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Migration of Birds
Evolution of Migration
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The rigors of the annual migratory journey are balanced by benefits derived from species being able to inhabit two different areas during seasons when each region provides favorable conditions. Upland Sandpipers breeding in the grasslands of North America and wintering on the pampas of Argentina never experience winter. If it were not advantageous to make the trip twice a year, the behavior would not have evolved or if once typical under one set of conditions, natural selection would have eliminated the tendency once the environment changed. An example of the latter case is the European Starling which is migratory on the continent, but the population isolated in the British Isles by the rise in sea level after the end of Pleistocene glaciation and now living in a moderate maritime climate has secondarily evolved nonmigratory behavior.
By departing in the spring from their wintering ranges to breeding areas, migrant species are probably assured of reduced interspecific competition for adequate space and resources such as ample food for themselves and their offspring. Permanent residents in temperate zones, whose wintering and breeding areas are in the same region, also gain a net benefit by being nonmigratory. Although not suffering the metabolic demands and hazards of migration, the energetic demands for survival and reproduction in an environment with a greater annual range of climactic variation, and the need to adapt to the seasonal changes in the availability and kinds of foods, are comparable. Even for permanent residents in the tropics where climatic variation is relatively low, these benefits are offset by lower reproductive success resulting from higher nest predation.
While the various kinds of wood warblers and flycatchers are wholly migratory, other species like most woodpeckers are permanent residents. Some populations of species have individuals that are migratory while other individuals breeding in the same area are not. These partial migrant species, like Blue Jays, exemplify the difficulty in suggesting simple, singular explanations for the origin of migration.
Birds require specific environmental resources for reproduction. Among both migratory and nonmigratory species alike, adequate food for the young appears to be primary in determining where, as well as when, a species will breed. American Goldfinches and Pine Siskins are closely related and winter together in gregarious flocks. With the emergence of abundant insect food in the spring, siskins disperse and begin nesting while goldfinches postpone their reproduction until late summer when thistle seeds become available for feeding young. For other species, like waterfowl, the availability of suitable nest sites rather than food for the young appears to determine the timing of breeding.
The evolution of migration also involves adaptations that affect the timing of this behavior so that the species is in the breeding or wintering habitat under the most propitious conditions. For most migrants, especially long-distance migrants, the evolution of migratory behavior demands a physiological response to environmental cues in preparation for migration that are different from the environmental factors that ultimately determine their reproductive success on the breeding range or survival on the wintering range. Thus, in the fall swallows and other insectivorous species depart southward long before food resources or weather become critical for their survival. Factors other than a decrease in food availability or cold stress, for example, must prompt their migratory departure.
The verdant flush of regrowth in the spring is clearly associated with migratory movements of many species to higher latitudes where longer daylengths provide ample time for feeding young, permitting their rapid growth and shorter exposure in the nest to predation. But the higher the latitude the shorter the breeding season, so that while summer days may be long, the summer season is short and migrants in more northerly climes may have only one chance to breed before they must again travel southward. At lower latitudes, breeding seasons are longer, allowing multiple attempts to produce young. This longer breeding season, however, is related to a higher probability that nests will suffer losses to predators.
Fall departure from higher latitudes removes individuals from climatic conditions that will eventually exceed their physiological tolerance limits. The Dickcissel is a Neotropical migrant that breeds as far north as Winnipeg, but cannot survive environmental temperatures below freezing during the short days of winter at mid-temperate latitudes. The arrival of migrants on the winter range, however, increases the chances for greater interspecific competition with resident species in years when resource availability might be reduced. This cost, plus the hazards associated with the migratory journey, decreases adult survivorship. The evolution of migratory behavior must, on average, offer a favorable balance between these various costs and benefits.
Birds appear in the fossil record distinct from their reptilian ancestors about 150 million years ago. For the next 50 million years or so a relative uniform and benign maritime climate pervaded the Earth. Sometime around 65 million years ago, however, global climate abruptly changed, perhaps from impact by a large asteroid, and the biota of the planet suffered a major episode of extinction. But a remnant lineage of birds survived and gave rise to the modern groups of birds we see today. Yet with the slow, continuing drift of the continents into higher latitudes that began soon after the first appearance of birds, and the development of mountain ranges as a result of the collision between tectonic plates, climates became more latitudinally and often longitudinally differentiated. The resulting diversity in habitats provided the selective pressures that led to the evolution of migration again and again in different species.
The general model for the evolution of migratory behavior considers a permanent resident that expands its range due to intraspecific competition into an area that is seasonally variable, providing greater resources for reproduction but harsher climactic stress and reduced food availability in the non-breeding season. Individuals breeding in these new regions at the fringe of the species' distribution are more productive, but in order to increase non-breeding survival they return to the ancestral range. This results, however, in even greater intraspecific competition because of their higher productivity, so that survival is enhanced for individuals that winter in areas not inhabited by the resident population. The Common Yellowthroat of the Atlantic coast is a good example. Birds occupying the most southern part of the species' range in Florida are largely nonmigratory, whereas populations that breed as far north as Newfoundland migrate to the West Indies in the winter, well removed from the resident population in Florida. Because a migrant population gains an advantage on both its breeding and wintering range, it becomes more abundant, while the resident, non-migratory population becomes proportionately smaller and smaller in numbers. If changing environmental conditions become increasingly disadvantageous for the resident population or interspecific competition becomes more severe, the resident population could eventually disappear, leaving the migrant population as characteristic of the species. These stages in the evolution of migration are represented today by permanent resident populations, partial migrants, and fully migratory species. As for all adaptations, natural selection continues to mold and modify the migratory behavior of birds as environmental conditions perpetually change and species expand or retract their geographic ranges. Hence, the migratory patterns that we observe today will not be the migratory patterns of the future.
Migration involves not just the evolution of a specific behavioral pattern, but often morphological changes as well. The shape of the wing is a structural correlate with migratory behavior. Migratory species typically have proportionally longer wings, with a higher aspect ratio, than related nonmigratory species. This adaptation reduces the relative impact of wing-tip (induced) drag, resulting in greater effective lift as well as an often more efficient ratio between wing area and body weight. Furthermore, the outer primary feathers, which together with the inner primaries provide forward thrust in flapping flight, are often longer in migrants, giving the wing a pointed rather than a rounded shape. In Asia, the sedentary Black-headed Oriole has a rounded wing, whereas the closely related Black-naped Oriole with pointed wings is migratory between Siberia and India. Albatrosses, falcons, swifts, various shorebirds, and terns, many of which make long-distance journeys, have long, more pointed wings. Even among closely related migrants there is a difference. Thus the pointed wings of the Semipalmated Sandpiper, which migrates from the arctic to only northern South America has noticeably shorter wings than the Baird's and White-rumped sandpipers that fly from the arctic all the way to the southern tip of South America.
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