Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)
Wondering what the crows roosting in town eat in the winter, I collected bagfuls of their regurgitated pellets under the roost and picked the pellets apart. These undigested remains of what they had eaten that didn’t pass through their digestive tract contained mostly berry seeds. Some pellets consisted primarily of wild grape seeds. Others were masses of seeds from the common Viburnum species, and some contained wild holly seeds. In early spring many were almost pure bundles of undigested staghorn sumac seeds. Almost every kind of winter berry was represented in these crow pellets that can be as much fun to pick apart as owl pellets full of bones and fur.
Only nine of the thirty-eight local species of berries that I know of ripen and rot quickly. These are the summer berries, such as strawberries, June or serviceberries (Amelanchier), raspberries, black berries, blueberries, and chokecherries. They ripen in a progression from May through August and they spoil within several days. That leaves twenty-nine winter berries, which all ripen in the early to late fall. They are not sweet, but they last on the branch through the winter.
I have come to notice these winter berries as a consequence of watching birds. Berries and birds are intertwined in an ancient and complex mutual relationship that is as intricate and interesting as that of flowers and bees, although it is not always as visible or obvious because it proceeds over time spans measured by seasons rather than minutes.
Watching the different kinds of berries that are specific to winter can be a slow sort of sport, unless one has a good view and keeps a long-running score. I am lucky to have a beaver bog nearby which I routinely check to see what’s happening. Many berry species can be found there. The beaver pond is surrounded by a virtual hedge of arrowwood (Viburnum dentatum), a species whose dark-blue berries show up conspicuously because they are attached to the twigs by bright yellow stems.
Arrowwood berries provide a considerable bird feast in the fall. During the autumn of 2000, I counted and weighed the berries from twenty arrowwood bushes, estimated the number of bushes surrounding the pond, and calculated that the little beaver bog yielded slightly over a ton of bird food. From early September and into November, wave after wave of hundreds of robins came through, feeding for several days before moving on. Bluebirds, blue jays, starlings, grouse, and pileated woodpeckers also fed on these arrowwood berries, and I found not one berry remaining by early March, when the resident winter birds would presumably need them most. Indeed, by February arrowwood berries are dry, their yellow attachments to the twigs are brittle and faded, and the remaining berries then drop off. Thus, although fed on by many birds, these berries are apparently adapted for fall migrants.
Surrounding the beaver bog, though in much less abundance, are also maple-leaved arrowwood (Viburnum acerifolium) and nannyberry (Viburnum lentago). Both species have blue to black raisinlike berries, but unlike the V. dentatum, these stay attached by stalks that do not turn brittle. Furthermore, these berries are not consumed by the fall migrants. They can stay on the bushes all winter and they are fodder for resident winter birds such as grouse, purple finches, blue jays, and crows. If not fully removed, they then also feed spring migrants.
Winterberry, the common name for a holly (Ilex verticillata), should presumably also feed winter birds. It grows in swamps and wet places, bearing crimson berries. A bush laden with these berries, after the leaves are dropped in the fall, shines like a red torch that signals birds to come eat and ultimately to disperse the seeds. Yet, I had routinely seen resident winter birds, such as woodpeckers, chickadees, kinglets, blue jays, and grouse, in and around these bushes, and the berries stayed for months, untouched.
One early November day a few years ago, I saw a flock of about two dozen migrating robins descend on a large holly bush covered in thousands of berries, and when they flew on they had not left a single berry uneaten. Having been alerted, I now routinely see robins (and no other birds) eat these berries in the fall. The few berries that don’t get taken in the fall remain until about February, when their color fades to yellow-brown and they start to drop off. Thus, even the quintessential “winterberry” caters to a select clientele of fall migrants and not to the resident winter birds at all.
One of the most enigmatic berries to me is the highbush “cranberry” (Viburnum opulus). This berry is bright red, like those of holly in late fall, but the V. opulus berries decorate bushes through winter and well into early spring, which is why they are often planted as ornamentals. The bright red is a conspicuous enough signal to birds, yet year in, year out the berries stay on the bushes for months. An abundance of highbush cranberries grow among the other berries in a winter berry patch I have cultivated next to our house in Vermont, and while birds have routinely feasted on arrowwood and nannyberry, they have always left the highbush cranberries alone. They are not poisonous—they are depicted as suitable fare in recipe books on wild fruit. Taken raw, they taste sour, but when boiled and sugared they are not objectionable, at least to humans. I was perplexed that such a “typical” bird berry was not eaten by any of the fall migrants nor by any of the other birds that were around it every day, for months. Surely, some bird must find them palatable.
Then on February 23, 2000, the mystery was solved. A flock of eighty to one hundred Bohemian waxwings (only sporadically seen here in winter) arrived and landed on a sugar maple tree above the large highbush cranberry bush at the edge of the field near the house. One bird dove down to the berries, several flock members followed, and then the whole flock descended en masse. Within a half hour the large clump of bushes that had been heavily laden with berries was stripped totally bare.
Flocks of both Bohemian and cedar waxwings were locally common that winter. Both species of waxwings were also feasting, separately, on the berries of ornamental hawthorn on the University of Vermont campus. Both waxwings live entirely from berries in the winter, but unlike most other birds, their summer diet is frugivorous as well. Thus, the highbush cranberry is perhaps the choice of the discriminating berry-bush specialists, and highbush cranberries remain bright and juicy throughout the winter as though they were being preserved specifically for late winter or early spring migrants.
In the following summer none of the Viburnum bushes flowered and thus no berries were produced in the fall. However, I again saw flocks of Bohemian waxwings in February. This time I found a large flock in brushy young woods. I was surprised to see them there, because I saw no berries. The birds seemed to be picking buds, or so I at first presumed. Looking closer I suddenly discovered that they were picking sparse, hard-to-see, dry, shriveled, black-blue buckthorn (Ramnis cathartica) berries. Therefore, I did not find the berries by myself. I found them by joining the flock, and birds probably find berries similarly.
Staghorn sumac is also available all winter long. It has tightly packed small dry fruit that have no visible meat. The massed fruit are almost bare seeds covered with a hairy fuzz as though designed to be unpalatable. Like highbush cranberry, this sumac ripens in late summer and remains uneaten on the bush almost all winter. The berries are preserved not only by acid, as is highbush cranberry, but by dryness. Yet, in early spring, when few other berries are left and choice is correspondingly limited, they are finally eaten by robins, starlings, crows, and a variety of returning migrants including flickers, thrushes, and catbirds. Apparently sumac saves energy by not offering a fleshy fruit with sugar, fats, and protein, and it gets away with that stinginess both proximately and ultimately because the fruit’s very long branch life extends beyond that of the more perishable competition. (Sumac fruit have, to human palates, a lemony taste and are a good source of vitamin C, if boiled in a tea.)
Many birds require berry crops for fattening up on their long fall migratory flights, putting on as much as 10 percent body weight per day. That impressive fattening feat involves adjustments of gut length and other digestive adaptations for berries that allow for rapid food processing (Karasov). The fruit’s nutritional content depends on the season for which their dispersal is tailored. Thus although the highest-quality (highest energy content) fruits contain fat and sugars, that food (especially fat) causes rapid fruit spoilage due to microbes (Stiles). Low fat and sugar contents, as well as high acidity and low water content all help to prolong branch life, with staghorn sumac being the extreme of that strategy. Its fruit can be retained without spoilage for about eight months after being produced. Extreme? Well, maybe not entirely. The tomatoes we get at the supermarket may be a close analogy. They are selected for long-distance travel from California and for long shelf life, unlike the garden variety we grow for good taste. As with wild fruit, the nutrients that make them taste good also cause their rapid spoilage, and our commercial varieties of fruit are selected, like many winter fruits, for longevity.
If the berry is adapted to the bird, then it stands to reason that the bird is adapted to the berry. But aside from specific digestive physiology, there is also the perhaps even greater problem for the bird of locating the often widely dispersed berry bushes.
Seeing robins, bluebirds, starlings, crows, and waxwings descend on their berry bonanzas suggests why these winter frugivores fly in large flocks. The “many eyes” hypothesis posits that animals in groups can more easily spot danger. Might it also allow them to more easily locate the scattered food bonanzas? The latter has a cost, namely that any one individual who finds a berry clump has to share it. However, that cost is not great if the clump is ample and the flock has to keep moving. So, joining up with a hundred pair of other eyes can pay off in more ways than one, especially if everyone is in a rush and not interested in staying around.
Many migrants fly to staging areas in between their migratory endpoints where they have previously found food and where they fatten up for the long journeys ahead. After seeing the berry-gulping flocks on their migrations, I started to cultivate winter berry bushes despite the fact that most don’t feed the resident winter birds. It pleases me to think that after the bears have fattened up on the blueberries, chokecherries, and wild apples, there are still plenty of winter berries to go around, providing fuel for not only crows, but also the long-distance wanderers on their stopover from the north.