Birds that migrate typically have longer wings than birds that don't migrate. Their wings have a higher ratio of wing length to wing breadth than non-migratory species or resident species like the Northern Cardinal. This adaptation reduces the relative impact of drag (air resistance), resulting in greater lift (upward force that opposes the pull of gravity).
Feathers also factor in. A bird's outer primary feathers work with its inner primaries to provide forward thrust in flapping flight. In migrants, these outer primary feathers are often longer, giving the wing a shape that's pointed rather than rounded. Albatrosses, falcons, swifts, various shorebirds, and terns – any of which make long-distance journeys – have longer, more pointed wings.
Other features that aid a migrating bird include the bird's:
Experiments have shown that day length is the environmental stimulus that triggers a bird's weight gain before migration. Light not only directly affects the feeding centers in the bird's hypothalamus (part of the brain that regulates body temperature, hunger, thirst, and more), but also stimulates nearby centers in the brain to shift the bird's secretion of hormones. These hormonal changes give the bird an increased appetite and let the bird develop fat deposits resulting from its greater food intake.
In the spring, before migration, the bird experiences a change in its neural (brain) centers that control hunger and satiety (feeling of fullness) and as a result, the bird gains weight by overeating. This increased energy intake is stored as large fat deposits under the skin, in the muscles the bird uses for flight and in the bird's abdominal cavity. Small perching birds like sparrows and warblers gain about 1 to 1.5 grams per day. This increased appetite continues over a period of about two weeks prior to migration.
Once migration has begun, birds retain the ability to rapidly gain weight during stopover periods in the course of their journey. When a bird isn't migrating, fat comprises about three to five percent of its body weight. Short and middle distance migrants increase their fat load to about 15 percent of their weight, while long-distance migrants increase their fat to 30 to 50 percent of their weight. They are literally obese. These fat stores fuel the aerobic contraction of flight muscles, allowing the bird to make long flights with minimal fatigue.
For example, one year songbirds that normally nest on Jan Mayen Island in the Arctic Ocean arrived during a late spring to find that their breeding grounds were still snowbound. The songbirds' hormone development ceased immediately and the birds left.
We don't fully understand what stimulates a bird's preparations for migration in autumn. The working theory is that what takes place in the spring sets an internal timer for the bird that "goes off" when it's time to prepare for migration in the fall.
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