Heading South: An Excerpt from Autumn: A Season of Change
There is no aspect of migratory behavior in birds that fails to elicit wonder. The unpretentious wheatear, ready to depart the northern tundra on a marathon journey, has acquired in the short season between brood rearing (or its own birth) and migration a readiness equal to that of a great athlete and explorer combined. From the physiological demands of long-distance flight to the challenges of navigating unfamiliar routes, migration is an extraordinary undertaking.
If there is a clear beginning to preparations for migration, it is the noticeable increase in feeding activity that attends the shortening days of autumn. . . . Migratory birds enter a period of hy- perphagia. . . . An abundance of high-carbohydrate berries at this time of year often contributes to their cause, but, interestingly, it is not carbohydrates that they store. Long-distance migrants must be able to extract maximum energy from the relatively small fuel load that they are able to carry, and ounce for ounce carbohydrates do not deliver the punch. Because carbohydrates cannot be stored without water, they are not weight efficient. The advantage of eating fruits is that their high content of sugar can be quickly converted by the liver into energy-dense lipids, mainly triglycerides, and distributed to any one of approximately 15 fat storage depots. . . .
The result of all this feeding activity is, not surprisingly, a remarkable weight gain in long-distance migrants prior to their departure. While the normal fat load for nonmigratory songbirds is about 3 to 5% of their lean body weight (increasing slightly in winter), migrants of comparable size may add an amount up to 100% of their fat-free weight, effectively doubling their mass before takeoff. Some species of waterfowl are able to build fat stores at a rate approaching 10% of their lean body weight each day, reaching their maximum load capacity in less than 2 weeks.
Autumn: A Season of Change
Peter J. Marchand
University Press of New England, $17.95 (paper)