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　　Migration of Birds

　　Orientation andNavigation

　　A .Factors in a bird'senvironment select for the expression of migratory behavior, leading to theevolution of a migratory pattern or, on the other hand, to the loss ofmigratory abilities. Factors in the environment function to provide direct,proximal stimulation for the physiological preparation for migration. Factorsin the environment also provide information that allows birds to navigateduring migratory passage. Navigation requires knowing three things: currentlocation, destination, and the direction to travel to get from the currentlocation to the destination. Humans eventually learned to use both the sun andthe stars to obtain this information. Recently we invented more precisesatellite-based technologies that have made these celestial cues fordetermining geographic positions superfluous and developed electronic aids tonavigation that allow orientation without reference to the natural environment.Birds have successfully navigated for eons using environmental information.

　　B .Birds are not alone in their ability to navigatelong distances. Fish, mammals, and even insects make migratory journeys. Butthe clarion honking of geese moving in huge skeins across the vault of theheavens, the twittering of migrants filtering down out of the night sky, theflocks of newly arrived birds filling woodlands, fields, and mudflats makes usmost aware of the seasonal movements of birds and fills us with awe and wonderas to how such a magnificent event can be accomplished season after season,year after year, with such unerring precision.

　　C .Of the three kinds of information necessary fornavigation, we know something about the environmental cues that birds use to orienttheir migratory flight in the proper direction. On the other hand, there alsois well-supported experimental evidence that birds use neither the positions ofthe sun or the stars to know where they are or where they are to go. It hasbeen shown, however, that birds must learn both the location of the winteringarea as well as the location of the breeding area in order to navigateproperly, but we have no idea what information they are learning. Nor do weknow what cues birds use to know the location of their migratory destinationwhen they are in their wintering locale, often thousands of miles away. Therecapture of banded birds at the same places along the route of the migratoryjourney in subsequent years suggests that some species also learn the locationof traditional stop-over sites, but how they do that remains a mystery.

　　Vector Navigation

　　D.European Starlings passthrough Holland on their migration from Sweden, Finland, and northwesternRussia to their wintering grounds on the channel coast of France and thesouthern British Isles. Perdeck transported thousands of starlings from TheHague to Switzerland, releasing these banded birds in a geographic location inwhich the population had never had any previous experience. The subsequentrecapture of many of these banded birds demonstrated that the adults, which hadpreviously made the migratory flight, knew they had been displaced and returnedto their normal wintering range by flying a direction approximately ninetydegrees to their usual southwesterly course. The juveniles, which had nevermade the trip before, in contrast, continued to fly southwest and wererecaptured on the Iberian peninsula. These first-year birds "knew"what direction to fly, but did not recognize they had been displaced, thusending up in an atypical wintering range. In subsequent years these now adultbirds returned to again winter in Spain and Portugal. Coupled with anotherdisplacement of starlings to the Barcelona coast in Spain, Perdeck concludedthat the proper direction of the migratory flight was innate, that is,inherited in their DNA, since the naive juveniles could fly that direction, andthat the birds were also genetically programmed to fly a set distance. This isthe same vector or dead-reckoning navigation program Lindberg used to fly fromNew York to Paris by maintaining a given compass direction (or directions) fora predetermined time (i.e., distance). But this study demonstrated that thisnavigation system is modified by experience, since adults knew they were not inHolland any longer and knew that in order to get to their normal winteringgrounds they needed to fly a direction that they had never flown before! Theseresults are truly amazing. And we don't know how they did it.

　　E.Displacement studies in the Western Hemisphere usingseveral species of buntings also demonstrated that birds recognized they hadbeen moved and could fly appropriate, yet unique, routes to return to theirnormal range. Yet adult Hooded Crows transported latitudinally by over 600 km fromwintering grounds in the eastern Baltic to northwestern Germany failed torecognize this displacement. In the spring they oriented properly but migratedto Sweden, west of their normal breeding range. This species used vectornavigation, but did not know the location of its traditional destination. Sinceit is generally accepted that migratory behavior evolved independently againand again in different bird populations, a single explanation to fit all casesperhaps should not be expected.

　　Orientation Cues

　　F .Most of theeffort applied to understanding how birds make a migratory flight has beendirected toward environmental cues that birds use to maintain a particularflight direction. These cues are landmarks on the Earth's surface, the magneticlines of flux that longitudinally encircle the Earth, both the sun and thestars in the celestial sphere arching over the Earth, and perhaps prevailingwind direction and odors.

　　Landmarks are useful as a primarynavigation reference only if the bird has been there before. For cranes, swans,and geese that migrate in family groups, young of the year could learn thegeographic map for their migratory journey from their parents. But most birdsdo not migrate in family flocks, and on their initial flight south to thewintering range or back north in the spring must use other cues. Yet birds areaware of the landscape over which they are crossing and appear to use landmarksfor orientation purposes. Radar images of migrating birds subject to a strongcrosswind were seen to drift off course, except for flocks migrating parallelto a major river. These birds used the river as a reference to shift theirorientation and correct for drift in order to maintain the proper ground track.That major geographic features like Point Pelee jutting into Lake Erie or CapeMay at the tip of New Jersey are meccas for bird-watchers only reflects thefact that migrating birds recognize these peninsulas during their migration.Migrating hawks seeking updrafts along the north shore of Lake Superior or theridges of the Appalachians must pay attention to the terrain below them inorder to take advantage of the energetic savings afforded by these topographicstructures.

　　G.Since humans learned to use celestialcues, it was only natural that studies were undertaken to demonstrate thatbirds could use them as well. Soon after the end of the Second World War,Gustav Kramer showed that migratory European Starlings oriented to the azimuthof the sun when he used mirrors to shift the sun's image by ninety degrees inthe laboratory and obtained a corresponding shift in the birds' orientation.Furthermore, since the birds would maintain a constant direction even thoughthe sun traversed from east to west during the day, the compensation for thismovement demonstrated that the birds were keeping time. They knew whatorientation to the sun was appropriate at 9 a.m. They knew what different anglewas appropriate at noon, and again at 4 p.m. It has been recently shown thatmelatonin secretions from the light-sensitive pineal gland on the top of thebird's brain are involved in this response. Not only starlings but homingpigeons, penguins, waterfowl, and many species of perching birds have beenshown to use solar orientation. Even nocturnal migrants take directionalinformation from the sun. European Robins and Savannah Sparrows that wereprevented from seeing the setting sun did not orient under the stars as well asbirds that were allowed to see the sun set. Birds can detect polarized lightfrom sunlight's penetration through the atmosphere, and it has beenhypothesized that the pattern of polarized light in the evening sky is theprimary cue that provides a reference for their orientation.

　　Using the artificial night sky provided byplanetariums demonstrated that nocturnal migrants respond to star patterns.(quite analogous to Kramer's work on solar orientation, Franz Sauerdemonstrated that if the planetarium sky is shifted, the birds make acorresponding shift in their orientation azimuth. Steve Emlen was able to showthat the orientation was not dependent upon a single star, like Polaris, but tothe general sky pattern. As he would turn off more and more stars so that theywere no longer being projected in the planetarium, the bird's orientationbecame poorer and poorer. While the proper direction for orientation at a giventime is probably innate, Emlen was able to show that knowing the location of"north" must be learned. When young birds were raised under aplanetarium sky in which Betelgeuse, a star in Orion of the southern sky, wasprojected to the celestial north pole, the birds oriented as if Betelgeuse was"north" when they were later placed under the normally orientatednight sky, even though in reality it was south!

　　H.Radar studies have shown that birds domigrate above cloud decks where landmarks are not visible, under overcast skieswhere celestial cues are not visible, and even within cloud layers whereneither set of cues is available. The nomadic horsemen of the steppes of Asiaused the response of lodestones to the Earth's magnetic field to find theirway, and the hypothesis that migrating birds might do the same was suggested asearly as the middle of the nineteenth century. Yet it was not until themid-twentieth century that Merkel and Wiltschko demonstrated in a laboratoryenvironment devoid of any other cues that European Robins would change theirorientation in response to shifts in an artificial magnetic field that was asweak as the Earth's natural field. Although iron-containing magnetite crystalsare associated with the nervous system in homing pigeons, Northern Bobwhite,and several species of perching birds, it is unknown whether they areassociated with the sensory receptor for the geomagnetic cue. An alternatehypothesis for the sensory receptor suggests that response of visual pigmentsin the eye to electromagnetic energy is the basis for geomagnetic orientation.It has been shown, however, that previous exposure to celestial orientationcues enhances the ability of a bird to respond more appropriately when onlygeomagnetic cues are available.

　　Radar observations indicate that birdswill decrease their air speed when their ground speed is augmented by a strongtail wind. We also know that birds can sense wind direction as gusts rufflingthe feathers stimulate sensory receptors located in the skin around the base ofthe feather. Since there are characteristic patterns of wind circulation aroundhigh and low pressure centers at the altitude most birds migrate, it has beenhypothesized that birds could use these prevailing wind directions as anorientation cue. However, there presently is no experimental support for thishypothesis.

　　I.The sense of smell in birds wasconsidered for a long time to be poorly developed, but more recent evidencesuggests that some species can discriminate odors quite well. If the olfactorynerves of homing pigeons are cut, the birds do not return to their home loft aswell as birds whose olfactory nerves were left intact. A similar experiment hasdemonstrated that European Starlings with severed olfactory nerves returnedless often than unaffected control birds even at distances as great as 240 kmfrom their home roosts. And even more interesting, when these starlingsreturned to the nesting area the following spring, the starlings withnonfunctioning olfactory nerves returned at a significantly lower frequencythan the other starlings.

　　J.Considering the array of demonstratedand suggested cues that birds might use in their orientation, it is clear thatthey rely upon a suite of cues rather than a single cue. For a migrating birdthis redundancy is critical, since not all sources of orientation informationare equally available at a given time, nor are all sources of informationequally useful in a given situation.

　　Questions 1—8

　　The passage on the previous pages has eight sections labeled A-J

　　Which section mentions the following?

　　Write the correct letter A-Jin boxes1—8 on your answersheet.

　　1. The possible conclusion for migrating birds.

　　2. A description of olfactory nerves about birds’sdistance.

　　3. A description of Latitude about several species.

　　4. Insights from studies how young birds finddirection.

　　5. The ways birds can use for navigation.

　　6. Classes of animals for migratory movements.

　　7. The elements that birds have to navigation.

　　8. The birds use different cues to cope with weather.

　　Questions 9—11

　　Choose THREE letters A-F

　　Write your answers in boxes 9—11 on your answer sheet.

　　The list below gives some ways of regarding navigation.

　　Which THREE ways are mentioned by the writer of the text?

　　A. a exercise for young adult activity

　　B. Latitude for wintering ground

　　C. Physical surrounding

　　D. Weather cues

　　E. Satellite-based technologies

　　F. Places destination when they are in winter

　　Questions 12—14

　　Do the following statements agree with the information given in thepassage?

　　In boxes 12—14 on your answer sheet, write

　　TRUE ifthe statement agrees with the information

　　FALSE ifthe statement contradicts the information

　　NOT GIVEN if thereis no information on this

　　12. Birds’ migratory flight is affected by using natural environment.

　　13. Fish, mammals and insects are not in their ability to makemigratory journeys.

　　14. Birds use cues to find their destination thousands of miles away.

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