Featured Research

Photoperiodism in Birds

Stanley D. Beck.

from "Animal Photoperiodism: Relationship of Daylength to Animal Growth, Development, and Behavior".  1963. New York: Holt, Rinehart and Winston.

Flights of cliff swallows begin arriving at San Juan Capistrano on about the 19th of March each year. The yearly return of the swallows to Capistrano is frequently reported in newspapers and newscasts and has even been eulogized in a popular song. The precision with which these birds time their arrival at their summer breeding grounds has, over many years, become somewhat exaggerated. It is sometimes said that they arrive not only on exactly the 19th of March, but also at exactly the same hour of that day. Careful observations, however, have shown that the time of the spring flight varies by a few days from year to year. But even so, the swallows show a high degree of precision in the time that they complete their long migration from south of the equator to their California breeding area.

The Capistrano swallow is by no means limited in its summer range to just San Juan Capistrano, California. This bird (Fig. 4-1) is the Northern Cliff Swallow (Pterochelidon albifrons albifrons), and is widely distributed through North America. During the summer months it is found as far north as the State of Alaska, eastward to Nova Scotia, and as far south as Guatemala. Before winter the swallows migrate to Brazil, and some go as far south as Argentina. They do not breed in the southern latitudes.

Late in the winter (according to northern seasons) the birds gather into large bands. They then undertake the long flight north. They fly across the equator and along the eastern coast of Central America (Fig. 4-2). Over the mainland of North America, the groups break up into smaller flights, each one heading for its own home territory. The Capistrano swallows make up one such colony or local population.

Once in their home territory, the swallows build their mud nests, mate, lay eggs, and rear their young. As autumn begins, the birds again gather into bands, and by about October 23 they have flown away toward their wintering grounds in southern latitudes. Year after year the cycle is repeated.

Seasonal migrations are part of the biological make-up of thousands of bird species. Not all birds migrate, and not all migrant species cross the equator in their travels. In fact, most North American birds are confined to the North American continent. The Canada Goose, for instance, has summer breeding grounds from northern Minnesota northward to Alaska and Labrador. Its wintering territory extends from southern Illinois southward to Mexico. The high-flying stately V-formation of migrating geese is well known to most everyone. The precise formation and eerie distant honking stirs a primitive beat in the human breast.

A whole host of common birds also make regular annual migrations, including sparrows, robins, juncoes, and that avian weed the starling. Most breed in northern areas, and take winter vacations from such household duties in the south.

Photoperiod and Bird Biology

Daylength plays an important role in the seasonal biology of most birds. The timing of their migrations and their annual breeding cycles is closely linked to seasonal changes in daylength. The breeding activities of some birds, especially tropical and desert species, appear to be guided and controlled mainly by temperature and rainfall rather than by photoperiod. But even in some of these species, photoperiod has been found to be of at least minor importance. Research biologists studying bird migration and breeding are not in complete agreement as to the importance of photoperiod in all cases. There is general agreement, however, that photoperiod is a major factor in the seasonal biology of nearly all migrating birds of the temperate zones.

The fact that birds undergo seasonal cycles of migration and reproduction poses some intriguing and challenging problems. We are faced with the problem of trying to find out bow the growth and physiology of the bird's reproductive organs can be adjusted by daylength so that the bird is ready to reproduce at the right time of the year. We must also face the question of what causes the bird to migrate.

The Annual Cycle. Let us begin our exploration of these problems with the changes that occur within the bird during the course of a year. Only after we understand what is known about the breeding cycle will we be in a position to consider the effects of photoperiod on these events. Cliff swallows cannot be maintained in captivity, and the Canada Goose is not a good experimental animal. So the research biologist has used less spectacular, but more easily handled, species for research on the breeding cycles of birds. Slate-Colored Juncoes (Junco hyemalis) and White-Throated Sparrows (Zonotrichia albicollis) have been studied in detail. Except for some local nonmigrating populations: of the junco, both of these species breed in the northern half of North America but spend the winter as far south as Mexico.

The annual breeding cycle is shown in a schematic fashion in Fig. 4-3. During the middle of the winter, the birds are in a nonbreeding condition. Their reproductive organs are small in, size and are nonfunctional. This shutdown of the reproductive organs is caused by a lack of the hormones' required for sexual development. Although the birds cannot reproduce, the winter period is an important stage in the breeding cycle. In some way that we do not yet understand, it, is a period of preparation for the breeding activity that will occur in 'the spring and summer. For this reason, the winter stage is called the preparation stage. Almost half of the year is spent in the preparation stage.

Beginning in late February and March, the birds begin to change. First, they gain weight by the accumulation of large amounts of fat. The birds always fatten up before they start their spring migration. The fat serves as an important energy supply during the long migration. At about the time the birds are getting quite fat, their reproductive organs begin to develop again. The hormones needed for reproduction are again being produced by the pituitary gland. Because of the progressive maturation of the reproductive organs, this stage of the breeding cycle is called the progressive stage.

The long migration northward begins during the progressive stage. By the time the birds reach their summer breeding grounds, their extra fat has been used up and their reproductive organs are nearly' functional. The birds now enter the reproductive stage. They build nests, mate, lay eggs, and rear their young. These activities take up most of June and July. At about the end of July, the reproductive organs begin to regress toward the inactive winter condition. Reproductive hormones are no longer produced in large quantity by the pituitary gland. The birds are now in the next phase of the breeding cycle-the regressive stage. During this stage they molt (shed old feathers). After their new feathers are well grown, the birds again become quite fat in preparation for migration.

By the time all these events have occurred, summer has faded, and it is September. During September and October southward migration occurs. The birds arrive in their winter territory by the first part of November. Their fat supplies depleted and reproductive organs fully regressed, the birds are again in the preparation stage, ready to start a new breeding cycle.

Photoperiod and the Breeding Cycle

We are now ready to deal with the question of what role photoperiod plays in the annual cycle of bird activity. Suppose that we catch a number of white-throated sparrows that are about ready to migrate south. Their reproductive organs have regressed and the birds have fattened. They are in the early phases of the preparation stage. Of course, these events occur -in the autumn, and the days are getting shorter and shorter.

Can we force our captive sparrows back into the reproductive stage by exposing them to long-day photoperiods? To test this possibility, we will put the birds into big indoor cages equipped with lights and timers. One group of sparrows will be kept under a long-day schedule of 16 hours of light and 8 hours of darkness. The rest of the birds will be held in a short-day photoperiod consisting of 11 hours of light and 13 hours of darkness per day.

The sparrows in the long-day cage will not show any signs of reproductive activity, even after many months of such long daylengths. Contrary to our expectation, long-day photoperiods will not force the birds into the reproductive stage. On the other hand, the birds that were held in the short photoperiod cage will show signs of being in the progressive stage after a few months. They will begin to grow fat, and their reproductive organs will begin to mature.

Detailed experiments of this type have shown that the completion of the preparation stage is a short-day reaction. The physiological changes that occur during this stage appear to require that the bird be in a short-day situation. In fact, it has been found that the birds require a long night; the number of hours of light is less important than the number of hours of darkness each day. About six weeks of long. nights are needed to complete the preparation stage and to advance to the progressive stage of the cycle.

Is the progressive stage also sensitive to daylength? It is, but in a way that is different from that of 'the preparation stage. As we have seen, during the progressive stage the birds undergo fattening, migration, and growth of the reproductive organs. These processes can be greatly speeded up by exposing the birds to long-day pbotoperiods. For instance, Fig. 4-4 shows the effect of day-length on the growth of the testis (male reproductive organ). Here we have plotted the number of days required for the male organs to reach a fully functional state in which mature sperm cells are produced. This process occurs much sooner if the bird is in a long-day situation than it does when the daylengths are short. Similar effects have been shown in regard to the fattening process.

Whereas the preparation stage is influenced by the number of hours of darkness occurring each day, the progessive stage has been found to be dependent on light. The length of the night is of less importance during this stage. In some unknown way, the daily amounts of light add up to determine the rate of development. Less than about 9 hours of light per day halts development, and fastest development occurs with a daylength of 16 or more hours.

Completion of the progressive stage finds the bird ready for reproduction-the reproductive stage. And in this stage, too, photoperiod plays an important role. Once again we find that photoperiod influences the rate at which the stage is completed. In caged experimental birds the reproductive stage can be greatly prolonged by keeping the birds under a shortday photoperiod of about 12 hours of light and 12 hours of dark per day. Under such conditions, the birds do not lay a greater number of eggs; they simply take longer to produce their usual seasonal quota. On the other hand, long days, such as 16 hours of light and 8 hours of dark, have the effect of intensifying and shortening the reproductive stage.

The regressive stage does not seem to be sensitive to photoperiod. However, the time of year that it occurs is determined by the effects of photoperiod on the previous stages of the cycle. By means of artificial lighting, can birds be forced through more than one breeding cycle in a single year? Yes, this has been accomplished experimentally. About two months of short days followed by four months of very long days will carry the birds through the breeding cycle. Such a schedule adds up to only six months; so two cycles can be run through in one calendar year.

From our brief discussion of the breeding cycle, we have learned that the cycle itself is not caused by photoperiod. The breeding cycle is obviously the result of physiological functions within the bird. Photoperiod is important in synchronizing the breeding cycle with the changing seasons of the year.

Photoperiod and Seasonal Migrations

How does photoperiod play a part in the seasonal cycles of birds that winter near the equator? At the equator, daylengths are constant and show no seasonal changes. For many years it was thought that daylength could not influence birds that winter in the tropics. But this proved to be a mistaken idea. A bird that migrates from northern areas has been exposed to many weeks of short autumn days before it arrives in its equatorial winter territory. The short daylengths of the tropics will complete the preparation stage of the bird's breeding cycle. As we have already seen, the progressive stage will occur even under short-day conditions. Only the rate of development is adjusted by daylength. The day-after-day cycle of light and dark eventually brings the bird into the migratory physiological state, even at the equator. Thus, the equatorial photoperiod plays a definite role in determining how long the bird stays in the tropics. Although it is probably an over simplification, one might say that the bird is capable of counting the daylight hours, and when the required total is reached, the bird is ready to migrate north. And by that time, the northern spring is approaching.

The same system is important in the timing of the seasonal cycles in forms, such as the bobolink, that spend the winter in latitudes as far south as Argentina. These birds require many weeks to make the long flight from North America. The short-day requirement of the preparation stage is probably met during the migration. The birds will reach their winter territories during the long days of the southern hemisphere's summer. Since long photoperiods hasten development during the progressive stage, the bobolinks will be ready to begin their northward migration comparatively early. Because of the distances involved, they need to leave earlier than equatorial birds, in order to arrive at their northern breeding grounds at the right time of the year.

Birds that winter in northern latitudes, such as the southern United States and Mexico, will experience very short days. In fact, the winter days will be as short as 10 hours of daylight and 14 hours of night. Such daylengths are appreciably shorter than the equatorial daylength of 12 hours of daylight and 12 hours of night. Birds wintering in Mexico will be very slow in completing the progressive stage, because of the very short days. Their spring migration will, therefore, be delayed. Since their migrations are over shorter distances, they do not need to start north as early as do birds that winter in the tropics.

Birds whose summertime breeding grounds are in the higher northern latitudes will experience very long days during the reproductive stage of their breeding cycle. But although the days will be long, the summer season will be short from the standpoint of available food and severe temperatures. However the long daylengths will tend to speed the birds' reproductive activity. The reproductive stage is short and intense. The birds will lay their eggs and rear their young with a minimum of delay. They will enter the regressive stage and be ready to migrate earlier in the summer than will birds that breed farther south. Thus, we see that the effect of photoperiod on the reproductive stage of the breeding cycle is also important from the standpoint of seasonal timing.

These examples should be sufficient to illustrate the importance of photoperiod on the seasonal biology of birds. The long migratory flights and the precisely timed seasonal activities have long intrigued the imagination and curiosity of naturalists. A few hundred years ago it was not known that birds migrate every year. It was believed that they hid in very strange places to spend the winter. Swallows, for instance, were thought to dive into ponds and lakes and spend the winter buried in the bottom mud. After people learned to explore the earth and to observe nature more carefully, it was discovered that birds migrate. But only within comparatively recent years have we been able to learn something about what makes the birds migrate.

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Last Updated 01/07/99