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Migration of Birds

Stimulus for Migration

The environmental factors that have resulted in the evolution of migratory behavior are not the environmental factors that stimulate development of the migratory condition or actually cause birds to embark on migratory flights. If a bird would wait until food on its breeding range became abundant to begin its vernal migratory preparation, it would have insufficient time to migrate, establish a territory, mate, incubate eggs, and raise young to take advantage of this abundance. The timing of its entire annual cycle must result in young in the nest coincident with an optimal abundance of food or other environmental factor that has a critical effect on productivity. Similarly, if birds waited until the climate became no longer tolerable to begin preparations for fall departure from breeding areas, it would be too late to gain the necessary energy surplus above the demands of thermoregulation to allow the required physiological changes associated with migration. The stimulus for development of the migratory state must be related to the eventual advent of suitable environmental conditions for reproduction or winter survival.

In the spring, the premigratory state is characterized by a change in neural centers in the lower part of the brain (the hypothalamus) controlling hunger and satiety so that the bird gains weight by overeating. This increased energy income, a food intake that is as much as 40% greater than during other times of the year, is stored as large fat deposits under the skin, in flight musculature, and in the abdominal cavity. Small perching birds like sparrows and warblers gain about 1 to 1.5 g per day, and this increased appetite continues over a period of about two weeks prior to migration. Furthermore, these birds retain the ability to rapidly gain weight during stopover periods in the course of their migratory journey. While during nonmigratory periods fat comprises about 3-5% of a bird's body weight, short and middle distance migrants increase their fat load to about 15% of their weight, while in long-distance migrants fat is 30-50% of their weight. They are literally obese. These fat stores fuel the aerobic contraction of flight muscles, permitting flights of long duration with minimal fatigue.

Experiments have demonstrated that day length is the environmental stimulus that results in vernal premigratory weight gain. Light not only directly affects the hypothalamic feeding centers but stimulates adjacent centers in the brain to affect a shift in the bird's endocrine secretions, specifically increasing prolactin from the pituitary, corticosterone from the adrenal gland, and the sex steroids (e.g., testosterone) from the gonads. These hormonal changes facilitate the development of fat deposits resulting from the greater food intake caused by increased appetite.

The premigratory state is also characterized by increased activity during the night, which is when most birds migrate. They become restless, perhaps in anticipation of the migratory flight. This behavior is seldom observed in the wild, but has been carefully evaluated in captive migrants. It has been shown, for example, that the intensity and duration of migratory restlessness in captives are correlated with the distance and period of migration in the wild population. Like premigratory weight gain, migratory restlessness is stimulated by long days through the effect of light on the hypothalamus, causing increased secretions of prolactin, corticosterone, and the sex steroids. Additionally, light stimulates the release of melatonin, a hormone produced in the pineal body on the top of the brain, which has also been shown to be necessary for the expression of this behavior.

It is important to emphasize that the light stimulus is a function of length of the light period rather than because of the change in daylengths. It is also clear that the absolute length of the daylight period that is considered "long" varies with species, not only in terms of the daylength characteristics of their environments but in the daily period when a species' brain is receptive to the effects of light. Both the external and internal aspects of light stimulation reflect their geographic distributions. Thus, birds wintering in the tropics have evolved a response to that photoperiod which results in premigratory changes similar to that of birds wintering in the North Temperate zone under increasing daylength. Even birds wintering in South America initiate premigratory preparation in March and April under the decreasing daylengths of the austral fall.

The adaptation of migrants to the temporal control daylength is amazing. Consider the transequatorial migrant Bobolink. This species initiates premigratory preparation under decreasing daylengths in the South Temperate Zone, migrates northward toward the equator, experiencing lengthening daylengths but decreasing daily variation in daylength, then crosses the equator and experiences rapidly increasing daylength until it finally arrives on its previous year's territory somewhere north of the fortieth parallel. That birds, many plants, and other animals depend upon daylength to regulate their annual cycles is not surprising. Of all the variables in the environment, only seasonal daylength variation has remained constant since the formation of the planet because of Earth's rotation on an axis inclined to the plane of its revolution around the Sun.

Yet the development of the migratory state is not completely driven by daylength. Birds have evolved closer control of this process by responding to other environmental stimuli, either accelerating or inhibiting the rate of response to the primary daylength stimulus. Temperature is one of the environmental factors involved. Thus, when spring is late birds do not arrive too early; similarly, when spring is advanced the birds arrive early to take advantage of the precocious environmental resources. There is also evidence that development of vegetative cover can influence light-caused reproductive development. When songbirds normally nesting on Jan Mayen Island in the Arctic Ocean arrived during a late spring to find their breeding grounds still snowbound, gonadal development was immediately truncated and the birds left, even though daylength was stimulatory.

The stimulus for autumnal premigratory preparation is not well understood. The current working hypothesis suggests that the spring photoperiod sets an internal timer that allows the expression of fall premigratory preparation after the cessation of a reproductive period which has evolved to be commensurate with species-specific environmental resources. Perhaps hormonal changes following breeding release the expression of these preset events. In many species, the postnuptual (or pre-basic) molt may inhibit the development of the premigratory state. In other species, however, migration precedes the fall molt. And some species, like Barn Swallows, molt while migrating.

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