With the welcome arrival of spring, each day brings new birds, many of them from far away. Their appearance - sometimes all the members of a species arrive within a few days - is the result of many complex factors.
Underlying everything is inheritance. A bird must prepare well in advance for a long journey: something has to trigger its takeoff; something else must point it in the right direction; there must be enough fuel aboard and reliable fueling stops along the way to sustain it; and something must indicate when it finally arrives at a traditionally established ground.
Take three of our most colorful birds, the rose-breasted grosbeak, scarlet tanager, and indigo bunting. They are affected by built-in daily and annual biological clocks, food supplies, and changes in daylight hours in the northern part of their range. Temperature modulates their arrival and departure: if it’s an early spring, the birds arrive early to take advantage of conditions, but they delay if it’s colder than usual.
Raising young up here is safe and profitable in summertime, with few predators and abundant food. Our long summer days offer more hours than the tropics for foraging, nest building, and feeding nestlings. But winters are harsh with little available food; flying to the tropics in winter means migratory birds can live in endless summer. Why not stay there year-round to raise their broods? Competition in the tropics is severe and predators abound, with flightless nestlings vulnerable to tree-climbing snakes.
We are close to the northern range limits for scarlet tanagers and indigo buntings, and since the readiness of their summer habitat here is later than farther south in their range, these birds are among the last to arrive in Vermont and New Hampshire and the first to leave. The rose-breasted grosbeak, meanwhile, with a range that extends into northern Canada, is among the first to arrive and last to leave.
Migratory behavior is affected by a bird’s sensory, nervous, endocrine, digestive, and reproductive systems. The hypothalamus and pituitary glands change significantly as migration draws near, invoking a response to available food and encouraging increased eating to build up reserves of fat for energy during a long flight. Day length stimulates the pineal body to secrete melatonin, creating a pre-migration restlessness and capacity for nocturnal flight.
The change in day length as spring approaches is far less noticeable in the tropics, where days and nights are always of roughly equal length, than here in Vermont and New Hampshire. Nevertheless, the birds’ internal biological clock senses the change, anticipating forthcoming reproduction in the North and even setting the stage for the return south months later. Flowering, seed production, and variations of insect, invertebrate, and small mammal populations are similar manifestations of the biological rhythms that affect birds.
Ancient birds probably migrated only short distances, up and down mountains as some do today, or a little south, then a little north. Long-distance migration involves geological history with planetary plates constantly on the move. Land masses separate, splitting animal populations. When an ocean gap broadens, natural selection favors birds that fly nonstop across open water to satisfy their biological needs. Eventually a gap may require flying from the northern hemisphere to the southern and back again.
Much research has centered upon homing, a directional sense that fine-tunes the final stages of migration, saving time and energy when great distances are involved. Birds often use visual clues to find their way in familiar territory, and in unfamiliar surroundings they search in random fashion until they recognize known landmarks.
Scientists have found that birds can detect the earth’s magnetic field for finding both direction and position. Some birds home by flying in a certain compass direction no matter where they are released, until they reach familiar landmarks. Others find their way home by flying in an unfamiliar direction over unfamiliar territory until they recognize something - they do well if the sun is shining, but on overcast days when polarized light is weak, they scatter widely. Some birds compensate for the time of day when using solar orientation, and most have a precise sense of time. Others, navigating by celestial patterns of stars imprinted in their brains, migrate at night, often far out at sea.
We still have little idea how complex genetic behavioral messages are passed to the next generation. How to explain young birds, in their first summer, departing for the south before or after their parents? Following traditional routes they have never seen before, something inside tells them, “All right, you’re on course,” or, “Swing three degrees to the west for the next leg of the flight,” or “O.K., now you’re over Yucatan.” Understanding avian navigation has barely begun.
Nevertheless, whenever I see these long-distance travelers return to New Hampshire and Vermont, I think, “Way to go!”