Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [102]
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.
21
BEARS IN WINTER
While searching for kinglets in the Maine winter woods during mid-December 2000, my students and I found the fresh spoor of a black bear on new snow. We had never seen a bear track so late in the year. When there is little food, the bears den up as early as mid-October. It had, however, been a fall with a heavy crop of both beechnuts and acorns. Wild apples had also been abundant on the old overgrown farms in the surrounding hills.
The bear had passed by only hours before, and we took up the chase, hoping to find its wintering lair. The bear had traveled without stopping to rest, walking past a calf carcass that we had laid out to feed ravens. Normally such prime veal would attract a bear miles away and then cause it to gorge. But the fact that this bear did not stop to feed prior to its long winter fast was not surprising. By December it had probably stopped looking