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What is migration?Ornithologists typically think of migration in terms of the

dramatic round-trip journeys undertaken by species that movebetween high and low latitudes. Even in birds, however, mi-grations of many types occur that vary in regularity of oc-currence, duration, and distance covered. The theme that tiesthe various types of migration together is that they are allevolved adaptations to fluctuating environmental conditionsthat render some areas uninhabitable during some portion ofthe year.

The adaptations to fluctuating environmental conditionsthat render some areas uninhabitable range from irruptivemovements to true migration. Irruptive movements involveirregular dispersal from an unfavorable area to a more favor-able area. In contrast, true migration characteristically in-volves return to the place of origin when conditions improve.

Bird migration includes a broad spectrum of movementsby individuals that range from irregular eruptions of individ-uals to the long-distance round-trip flights that we typicallythink of when migration is mentioned. There are between9,000 and 10,000 species of birds and more than half of thesemigrate regularly. Billions of individual birds are involved inthese migrations. Depending upon species, the migrationmight comprise a journey on foot up and down a mountain(as in some grouse), or it might involve a flight that literallyspans the globe. Some species fly by day, others almost ex-clusively at night; some migrate alone, others in flocks thatmay reach immense size; many migrations involve a return,often with uncanny precision, to localities previously occu-pied.

In terms of sheer magnitude, the migrations of manyseabirds are the most impressive. The famous Arctic tern(Sterna paradisea) nests as far north as open ground exists andmigrates the length of the oceans to spend the winter inAntarctic waters, a round-trip of some 25,000 mi (40,000 km)performed every year of the bird’s life. Some of the great al-batrosses, such as the wandering albatross (Diomedia exulans),circumnavigate the globe by moving west to east over the tur-bulent oceans within the “roaring forties” latitudes south ofthe tips of the southern continents. Sooty shearwaters (Puffi-nus griseus) are extremely abundant seabirds that breed on is-lands deep in the Southern Hemisphere, mostly around New

Zealand and the southern tip of South America. In late springand throughout the northern summer, sooty shearwaters mi-grate northward and circle the basins of the northern Pacificand northern Atlantic Oceans. Flocks of many thousands ofindividuals may be seen along the Pacific coast of NorthAmerica. By late summer they are headed back across theocean to their distant nesting islands, having circled the oceanin the process.

Many shorebirds nest at high latitudes in the Arctic andspend the winter far into the Southern Hemisphere. Theirchicks are precocial and thus require relatively little parentalcare. Adult birds often depart on autumn migration before ju-veniles, leaving the inexperienced youngsters to make theirway to the wintering grounds on their own. Typically, shore-birds migrate in flocks, often at night, but also during the daywhen crossing large ecological barriers such as oceans, gulfs,or deserts requires extremely long flights. American golden-plovers (Pluvialis dominica) make a non-stop flight from theMaritime Provinces of Canada across the western AtlanticOcean to South America, often flying at altitudes that exceed20,000 ft (6,000 m). In spring, the species follows a differentroute northward, crossing the Caribbean and Gulf of Mexicoand then heading north through the interior of North Amer-ica. Its western cousin, the Pacific golden-plover (P. fulva),departs its Alaskan nesting areas and flies over the Pacific toHawaii and beyond, performing single flights of 25,000 mi(7,500 km) or more.

Most waterfowl (swans, geese, and ducks) are shorter-dis-tance migrants, typically nesting and overwintering on thesame continent. They tend to migrate in cohesive flocks thatoften contain family groups and repeatedly use traditionalstop-over locations to rest and refuel. Often flying both dayand night, migrating waterfowl are strongly influenced byweather conditions. When the conditions are right, they cancover many hundreds of miles in a single, high-altitude flight.

Soaring birds can take advantage of a free energy-subsidyfrom the atmosphere. Warming of the earth’s surface inducescolumns of rising warmer air (thermals). Hawks, eagles, vul-tures, storks, and cranes use this atmospheric structure byfinding a thermal and then circling within the column of ris-ing air, gaining altitude with almost no expenditure of energy.Once a substantial altitude has been reached, the birds glide

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Avian migration and navigation

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off, covering ground as their path slowly descends. After cov-ering a considerable distance, often without the need to flaptheir wings, the birds need to locate another thermal and re-peat the process. Under the right weather conditions, largedistances over the ground can be covered with very little en-ergetic cost. Because thermals are present only during thewarmer portions of the day, soaring migrants are almost ex-clusively diurnal and selective in terms of the weather condi-tions under which they migrate.

Some common landbirds (including the passerines) mi-grate during daylight hours. These include swifts, some wood-peckers, swallows, some New World flycatchers, jays, crows,bluebirds, American robin (Turdus migratorius), New Worldblackbirds, European starling (Sturnus vulgaris), larks, pipits,some buntings, cardueline finches, and others. Most song-birds, however, migrate almost exclusively at night. Noctur-nal migrants include many thrushes, flycatchers, sylviid andparulid warblers, vireos, orioles, tanagers, and many buntingsand New World sparrows. Night migrants typically fly aloneor in only very loosely organized flocks and because of theirlesser powers of flight, most make shorter individual flightsand overall migrations than larger, stronger fliers. A typicalnight migration under good flying conditions might encom-pass 200 mi (320 km) and be followed by two or three dayson the ground during which the birds rest, feed, and depositfat supplies that will fuel the next leg of the migration. Somesmall songbirds regularly cross the Gulf of Mexico and theMediterranean Sea-Sahara Desert. Such flights, initiated atdusk, often require more than the night to complete, thus thebirds continue to fly into the following day until a suitablelanding place is reached. If bad weather is encountered, es-pecially over water, many birds may become exhausted andperish.

The blackpoll warbler (Dendroica striata) of North Amer-ica is exceptional. Weighing about 0.4 oz (11 g) at the end ofits breeding season, blackpolls may accumulate enough sub-cutaneous fat in autumn to increase its body weight to ap-proximately 0.7 oz (21 g) before departing from northeasternNorth America on a non-stop flight over the western AtlanticOcean. The trip takes from three to four days to complete,and with an in-flight fat consumption rate of 0.6% of its bodyweight per hour, the blackpoll has enough added fuel for ap-proximately 90 hours of flight. There is essentially no placeto stop enroute and most individuals overfly the Antilles andmake first landfall on the continent of South America. Theirautumn journey is surely among the greatest feats of en-durance in the bird world.

Why migrate?Despite the obvious diversity in migration strategies

among birds, they all represent adaptations to variability inresources and the predictability of that variability. These twovariables determine to a large extent what kind of migratorybehavior will evolve. Resources are the necessities of life: food,water, shelter, etc. Many animals respond to the seasonal dis-appearance of some essential resource by entering an inactiveor dormant state (e.g., hibernation). Most birds, however,preadapted by the ability to fly long distances, have respondedto variable resources by moving to more hospitable areas.

The more variable the resources on which a bird depends,the stronger will be the selection pressure favoring the evo-lution of migration. Another important consideration is thepredictability with which the resources fluctuate. There is nooption for an insectivorous bird such as the blackpoll warblerto spend the winter in its breeding range. Selection favoringobligatory migratory behavior will be strong because any in-dividual that fails to migrate will not survive the winter. Inmore temperate areas, differences in the severity of winterfrom year to year might make it possible for a bird to over-winter in some years, but not in others. Selection in these lesspredictable environments should favor the evolution of moreflexible strategies such as partial migration in which some in-dividuals of a population migrate whereas others remain yearround in the breeding area. Some fluctuations in resources donot always follow regular seasonal changes in climate. Theconiferous seeds and buds eaten by various cardueline finches,such as crossbills, fluctuate not only seasonally, but also fromyear to year and region to region. These fluctuations may be

4 Grzimek’s Animal Life Encyclopedia

Avian migration and navigation Birds


Blackpoll Warbler

SouthAmerica

Migration route of the blackpoll warbler (Dendroica striata). (Illustra-tion by Emily Damstra. Reproduced by permission.)

quite unpredictable, so migration in these species must be fac-ultative, responding directly to local conditions. Thesemovements are termed irruptive because large numbers ofbirds emigrate from the boreal forests in some years, but re-main there in others. Nomadism implies that individuals areperpetually on the move. It is not clear that any bird speciesare truly nomadic, although birds of the desert interior of Aus-tralia are often cited as examples.

Origin and evolution of migrationSeasonal migration is found on all the continents and

among species as diverse as penguins, owls, parrots, and hum-mingbirds. Evidence of the first origins of migration are prob-ably lost forever, but recent phylogenetic reconstructionssuggest that migratory behavior has appeared and disappearedrepeatedly in avian lineages. Its first appearance may well havecoincided with the acquisition of efficient long-distance flightcapability. Although present patterns of migration may havebeen influenced by global climatic events such as glaciation,migration on a large scale probably predated these events.

There is considerable evidence that migratory behavior canappear and disappear quite rapidly in populations. Housefinches (Carpodacus mexicanus) introduced into the more sea-sonal climate of the northeastern United States from a seden-tary population in southern California have evolved full-scalepartial migration in fewer than 50 years. Experiments with asylviid warbler, the blackcap (Sylvia atricapilla), have demon-strated strong genetic control over several aspects of migra-tory behavior. The blackcap is widespread in western Europefrom Scandinavia south to areas around the Mediterranean,and island populations live on the Canary and Cape Verde is-lands. This single species exhibits a wide range of migratorybehavior from obligate long distance migrants in the north tonon-migratory populations on the Cape Verdes. Cross-breed-ing of Canary Island birds with northern obligate migrantsproduced individuals that showed almost exactly intermedi-ate levels of migratory activity in captivity. This activity,known as Zugunruhe or migratory restlessness, is exhibitedtwice a year in captive migratory birds. It is characterized bya daily rhythm in which these birds rest for a short period inthe evening and then awaken and flutter vigorously on theirperches throughout much of the night.

Control of the annual cycleMigratory behavior is an integral part of the annual cycle

of those species that are obligate migrants. Like other eventsthat occur during each year of a bird’s life (e.g., molt, breed-ing), migration is under the control of an endogenous clockcalled a circannual rhythm, a clock that runs with a periodic-ity of approximately one year. In birds held under constantenvironmental conditions (constant temperature, constantdim light), the events of the annual cycle, including migra-tory behavior, continue to recur in the proper sequence foryears. Thus, although migration in the real world is certainlyinfluenced by ambient environmental conditions (climate,weather, food supply, changes in photoperiod), the underly-ing stimulus comes from within. In nature, circannualrhythms are synchronized with the real world changes in sea-sons through the associated systematic changes in day-length(photoperiod).

Energetics of migrationBirds are remarkably adapted to flight by virtue of a lung

and air-sac system that permits maximal oxygen uptake, hol-low bones and other weight-reduction adaptations, and ex-traordinarily efficient hemoglobin. Nonetheless, longmigratory flights are extremely strenuous and energy-de-manding. Gram for gram, fat is the most calorie-rich sub-stance that animals can sequester in their bodies. It providesabout twice the calories per gram as a carbohydrate or pro-tein, and oxidation of fat is a more efficient metabolic process.Fat deposition prior to migration results from changes indiet, increases in food intake, and changes in metabolism.Migratory fat is deposited throughout a bird’s body, includ-ing within internal organs such as the heart and liver, butmost is laid down in subcutaneous “fat bodies.” The amountof fat deposited varies greatly among migratory species.Long-distance migrants and those that must cross large eco-logical barriers (e.g., the blackpoll warbler) deposit largeramounts of fat prior to initiating flight. Typical non-migra-tory birds carry 3–5% of fat-free mass as fat; in passerine mi-grants this figure may reach 60–100% at the beginning of along flight.

Once in flight, the migrant’s fat deposits are depleted. Fuelstores must be replenished during a migratory stop-over be-fore the bird will be able to execute another flight segment.The rate of fuel recovery will depend upon the quality of thehabitat in which the bird lands and lays over. Typically, song-birds are able to deposit fat at rates of 2–3% per day and un-der optimal conditions the rate may reach 10% of fat-freebody mass. Given the obvious importance of stored fat to thesuccess of a continuing migration, finding stop-over habitatin which fuel supplies can be quickly replenished is critical toa migrant’s success.

Altitude of migration and flight speedMost bird migration proceeds at rather low altitudes, as

has been revealed by radar studies. Larger, faster-flyingspecies such as shorebirds and waterfowl tend to fly at thehighest altitudes. It is usual to find them up to 15,000 ft (4,500m) or more when migrating overland. Shorebirds on a trans-oceanic flight have been detected passing over Puerto Rico at23,000 ft (7,000 m). Bar-headed geese (Anser indicus), regu-larly migrate over the tops of the highest Himalayas. Song-birds migrate at substantially lower altitudes. At night, mosttypically fly below 2,000 ft (610 m) and nearly all will be be-low 6,500 ft (1,980 m).

The speed at which a migrating bird makes progress overthe ground depends upon how fast it flies (its air speed) andthe wind direction and speed where it is flying. Most passer-ine migrants fly at relatively slow air speeds of 20–30 mph(32–48 km/hr). Flying in a tail wind could easily double theirspeed over the ground; likewise a head wind could substan-tially retard progress and a cross-wind could cause the birdto drift from its preferred heading direction. Waterfowl andshorebirds fly faster, with air speeds in the 30–50 mph (48–80km/hr) range. Given the potential influence of winds on theprogress and success of migration, it is not surprising that mi-grating birds show quite refined selectivity in terms of theweather conditions under which they initiate flight.


Birds Avian migration and navigation


Orientation and navigationOne of the most remarkable things about birds, and mi-

grating birds in particular, is their ability to return to specificspots on the earth with amazing precision. This phenomenonis termed homing. Many different kinds of animals exhibithoming ability, but the behavior reaches its pinnacle in birdswhere the scale of homing flights may reach thousands ofmiles in a long-distance migrant. Not all migratory speciesshow strong fidelity to previously occupied places, but manydo, returning year after year to exactly the same place to nestand to exactly the same place to spend the winter. How theyare able to navigate with such precision over great distancesis a question that we still cannot answer completely.

Many young birds on their first migration travel alone,without the potential aid of more experienced parents or otherolder birds. Having never been to the winter range of theirpopulation before, they are not navigating toward a preciselocality, but rather toward the general region in which theirrelatives spend the winter. But, how do they know in whichdirection the wintering place lies and how far it is from their

birthplace? At least some of this information is geneticallycoded and controlled by the endogenous circannual rhythmsdiscussed above. This migratory program provides the youngbird with information about the direction it should fly on itsfirst migration. This can be demonstrated readily by captivenocturnal migrants during the seasons of migration when thebirds become restless and active at night. Displaying a be-havior known as Zugunruhe, birds placed in a circular orien-tation cage will exhibit hopping that indicates the directionin which they would migrate if free-flying. Cross-breedingexperiments with European blackcaps from populations withquite different migration directions have shown that the bear-ing taken up by inexperienced first-time migrants is stronglyheritable and controlled by a small number of genes. Somespecies have complicated migration routes involving largechanges in direction. The garden warbler (Sylvia borin) andblackcap are examples that have been studied. Western Eu-ropean populations initially mig\rate southwestward towardthe Iberian Peninsula and then change direction to a moresouthward heading that takes them into Africa. Hand-raisedwarblers kept in Germany for the entire season and tested re-




Arctic Tern Sooty Shearwater

Wandering Albatross

The migration routes of several birds. (Illustration by Emily Damstra. Reproduced by permission.)

peatedly in orientation cages showed this change in directionat approximately the right time during the migration season.

It is less clear how the distance of the first migration mightbe coded. It is known that the amount and duration of mi-gratory restlessness exhibited by different species and popu-lations is correlated with the distances that they migrate andthe lengths of their migration seasons, suggesting that somecomponent of the endogenous time-based program also con-trols the distance migrated. However, migrants are oftenstopped and held up by bad weather and it is not clear howthe endogenous program might adjust for these changes.

Once having bred or spent the winter in a specific place,many birds will show strong site fidelity to those places. Tohome to a specific site requires more than a simple directionand distance program. The animal must be able to navigateto a particular goal, compensating for errors made along theway or for departures from course caused by wind. This hasbeen demonstrated with individuals of many species that havehomed after being artificially transported to places where theyhave never been before. In the case of various seabirds, hom-ing distances were thousands of miles. Animals employ a va-riety of different mechanisms in order to home. For shortdistances, a number of relatively simple strategies will work:laying down a trail that is retraced; maintaining sensory con-tact with the goal by sight, sound, or smell; inertial naviga-tion; referring to learned landmarks; or even random orsystematic search. But for the very large distances involved insome cases of goal navigation in birds, including long-distancehoming by pigeons, some other explanation is required. Thebest evidence suggests that birds employ a navigational mech-anism based on possessing an extensive map that provides theindividual with information about its spatial location relativeto its goal (home), and a compass that is used to identify thecourse direction indicated by the map. Obviously, neithercomponent will work by itself. A compass will be useless with-out information about the direction toward home from themap. Likewise, the map, telling the bird that home lies to thenorth of its present location, will also be ineffective in the ab-sence of a compass to indicate the direction “north.”

Homing pigeons, and probably other birds as well, employa number of strategies when attempting to return to a spe-cific goal. There is good evidence that they use informationperceived during the displacement journey to the release siteto determine the direction of displacement (route-based nav-igation). They probably also use familiar landmarks when nav-igating close to home. These means will be of limitedeffectiveness at great distances, yet even when released at un-familiar sites hundreds of miles from their home loft, pigeonswith some homing experience return directly and rapidly.Therefore, whereas pigeons may make use of various strate-gies when homing, it appears that a map based solely on stim-uli perceived at a distant and unfamiliar location are sufficientto provide the requisite homing information.

During the last half of the twentieth century, a great dealof experimental effort was devoted to discovering the physicalbasis of the compass and map employed in homing navigationby birds. Interestingly, compasses have been studied primar-ily in migratory birds, especially those species that migrate at

night, whereas the map component of navigation has beenstudied almost exclusively in non-migratory homing pigeons.

Bird compassesA compass is involved not only in map and compass nav-

igation such as occurs when a homing pigeon is taken awayfrom its loft; it is also a fundamental component of orienta-tion by migrating birds. Two main experimental approacheshave been used to study bird compasses. First, various ma-nipulations have been performed on homing pigeons and theireffects monitored by observing the initial flight directiontaken by pigeons when they were released at distant sites. Sec-ond, orientation in migratory birds has been studied by ob-serving captive birds in a migratory condition. These birdswill hop and flutter in the intended migratory direction whenplaced in a circular orientation cage (a behavior known as Zu-gunruhe). The studies on pigeons and captive migrants re-vealed that birds possess several different compass capabilities.

Magnetic compassThe ability to detect the earth’s magnetic field is wide-

spread among animals and microbes. The magnetic compassin birds has been demonstrated by predictably altering themigratory orientation of birds through manipulating the mag-netic field within their cages, and by altering the initial hom-ing bearings of pigeons by changing the magnetic field arounda bird’s head with miniature magnetic coils. By independentlymanipulating the horizontal and vertical components of anearth-strength magnetic field, it has been shown that the mag-netic compass of birds is not based on the polarity of the fieldas is our technical compass. Rather, the bird compass relieson the inclination or dip angle of magnetic field lines to de-termine the direction toward the pole or toward the equator.Within both the Northern and Southern Hemispheres, mag-netic lines of force dip downward toward the poles, so thesame magnetic compass mechanism is effective for species liv-ing within either hemisphere. Birds that cross the magneticequator will face a problem and in two species of trans-equa-torial migrants it has been shown that the birds’ directionalresponse, with respect to magnetic inclination, reverses afterthey experience a period in a magnetic field without inclina-tion, simulating an equatorial field.

A magnetic orientation capability develops spontaneouslyin young birds. The mechanism by which birds perceive themagnetic field remains uncertain. There is experimental evi-dence to support a receptor based on tiny particles of mag-netite located in the anterior region of the head and evidencesuggesting that photoreceptive pigments in the eye providethe sensor.

Sun compassThe sun compass is found in many animals. To use the

sun as a compass, of course, one must take time of day intoaccount. Animals accomplish this because the sun compass islinked to the internal circadian clock possessed by all animals.With this time-compensation mechanism in place, a bird canorient in a given compass direction at any time of day. Thelink between the circadian clock and sun-compass orientationcan be demonstrated by changing the bird’s internal clock byconfining it in a room with a light:dark cycle that differs by




several hours from that outside. If this is done with a hom-ing pigeon that is then released on a sunny day, it will makea predictable error in identifying the direction in which to fly.It will take the time indicated by its internal clock to be cor-rect and thus misinterpret the direction of the sun. With thissort of experiment it can be shown, for example, that a pi-geon will mistake a midday sun, high in the sky, for a risingor setting sun that would be near the horizon. This is becauseonly the azimuth direction of the sun is used in sun compassorientation; the sun’s elevation is ignored. Birds apparentlyhave to learn the sun’s path across the sky and that path variesconsiderably with latitude and with season. Once the path islearned, compass directions must be assigned to it, i.e., thepigeon must learn that the sun rises in the east, etc. There isevidence that the magnetic compass provides the compass di-rections that are then associated with the azimuths of the sun’spath. The sun compass is the primary compass employed byhoming pigeons to identify the direction that its map indi-cates is homeward. Its role in migratory birds is less clear, butit may be important for species that migrate at very high lat-itudes in the Arctic where daylight is nearly continuous andmagnetic directions often unreliable.

Star-pattern compassNight-migrating birds are the only animals known to use

the stars as a compass. Once learned, orientation is based onthe fixed spatial relationships among stars and groups of stars:birds can orient even under the fixed sky of a planetarium.Young birds learn the relationships among star patterns byobserving the rotation of the night sky that results from theearth’s rotation on its axis. In this way they are able to local-ize the center of rotation (Polaris) and thus true north. Theinnate migratory direction is coded with respect to stellar ro-tation in young birds.

Polarized-light compassMost nocturnal migrants initiate flights shortly after sun-

set and diurnal migrants often take flight around sunrise. Atthese times of day, patterns of polarized skylight, resulting

from the sun’s light passing through the earth’s atmosphereare very conspicuous across the vault of the sky. Because it issunlight that is being polarized, the patterns of polarized lightin the sky change in parallel with the movement of the sunand rotate around the celestial pole similar to stars at night.Thus observation of celestial rotation via polarized light inthe sky can also reveal true north. Experiments with migrantsin orientation cages in which polarized light has been ma-nipulated and skylight polarization patterns have been elimi-nated show that the birds employ a polarized light compass.

Multiple compasses and interactionsIt is impossible to know why evolution has endowed birds

with so many compasses, but presumably it is advantageousto have back-up systems available when navigational problemssuch as overcast skies or magnetic anomalies are encounteredenroute. The compasses appear to be related to one anotherhierarchically. In homing pigeons, for example, the sun com-pass is used preferentially whenever the sun is visible; the mag-netic compass seems to provide an overcast sky back-up. Inadult migrants, the relationship among compasses is some-what less clear and there may be differences among species.Based on the information available as of 2001, the magneticcompass appears to be central to migratory orientation, pro-viding the primary directional information. When an imme-diate choice of direction needs to be made, visual compassesmay be used, but ultimately the birds seem to rely on theirmagnetic compass.

In many areas, especially at high latitudes, magnetic and ge-ographic compass directions may differ substantially (magneticdeclination). In these situations, the true or geographic compassdirections will be the more reliable indicators of the directionsthat birds need to travel. During the development of compasscapabilities in young songbirds, true compass directions identi-fied by observing celestial rotation of stars at night and skylightpolarization patterns during the day are used to calibrate thepreferred magnetic migratory direction. In this way, the visualand non-visual compasses are brought into conformity.




WS

Using a star compass, migrating birds can navigate at night. (Illustra-tion by Emily Damstra. Reproduced by permission.)

WS

During daylight, birds can navigate using the sun. (Illustration by EmilyDamstra. Reproduced by permission.)

Navigational mapsThe search for the physical basis of the navigational map

sense has been one of the more controversial issues in the studyof animal behavior. There are currently two hypotheses to ex-plain the map sense of birds. They may not be mutually ex-clusive and as is the case with the compass, birds may havemore than one map to consult when faced with a navigationalproblem. Experimental studies of the navigational map havebeen confined almost exclusively to homing pigeons.

The olfactory map hypothesis is based almost entirely onwork done with homing pigeons. It states that pigeons learnan odor map of the region by associating different odors withthe different directions from which winds carry them past theloft. A large body of experimental evidence involving bothmanipulations of the pigeon’s olfactory system and manipu-lations of the odor environment supports the odor map. Ex-perienced pigeons seem to be able to use an odor-based mapover surprisingly large distances (up to 250 mi [400 km]). Howodorous substances could provide reliable spatial informationwhile being transported in an often turbulent atmosphere hasbeen a contentious issue. Very recently, analysis of trace gasesin the atmosphere has shown that spatially stable patterns ofodorants sufficient to account for the known levels of preci-sion exhibited by homing pigeons do exist, although the odorsactually used by the pigeons remain unknown.

The other hypothesis is that birds might use the earth’smagnetic field as a map. Components of the magnetic fieldvary systematically over the earth, particularly as a functionof latitude. There is good evidence that homing pigeons candetect very small differences in magnetic fields that would berequired to use the information as a map (a much more chal-lenging task than using the magnetic field as a compass). Thebest evidence supporting the idea of a magnetic map is indi-rect. Homing pigeons are often disoriented when released atmagnetic anomalies, places where the earth’s field is dis-turbed, usually as a result of large iron deposits underground.The effect occurs only when the pigeons are forced to maketheir navigational decisions right at the anomaly: if releasedoutside the anomaly, they are not disturbed by flying acrossit on the way home. That the pigeons are disturbed even un-der sunny skies when their sun compass is readily availablefurther suggests that the magnetic anomalies are somehow af-fecting the map step of navigation rather than the compass.

We know that migratory birds exhibit striking site accu-racy and therefore must possess sophisticated homing abili-ties. However, very little is known about navigational mapsin these species. Although many migrants return with preci-sion to previously occupied places, we do not know whetherthey are really goal-orienting throughout the migratory jour-ney or only when they get closer to the destination.




ResourcesBooksAble, Kenneth P., and Mary A. Able. “Migratory Orientation:

Learning Rules for a Complex Behaviour.” In Proceedings ofthe 22nd International Ornithological Congress, Durban, ed.Nigel J. Adams and Robert H. Slotow. Johannesburg:Birdlife South Africa, 1999.

Berthold, Peter. Bird Migration. 2nd ed. New York: OxfordUniversity Press, 2001.

Berthold, Peter. Control of Bird Migration. New York:Chapman and Hall, 1996.

Wiltschko, Roswitha, and Wolfgang Wiltschko. “CompassOrientation as a Basic Element in Avian Orientation andNavigation.” In Wayfinding Behavior: Cognitive Mapping andOther Spatial Processes, ed. R.G. Golledge. Baltimore: JohnsHopkins University Press, 1999.

Wiltschko, Roswitha, and Wolfgang Wiltschko. MagneticOrientation in Animals. Berlin: Springer-Verlag, 1995.

PeriodicalsAble, Kenneth P. “The Concepts and Terminology of Bird

Navigation.” Journal of Avian Biology 32 (2000): 174–183.

Able, Kenneth P. “The Debate Over Olfactory Navigation byHoming Pigeons.” Journal of Experimental Biology 199(1999): 121–124.

Phillips, John B. “Magnetic Navigation.” Journal of TheoreticalBiology 180 (1996): 309–319.

Wallraff, Hans G. “Navigation by Homing Pigeons: UpdatedPerspective.” Ethology, Ecology and Evolution 13 (2001): 1–48.

Wallraff, Hans G., and M.O. Andreae. “Spatial Gradients inRatios of Atmospheric Trace Gases: A Study Stimulated byExperiments on Bird Navigation.” Tellus 52B (2000):1138–1157.

Wiltschko, Wolfgang, et al. “Interaction of Magnetic andCelestial Cues in the Migratory Orientation of Passerines.”Journal of Avian Biology 29 (1998): 606–617.

OtherHave Wings, Will Travel: Avian Adaptations to Migration. 25 July

1997. <http://www.umd/umich.edu/dept/rouge_river/migration.html>

Bird, David M. “Migration: Your Questions Answered.” BirdWatcher’s Digest <http://www.petersononline.com/birds/bwd/backyard/watching.shtml>

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