The Geography of Bird Migration

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  • This article was downloaded by: [University of Missouri Columbia]On: 17 March 2013, At: 20:34Publisher: RoutledgeInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

    Journal of GeographyPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/rjog20

    The Geography of Bird MigrationPhilip R. PrydeVersion of record first published: 16 Aug 2007.

    To cite this article: Philip R. Pryde (2002): The Geography of Bird Migration, Journal of Geography, 101:5, 207-221

    To link to this article: http://dx.doi.org/10.1080/00221340208978501

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  • The Geography of Bird Migration

    Philip R. Pryde

    ABSTRACT The following material has been

    designed as a curriculum module to famil- iarize students with the geographic con- cepts associated with the migration of North American birds. These concepts include breeding territories, orientation, weather factors, mapping, and flyways. This module would be most suitable for grades 4 to 8. The subject matter is inher- ently interdisciplinary in nature, encom- passing concepts from geography, the nat- ural sciences, and environmental studies, and should help meet "teaching across the curriculum" requirements. It can also be adapted to extra-curricular offerings, such as special after-school classes or courses taught by organizations such as the Audubon Society. Teachers may modify the material to match the classroom time they have available.

    Key words: migration, breeding habitat, f l y - ways, orientation, wintering habitat

    Pkilip R. Pryde is. a professor in the Department of Geography at San Diego State University. His teaching and research inter- ests include geographical aspects of environ- mental policy, with an emphasis on preserved natural areas. He is also a past president of the San Diego Audubon Society, for which he leads field trips and teaches courses on the field identification of North American birds.

    INTRODUCTION

    nating subjects for adults and children alike. The migration of birds has intrigued people for thousands of years, having been discussed by Aristotle, among many others. North American interest in bird migration dates back at least to the observations of the artist Mark Catesby (1682-1749), and the writings of William Bartram (1739-1823) (Cruikshank 1957; Field 2000). In 1918 Congress passed the Migratory Bird Treaty Act, which gave the first legal protection to migratory birds.

    Many fascinating works exist on the topic of where and how birds migrate (Able 1999, ch. 2; Berger 1961; DeGraaf and Rappole 1995; Elphick 1995; Hagan and Johnston 1992; Mead 1983; Rappole 1983,1995). In recent years attention has been increasingly directed to the apparent reductions in numbers of many migratory bird species and to losses of their habitats (Martin and Finch 1995; Terborgh 1989). For a book on bird migration written in a more popular vein, try Weidensaul (1999).

    Here, the subject of bird migration is presented as a curriculum module designed to acquaint students with the geographic concepts inherent in patterns of bird migration in the Western Hemisphere. It is designed for use by students in grades 4 to 8. Components of the module include: Why birds migrate, The mechanics of migration, Other aspects of migration, Migration and habitat, and a classroom exercise and reading assignment. This curriculum module address- es National Geography Standards 1, 8, 14, and 26.

    reasons why birds migrate, how they do it, and what some of the most impor- tant geographic aspects of migration are. The emphasis is on neotropical migrants (birds that migrate from North America to Central and South America and the Caribbean - see Glossary in Appendix E), though other interesting migrants are discussed as well. Secondary goals are to acquaint students with a variety of bird families, to increase their familiarity with the political subdivi- sions of North and Middle America, and to help develop their atlas and map- ping skills.

    The material as presented here is designed as a guide for teachers. A one-page reading assignment for students in grades 4-6 is included as Appendix D. The homework exercise (Appendix A) is most suitable for grades 6-8, but can be attempted in grades 4-5 at the teacher's discretion. Teachers may modify the material as presented here to match the time available for presenting this unit.

    WHY Do BIRDS MIGRATE?

    excessively cold weather and to reach a more reliable food supply during the winter season. The farther north you live in North America, the shorter the growing season, and the Ionger the coId season with reduced food supply and higher energy requirements. Thus, for most birds, a reliable food base exists in the northern regions only in the summer months. In most cases this means

    Birds, and their manner of flight and migration, are intrinsically fasci-

    The primary goals of the module are to familiarize students with the

    The main reasons that birds migrate southward in winter are to avoid

    Journal of Geography 101: 207-221 02002 NationaI Council for Geographic Education

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  • 208 Pryde

    migrating to a warmer climate during the winter, where food will be more readily available.

    But if there is adequate food in their wintering region, why then do they fly north in summer? One expla- nation of the northward migration phenomenon is that for some species of birds, nesting in the more northerly forests or sub-arctic may have some distinct advantages. Among these are less competition for nesting sites and food and a more predator-free environment. Some types of carnivores that elsewhere would prey on juvenile birds (such as snakes) are relatively scarce in the far north. Also, summer days are longer in the north, and thus more feeding time is available. The longer days also produce smaller diurnal temperature changes, another advantage for newly-hatched chicks.

    long-distance migration to breeding grounds. For example, consider birds that nest in boreal conifer forests. At the end of the ice ages, the northern limit of conifers was prob- ably much farther south than it is today. As the glaciers retreated, and climate gradually became warmer, conifers began growing in increasingly higher latitudes. The birds took advantage of this, colonizing the new conifer forests as the trees advanced farther and farther north. With time, what might have been a relatively short migration to suit- able winter habitat became a journey of many hundreds, or even thousands, of miles. At the same time, a tendency towards over-population on traditional breeding grounds may have pushed some members of essentially tropical species towards the less heavily populated new forests to the north.

    incentive for migration than cold temperatures (Greenberg and Lumpkin 1991). Even small birds such as chickadees will remain in their breeding areas throughout the winter, despite occasional bitterly cold temperatures, because their food supply remains plentiful and accessible. In a similar manner, well-maintained bird feeders will induce some individuals of species that normally migrate to over-winter in a residential neighborhood. Note, though, that this prac- tice might prove harmful to some birds if the person tend- ing the feeder happens to move away in the middle of a particularly harsh winter.

    Long-term climatic change may also help explain

    A reduction in food supply is a more important

    Do all birds migrate?

    no need to migrate, as food and warmth are available year- round. Even in mid-latitude regions (such as most of the United States), some species (most types of woodpeckers, grouse, and jays, for example) can find adequate suste- nance throughout the year, and do not need to migrate to more tropical realms. But in the northern states and in Canada, most species do migrate. One study found that of 215 species of breeding birds in the state of Michigan, fewer than 20 did not migrate at all (Berger 1961).

    ber of species, such as snowy owls, ravens, and snow

    No, not all. Most species of birds in the tropics have

    In extreme northerly latitudes, only a small num-

    buntings, are able to adapt to winter-season conditions. There are various mechanisms that allow this to take place. In most cases, over-wintering birds have food preferences that are available year-round, such as seeds for chickadees. Thus, they have a reliable energy supply that enables them to survive the cold winter months. Their feather structure usually has excellent insulation properties, as well. In a few cases, certain species are able to engage in torpor (meaning their heart rate, body temperature, and metabolism markedly slow down) during long, cold winter nights, or even for a few days at a time. Only one species, the poor- will of the American West, has been suggested as engagng in all-winter hibernation (Jaeger 1948).

    Vertical migration Migration doesnt always have to be latitudinal, that

    is, occurring in basically a north-south direction. It can also be altitudinal, or vertical. In this latter type of migration, certain birds will move seasonally up and down the slopes of major mountain ranges. During summer, food sources and attractive habitat will be available at the higher eleva- tions; conversely, food is more readily available at the lower elevations in winter. Rosy finches, pine grosbeaks, and some species of chickadees, nuthatches, and jays are exam- ples of North American birds that engage in vertical migra- tion. Some populations of juncos migrate vertically, while other juncos travel hundreds of miles north in spring and south in fall (USFWS 1979, 91).

    Two anomolies are mountain quail and blue grouse. Mountain quail, apparently in no great hurry, walk down the mountainside in fall. Blue grouse breed at lower mon- tane elevations and winter in high elevation pine forests (Berger 1961, 119).

    tical feet up a mountainside is the equivalent of travelling about 300 miles northward. In both cases, the cooler cli- mate restricts vegetation. This is why the treeless tops of high mountains are called alpine tundra.

    area (about 5,500 feet elevation) to a meadow at 9,500 feet in the Rocky Mountains, that would be like flying from Denver to approximately where in Canada? How many miles would it have to fly to reach this Canadian location? At about what latitude would it be? (See Appendix F for further information on these exercises.)

    In addition, there are also a few examples of longi- tudinal migration, that is, instances where birds migrate mainly in an east-west direction. Species in which at least a part of the population does this include evening grosbeaks, California gulls, and white-winged scoters (Berger 1961). And in at least one case, Heermanns gull, the wintering region (the California coast) is north of the breeding regon (the Gulf of California).

    In terms of climatic variation, moving a thousand ver-

    Exercise #1: If in spring a bird flies from the Denver

    Dispersal and irruptions

    Migration means a regular seasonal movement to a more These terms describe other types of bird movements.

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  • The Geography of Bird Migration 209

    supportive biogeographic environment for the purpose of finding either suitable breeding habitat (summer) or a more reliable food supply (winter). As we have seen, migra- tion can be either horizontal or vertical.

    Dispersal refers to a somewhat more random move- ment at a particular time of year, most typically after the nesting season has ended and the young are now strong enough to fly on their own. This is called "post-breeding dispersal." The usual purpose of this is to find different and hopefully more abundant feeding areas for the newly expanded family. The direction of the dispersal may even be northward; many species of birds may head northward for a month or two in late summer before later heading south. Also, the nest itself is an attraction for predators; the sooner it can be abandoned, the better. Later, when the weather turns cooler and the young birds are stronger, the whole family may take part in migration, though not neces- sarily as a family group.

    means that in a particular year a large number of birds of a given species start showing up in locations (usually during the fall or winter) where they are normally rare or absent. One possible reason for this is an excess of younger birds in the customary breeding region. But some species, such as Lawrence's goldfinch, seem to wander irregularly for reasons that have not yet been identified.

    There is also the concept of an "irruption," which

    THE MECHANICS OF MIGRATION How far do birds migrate?

    migrate only a short ways, others cover huge distances. As noted above, birds that engage in vertical migration may need to travel a relatively few miles. Among the species that move southward in fall, some travel only a few hundred miles. A portion of the population of robins,

    There is a great deal of variation; some birds

    Figure 1. Black-and-white warbler (Mniotilta varia), a common migratory and breeding species over much of eastern United States and Canada. Drawing by David Stump, San Diego Audubon Society.

    '"$ I I I

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    Figure 2. Some major f lyways f o r migratory birds between Canada, the United States, and tropical America.

    meadowlarks, several species of sparrows, blackbirds, war- blers, and most ducks, for example, may go just from Canada and the northern tier of US. states in autumn to the Gulf Coast states, Arizona, or California. The winter range of many species, such as yellow-rumped warbler, white-crowned sparrow, Scott's oriole, white pelican, and many others, is partly in the U.S. and partly in Mexico. Some Arctic species, such as snowy owls, northern shrikes, and Bohemian waxwings, retreat southward only as far as the northern states of the US. in winter.

    winter and instinctively head for somewhere in tropical America. This includes most of...