep them from melting while also soft and malleable enough for shaping. Ventilation and proper humidity control are used to concentrate the honey and control molds. Special orientation mechanisms involving a superb internal time sense allow the bees to use the sun as a reference point to navigate quickly and efficiently between foraging and the food storage sites at the nest. As a result of their superb energy-storage mechanisms, honeybees are the only insects in the north that don’t hibernate and that maintain a high body temperature all winter long. Bumblebees, which also collect honey and pollen and which individually work longer and faster than honeybees, do not lay up food stores for winter. There is no need. They only store for a rainy day, converting their wealth directly into offspring, then the colony disintegrates in the fall and only the fertilized females (new queens) stay alive to hibernate in cold storage in the ground.
As I sit here at my house in Vermont, next to my four beehives in mid-August, the goldenrod in the neighbor’s fields is in full bloom and my bees are busily harvesting nectar from it to make honey to fuel their energy metabolism through the winter. Pollen is collected often simultaneously with nectar, and this pollen will be used in early March when the queen starts to lay eggs. It will feed the larvae that grow long before foragers can again leave the hive to hunt for food. Goldenrod is, next to asters, the bees’ last major crop of the year. Some late wild aster will top off their winter larder, but after that the bees here in northern New England will have to wait almost a half year before they’ll see another flower.
It is during this intervening time, when the honeybees are confined to their hives, or nests, and the bumblebees are hibernating in torpor underground, that the kinglets, not much bigger than big bumblebee queens, are busily foraging in the spruce thickets. As far as we know, they do not rely on or use any food larder. Kinglets have never been observed to cache food. This does not, however, totally exclude the possibility that they do it. Maybe they’ve not been observed under the right circumstances. Potentially, kinglets are at least as intelligent and programmable to store food as are bees. But they don’t live in a hive appropriate for making and storing honey. Kinglets diversify their options by being partial migrants, and animals that may have to leave their summer digs don’t lay up huge food stockpiles. Finally, as the birds travel in flocks with other birds that also feed on insects, caching might be a waste of energy, because a stockpile would be pilfered by other birds in the flock. Overall, food storage is an unlikely key to explain how kinglets survive long winter nights without freezing. The answer to that mystery must thus be elsewhere.
Honey bee carrying pollen.