BirdMigrationand GlobalChange
GeorgeW.Cox
Copyright©2010GeorgeW.Cox
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Includesbibliographicalreferencesandindex. ISBN-13:978-1-59726-687-1(cloth:alk.paper)
ISBN-10:1-59726-687-6(cloth:alk.paper)
ISBN-13:978-1-59726-688-8(pbk.:alk.paper)
ISBN-10:1-59726-688-4(pbk.:alk.paper)1.Migratorybirds—Adaptation. 2.Migratorybirds—Effectofhabitatmodificationon.3.Climaticchanges.4. Globalwarming.I.Title. QL698.9.C692010 598.156′8—dc22
2010004093
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ManufacturedintheUnitedStatesofAmerica 10987654321
Keywords:bird,migration,globalchange,climatechange,globalwarming,adaptation, evolutionarychange
Willthegreatmigrationsofbirdssurvive?Manybiologistsbelievethatmigratorybirdsareatgreateraverageriskofextinctionduetochangingclimatethanareresidentspecies.Thisideaisbasedonthefactthatmigratory birdsdependondifferenthabitatsandresourcesindifferentlocationsduringtheirannualcycleandonthefactthatthefailureofanyofthesecouldbe fatal.Asecondgeneralbeliefisthattheclimateisnowchangingatarate fasterthanbirdshaveexperiencedinthepastandthatevolutionaryadjustmentsbybirdsmaynotbeabletokeeppacewiththischange.
Thethesisofthisbookisthatmanymigratorybirdsexhibitahighdegreeofecologicalandevolutionaryadaptabilityandthatmanyarenow showingrapidadjustmenttoclimaticchanges.Asubsidiarythesis,however,isthatforotherspecies,climatechangecanseverelyconstrainorpreventfullecologicalandevolutionaryadjustment,puttingtheirsurvivalat risk.Theobjectiveofthisbookisthereforetoevaluatethecapacityofmigratorybirdstorespondtothechallengesofchangingclimate.
Thebookisareview,synthesis,andinterpretationofrecentscientific literatureonmigratorybirdsandtheirresponsestochangingclimate.We beginbycharacterizingthepatternsofmigrationshownbybirdsofdifferentecologicalandtaxonomicgroups,andthefrequencyofmigrationindifferentworldenvironments,frompolarregionstothetropics.Thenatureof climatechangethroughouttheworldisexamined,andtheimpactsofthis changeonbirdhabitatsareevaluated.Thespecialthreatsofchangetoparticularhabitattypesandresourcesutilizedbymigratorybirdsareexamined andtheconstraintsonresponsesbybirdsconsidered.Theparticularpatternsofinfluenceofclimatechangeonmigratorylandbirdsareconsidered fordifferentbiogeographicregionsandforspecialbirdgroups,including raptors,freshwaterbirds,andmarinebirds.Finally,thecapacityforecologicalandevolutionaryadjustmentisexamined,andtheadequacyofcurrent conservationprogramsisevaluated.Notreatmentofbirdmigrationhas pulledtogetherthesediversetopics,whichareofcriticalimportancetohumaneffortstoprotectbiodiversity.
Mostofthetopicsinthebookhavebeenaddressedforspecificgroups
ofbirdsorforbirdsinspecificgeographicalareas.Sometopics,suchasthe impactsofclimatechangeonmigratorytropicalbirds(Chapter10)and evolutionaryadaptabilityofwaterbirds(Chapter18),havereceivedonly spottyattentionotherthanthatdirectedatafewspecies.Othertopicsfor whichnorecentsyntheseshavebeenattemptedincludeworldwidealterationofmigratorybirdhabitats(Chapter3),migratorylandbirdsofthe temperateSouthernHemisphere(Chapter9),shorebirds(Chapter12), waterfowl(Chapter13),oceanicbirdsoftheNorthPacificandSouthern Hemisphere(Chapters15and16),evolutionaryabilityoflandbirds (Chapter17),andoverallcapacityforecologicalandevolutionaryadjustment(Chapter19).RecentsymposiahavetendedtofocusonlandbirdresponsestochangingclimateinNorthAmericaandEurope,butinterestis increasingrapidlyinAustralia.Wereallyknowlittleabouthowmigratory landbirdsofSouthAmerica,easternAsia,andAfricaarerespondingtoclimaticchange,butallindicationsarethatmajorchangesareoccurring.
Birdinghasbecomenotonlyahugelypopularoutdooractivityformillionsofpeoplebutalsoanactivitythatbringspeoplefacetofacewithbiodiversityandthethreatstoitssurvival.Ihavenotmetabirderwhoisnot concernedaboutprotectionofbirdhabitatsandabouttheimpactsof changingclimatesonbirds,whetherresidentormigratory.Ihopethatthis accountincreasesawarenessofthechangesthatmigratorybirdsareexperiencingandstimulatesreaderstoparticipateineffortstokeepthegreatmigrationsinmotion.Seetheappendix,aswellaswww.birdmigration.org, foralistofthecommonandscientificnamesofspeciesdiscussedinthe text.
Iamindebtedtomanyfriendsandcolleagueswhohavereviewedportionsofthebook,especiallyBenBecker,KeithBildstein,ChristianBoth, RichardBrewer,LyndaChambers,GlennConroy,HughDingle,George Divoky,PeterDunn,FredGehlbach,FrankGill,PhilHockey,SteveOberbauer,JanePhillips-Conroy,RobertRicklefs,KurtRiitters,TimSparks, NilsStenseth,NilsWarnock,andS.JosephWright.Manyothersgraciously tooktimetohelpmefindliteratureforparticularregionsortopics:Tatsuya Amano,KenChan,TimCoppack,VictorCueto,TomEbert,KenGreen, LouisHansen,StuartHurlbert,AlexJahn,LeoJoseph,BrianLinkhart, J.AlanPounds,MichaelScott,KimScribner,RobSimmons,Liliana Spescha,WayneTrivelpiece,RickWessels,EdWillis,andBenZuckerberg. IespeciallythankDarlaCox,mywife,forsharingmanybirdingexperiences andforreviewingmanyofthechaptersofthisbook.
GeorgeW.Cox SantaFe,NewMexico
MigratoryBirds:EcologicalPatterns
Migratorybirdsoccupyalloftheworld’smajorenvironments,andtheir movementsareextraordinarilydiverse.Land,freshwater,andmarinebirds showwell-developedmigrations,asdobirdsofclimaticzonesranging frompolarregionstothetropics.
Forlandandfreshwaterbirds,migrantsvaryenormouslyinthedistancetheytravel,intheregularityoftheirschedulesfromyeartoyear,and inthefractionofindividualsthataremigratoryindifferentpartsoftheir ranges.Short-distancemigrantsrangefromthosethatshowlocalhabitat shiftsoraltitudinalmovementswithinasmallgeographicalareatothose thatmakeintracontinentalmovementsofuptoafewhundredkilometers. WhereIliveinnorthernNewMexico,forexample,thewinterDark-eyed JuncosatourfeederareamixoflatitudinalmigrantsfromthePacific NorthwestandnorthernRockyMountainsandgray-headedbirdsthatare altitudinalmigrantsfromnearbymountains.Weatherconditionsofthe particularyearinfluencemanyoftheseshort-distancepatterns.Frequently, onlyaportionofthepopulationofaspeciesinagivenlocalityshowsshortdistancemovements,apatterntermedpartialmigration.Overthegeographicalrangeofmanyspecies,localpopulationsrangefrombeingcompletelyresidenttopartiallyorfullymigratory.
Birdsofmanydifferenttaxaandspeciesatalllatitudesexhibitshortdistancemovements.Birdsofmountainousregions,fromhighlatitudesto thetropics,showaltitudinalshifts,withmanysuchmovementsbytropical speciesyettobedocumented.ManybirdsofsemiaridregionsinAfrica, Asia,andAustralia,characterizedas“nomadic,”showopportunisticmovementpatternsthatenablethemtoutilizeareasthathaverecentlyexperiencedfavorableweatherconditions.
Long-distancemigrationsoflandandfreshwaterbirdsinvolveflights acrossmajorclimaticzones,andoftenbetweencontinentsorhemispheres. Thesemovementsareoftenmoreregularintimingthanthoseofshortdistancemigrants.Strong-flyingraptorsandshorebirdsperformsomeof thelongestmigrations.Radiotrackinghasshown,forexample,thatBartailedGodwitsflynonstopfordistancesof10,000kilometersormorebetweenbreedingareasinAlaskaandwinteringareasinNewZealandand Australia.TheSwainson’sHawksthatoccasionallyflyovermyhomein NewMexicoinspringspendthewinterontheArgentineanpampas.Many smallsongbirds,however,makeintercontinentalmigratoryflightsofthousandsofkilometers.
Seabirdsshowdiversepatternsofmigration,aswell.Someareshortdistancemigrantsthatdispersefromcoastalorinsularbreedingcoloniesto
neighboringoceanicregionsuptoonlyafewhundredkilometersdistant. Others,suchastheArcticTern,arelong-distancemigrantsthatflythousandsofkilometersfrombreedingareastononbreedingrangesindistant oceans,sometimesintheoppositehemisphere.
MigratoryBirds:TaxonomicPatterns
About9930speciesofbirdsexistworldwide,belongingto204families. Frommanysourcesintheliterature,Ihavecompiledapreliminarylistof migratorybirds,atasknotaseasyasitmightatfirstseem.Manyspecies thatarecommonlyregardedaspermanentresidentsarereallypartialmigrantsinsomepartsoftheirranges.InnorthernNewMexico,forexample, theSteller’sJay,consideredbymostreferencebookstobeapermanentresident,isapartialmigrant,withmanybirdsmovingtolowerelevationsin winter.Ourunderstandingoftheseasonalmovementsoftropicalspecies, especiallyaltitudinalmovementsofthoseofmountainousregions,isstill veryrudimentary.Mysurveyofmigratoryspeciesindicatesthatatleast 2600birdspeciesof141familiesshowsometypeofseasonalmigrationor substantialnomadism(Table1.1).Thiscorrespondstoabout26.2percent ofallbirdspecies,afigurethatissuretoincreaseaswelearnmoreabout birdmovementsinregionssuchaseasternAsia,muchofAfrica,andmountainareasthroughouttheworld.Myestimatealsosubstantiallyexceedsthe estimateof19percentofmigratoryspeciespresentedbyBirdLifeInternationalin2008.
Thefrequencyofmigrationvarieswidelyamongdifferentgroupsof birds.Membersofafewfamiliesoftropicalbirds,suchasthefamilyto whichtheantthrushesandantpittasbelong(Formicariidae),arealmost completelynonmigratory.Others,suchastheNewWorldandOldWorld warblerfamilies(ParulidaeandSylviidae,respectively),containsomespeciesthatarepermanentresidents,otherspartialmigrants,andstillothers long-distancetemperate–tropicalmigrants.Inmostfamiliesoftemperate zonesongbirds,suchasthetitmiceandchickadees(Paridae),somespecies areatleastpartialmigrants,thatis,withsomepopulationsthatconsist partlyorlargelyofmigratoryindividuals.Almostallseabirdsaremigratory,inthesenseofspendingnonbreedingperiodsatseaatfeedingareas substantiallydistantfromtheirnestingareas.Waterfowlthatbreedinthe temperatezonesortheArcticarealmostallmigratorytovaryingdegrees. Shorebirdsbreedingathighlatitudesarenearlyalllong-distancemigrants. FalconsandhawksthatbreedinthetemperatezonesortheArcticarealso mostlymigratory—someonlyshort-distancemigrantsbutothersshowing
Table1.1. Migratorybirdspeciessummarizedbyorder.
TotalMigratory
Orders FamiliesSpeciesSpecies
Tinamous(Tinamiformes) 1470
Rheas(Rheiformes) 120
Ostrich(Struthioniformes) 111
EmuandCassowaries(Casuariiformes) 242
Kiwis(Dinornithiformes) 150
Loons(Gaviiformes) 155
Grebes(Podicipediformes) 11915
Penguins(Sphenisciformes) 11715
PetrelsandAlbatrosses(Procellariiformes) 4114114
PelicansandRelatives(Pelecaniformes) 46748
DucksandGeese(Anseriformes) 2162124
Flamingos(Phoenicopteriformes) 166
WadingBirds(Ciconiiformes) 511754
Raptors(Falconiformes) 5313108
QuailandRelatives(Galliformes) 628413
CranesandRelatives(Gruiformes) 1220774
Shorebirds(Charadriiformes) 19356274
Sandgrouse(Pterocliformes) 11611
PigeonsandRelatives(Columbiformes) 130827
ParrotsandRelatives(Psittaciformes) 336837
Mousebirds(Coliiformes) 160
CuckoosandTuracos(Cuculiformes) 316450
Owls(Strigiformes) 221518
NightjarsandRelatives(Caprimulgiformes)512035
SwiftsandHummingbirds(Apodiformes) 344389
Trogons(Trogoniformes) 1405
MotmotsandRelatives(Coraciiformes) 1021948
Woodpeckers(Piciformes) 641227
PerchingBirds(Passeriformes) 9858931400
Total 20499302600
intercontinentalmovements.Owlsofthesesamelatitudes,ontheother hand,areinsomecasesmigrants,inothercasespermanentresidents.
MigratoryBirds:GeographicalPatterns
Therelativeabundanceofmigratoryspeciesalsodiffersmarkedlyamong majorgeographicalareas(Table1.2).ThelargelandmassesoftheNorth
Table1.2. Livingresidentandmigratorynonoceanicbirdspeciesinvariousworldregions.
PrincipalSources: Clements,J.F.2007. TheClementsChecklistofBirdsoftheWorld ,6thed.ComstockPublishingAssociates(CornellUniversityPress),Ithaca,NY.
AOUchecklistofNorthAmericanbirds.http://www.aou.org/checklist/north/index.php. Avibase.Birdchecklistsoftheworld.http://avibase.bsc-eoc.org/checklist.jsp?lang=EN. OrientialBirdClub.http://www.orientalbirdclub.org/publications/checklist/obcchecklist.txt. TheBirdsofAfrica .Vol.1–7.AcademicPress,NewYork.
TemperateandArcticzonescontainmanymigrantspecies,mostofwhich breedintheregion.Intropicalandsubtropicalregionsofthecontinents, winteringmigrantsarenumerous.Tropicalislandarchipelagossuchasthe EastandWestIndiesarealsowinteringareasformanymigrants,asare smallerislandareasthroughouttheAtlantic,Pacific,andIndianoceans. Manyofthemigrantstodistantoceanicareasarenonbreedingshorebirds andseabirds.SouthTemperateZoneregionsarealsohometomanybreedingmigrantspecies,andthebirdsthatbreedintheAntarcticarealmostall migratory.IntratropicalmigrationpatternsarewelldevelopedinAfrica, andtoalesserextentinAustraliaandtheNewWorld.Manybirdsofmountainousregions,regardlessoflatitude,showaltitudinalmovements.
Severalmajormigrationsystemscanberecognizedindifferentworld regions.IntheNewWorld,theseincludetheNearctic,Nearctic–Neotropical,Neotropical,andSouthAmericanmigrationsystems.The Nearcticmigrationsystemcompriseslandandfreshwaterbirdsthatbreed inCanadaandtheUnitedStatesandwinterprimarilynorthofcentralMexico.Thismigrationcomplexinvolvesabout423species,includingmany speciesofwaterfowlandotherfreshwaterbirds,hawksandowls,andsmall landbirds.About13oftheNearcticspeciesthatbreedathighlatitudesmigratetoAsiaratherthantomore-southernpartsofNorthAmerica.
TheNearctic–Neotropicalmigrationsystemincludesbirdspeciesthat breedinNorthAmericaandwinterinsouthernMexico,CentralAmerica, theWestIndies,andSouthAmerica.About184speciesofland,freshwater, andcoastalmarinebirdsareinvolved.About76ofthesespeciesspendthe nonbreedingseasoninMexico,theWestIndies,orCentralAmerica,but 108speciesextendtheirwinterrangesintoSouthAmerica.TheNeotropicalmigrationsystemcomprisesabout104species,mostofwhicharealtitudinalmigrants.About5speciesbreedintheNorthAmericantropicsand winterinSouthAmerica.WeknowmoreaboutpatternsofaltitudinalmigrationinMexicoandCentralAmericathananywhereelse,andwhatwe seeheresuggeststhatwehavemuchtolearnaboutaltitudinalmigrationin mountainousregionselsewhereintheworld.
TheSouthAmericanmigrationsystemcomprisesabout266land, freshwater,andcoastalmarinebirds.Mostofthesespeciesbreedinthe temperateregionofsouthernSouthAmericaandmigratenorthinthe australwinter,butatleast31speciesshowshort-distanceoraltitudinal movementsinthetropics.About77ofthespeciesbreedingintemperate SouthAmericaarefullymigratory.Some44species,mostlytyrannidflycatchersandswallows,winterinthehumidtropicalregioncenteredonthe Amazonbasin.Formorethantwo-thirdsofthesespecies,rangesofnonbreedingmigrantsoverlaprangesofresidentsofthesamespecies.
TheOldWorldexhibitsEuropean,European–African,andAsianmigrationsystems.InEurope,about116of524breedingspeciesofland, freshwater,andcoastalmarinebirdsareshort-distancemigrantsthatwinter withintheregion.TheEuropean–Africanmigrationsystemincludes185 speciesofland,freshwater,andshorebirdsofthirty-twofamiliesthatwinter inAfrica.Thesespeciesareprimarilyhawksandfalcons,waterfowl,shorebirds,OldWorldwarblers,thrushes,swallows,pipits,wagtails,andshrikes. Ofthese,62species,particularlymanyhawksandfalcons,shorebirds,Old Worldwarblers,thrushes,andshrikes,winterexclusivelyinsub-Saharan Africa.Oftheremainingspecies,somewinterinAfricaandsomeinareasof theMiddleEast.
TherichAsianmigrationsysteminvolvesbirdsthatmovefrombreedingareasineasternEurasiatowinteringareasinsouthernandsoutheastern Asia,thePhilippines,andtheEastIndies.About827speciesofforty-four familiesshowmigratorymovementswithinthisregion.About35species ofsandpipersandplovers,1tern,and10speciesoflandbirdscontinuefarthersouthtowinterintheAustralo–Papuanregion.InRussia,China,and otherpartsofeasternAsia,atleast66speciesofthrushes,flycatchers,Old Worldwarblers,finches,andotherpasserinesareshort-distanceoraltitudinalmigrantsthatdonotreachthetropics.
Africaexhibitstwowell-definedmigrationsystems.Thesouthern Africanmigrationsysteminvolves134fullyorpartiallymigratorylandand freshwaterbirdsthatbreedinsouthernAfricaandwinterfarthernorth.Waterfowl,rails,herons,kingfishers,cuckoos,swifts,swallows,andavariety ofpasserinebirdsareprominentinthissystem.ManybirdsbreedinginEuropeandAsiaappearinsouthernAfricaduringtheNorthernHemisphere winter.Theseincludemanyspeciesofsandpipersandplovers,gullsand terns,hawks,andOldWorldwarblers.
TheintratropicalAfricanmigrationsystemiscenteredontheequatorialregionandthesemiaridbeltstothenorthandsouth.About277species,rangingfromheronsandploverstolarksandfinches,moveprimarily northandsouthinthisregion,trackingfavorableconditionsrelatedtowet anddryseasonweatherpatterns.Somecuckoosbreedinthesemiaridzones bothnorthandsouthoftheequator,switchingplacesthroughmigration whennotbreeding.Inmore-aridregions,movementsofsomespeciesare bestdescribedasnomadic.AltitudinalmigrantsarefrequentinthemountainsofeasternAfrica.
TheAustralo–Papuanregion,comprisingAustraliaandNewGuinea, holdsalargelyself-containedmigrationsystem.Thismigrationsystemconsistsofabout272landandfreshwaterbirds.Somearemigrantsbetween AustraliaandNewGuinea,othersmovebetweenTasmaniaandmainland
Australia,andstillothersshowlatitudinalandaltitudinalmovements withinmainlandAustralia.Inaddition,manylandbirdsandwaterbirds performirregularmovementswithininteriormainlandAustralia.Some 169speciesshowpartialmigrationpatternsinAustraliaandTasmania. AmongTasmanianbirds,only20speciesaremigratory,andonly4arefully migratory,completelyleavingtheislandfortheAustralianmainland.
NorthernHemispheremigrantsalsoreachmanyPacificislands,includingNewZealand,althoughfewspeciesareinvolved.Long-distance migrantssuchasploversandsandpipersmostfrequentlyvisittheseisolated regions.
Seabirdmigrationsystemsrelatelesstolatitudinaltemperaturepatternsandmoretothelocationsofsuitablenestingareasandproductive oceanwaters.About234speciesofbirdsofeighteenfamiliesarelargelyor entirelypelagicintheirdistributionsinthenonbreedingseason(Table1.3). Onlyverygeneralmigrationpatternsareapparent.IntheeasternNorthAtlantic,about31speciesofloons,petrels,shearwaters,gannets,cormorants, seaducks,skuas,gulls,andalcidsbreedinArcticandsubarcticareasand winterinpelagicoroffshorecoastalareas.InthewesternAtlantic,about28 ofthespeciesofthesegroupsbreedinnorthernareasandwinterinwaters offthecoastsofCanadaandtheUnitedStates.TheNorthPacifichasa richerfauna.About53speciesoftheseseabirdswintersouthwardthrough theAleutianIslandsandalongtheeasternandwesterncoastsofthePacific. Tropicalandsubtropicaloceanshosttherichestandmostdiversefauna ofpelagicbirds,includingnumerouspetrels,shearwaters,terns,boobies, tropicbirds,cormorants,andfrigatebirds.
InthesouthernoceanssurroundingAntarctica,atleast94speciesof penguins,albatrosses,petrels,cormorants,skuas,andrelatedbirdsbreed andforagetoconsiderabledistancesfrombreedinglocalities.Intropical andwarmtemperateoceanareas,atleast97speciesofseabirdsofeleven familiesbreedandwanderwidelyoveroceanareasdistantfromtheirnestingislandsorcoastalrookeries.
TheChallengeofClimaticChange
Migratorybirdsfacemajorchallengesofsurvivalinthefaceofrapid,human-inducedglobalchange.Overthepastcentury,theearth’sclimatehas warmedby0.8°C,andbyAD2100,warmingwilllikelybebetween2.0°C and4.5°C.Becausetheydependonhabitatsandresourcesindifferentareas atdifferentstagesoftheannualcycle,populationsofseasonalmigrantsin
Table1.3. Worlddistributionofmigratoryoceanicbirds. 1
InterregionalMigrants
IntraregionalMigrants
Table1.3. Continued
InterregionalMigrants
IntraregionalMigrants TemperateWintertoWintertoTemperateWinterto
1 ZonalseparationsroughlycorrespondtoArcticandAntarcticcirclesandTropicsofCancerandCapricorn.Somespeciesarepartitionedbetweentwozones,giving risetothe0.5values.
everyworldregionwillbeaffectedbyclimaticchange.Theareasthatmigrantsuseseasonallyincludetheirbreedingranges,stagingandstopover locationsduringmigration,andareasoccupiedduringthenonbreedingperiod.Inonesense,becauseoftheirspecializationforuseofdifferenthabitatsorgeographicalareasatdifferenttimes,migratoryspeciesmightseem tobeatgreaterriskofextinctionthanpermanentresidents.Achangeinany oneoftheareasusedduringtheannualcyclemightcausetheirevolutionary strategytofail.Someornithologistsconcludethatsuchdependenceon multiplegeographicalareasplacesmigratoryspeciesatgreaterriskthanresidentspeciesinthefaceofglobalclimatechange.
Ontheotherhand,migratorycapabilityisanextensionofbasicphysiologicalandbehavioraladaptationsforlocalmovements,homing,andthe annualreproductivecycle,anditmustpossessadegreeofflexibility.Climaticchangesoverlonggeologicaltimehavetestedtheabilityofmigrants toadjustbreedingandnonbreedingrangesandaltermigrationroutes.Perhapsthisadaptabilityisadequatetorespondtotherapidenvironmental changesnowoccurring.Or,ofcourse,somemigratorybirdsmaybeableto adjustquickly,whileothersmaynot.
Severalfactorscouldconstraintheabilityofmigratorybirdstorespond tochangingclimate.Lackofgeneticvariabilityorphenotypicplasticity mightlimitthecapacityofspeciestorespondtochangingconditions.Limiteddispersalabilitymightalsoslowtheabilityofthespeciestoshifttheir geographicalrangesinresponsetochangingconditions.Anincreaseinfrequencyofextremeweatherevents,overalllossordegradationofoneor moreofthehabitatsonwhichtheydependthroughouttheannualcycle, andchangedinfluencesofcompetitorspecies,predators,ordiseasesmay alsoimpactmigratoryspeciesnegatively.
Isuspectthatpopulationlimitationofmigratoryspeciesisadynamic relationshipinvolvingbothbreedingandnonbreedingareas.Ifconditions inthewinteringrangefavorincreasedsurvival,expansionofthebreeding rangeintoregionswithlowerreproductivesuccesswilltendtooccur.If conditionsinthebreedingrangefavorincreasedreproductivesuccess,expansionofthenonbreedingrangeintoregionswithlowersurvivalwill likelyresult.Inasmuchasbreedingsuccessandnonbreedingsurvivalvary fromyeartoyear,theoutcomeofthisrelationshipchangesthroughtime. Bothhumanmodificationofthelandscapeandglobalclimaticchangeobviouslyinfluencethisprocess,aswell.
Issuesrelatingtomigratorybirdsattractenormousscientificconcern. TheSmithsonianInstitutionsponsoredsymposiaonmigratorybirdsinthe NewWorldin1977and1989.In2002,anotherSmithsoniansymposium,
entitledBirdsofTwoWorlds:AdvancesintheEcologyandEvolutionof Temperate–TropicalMigrationSystems,consideredmigrationpatterns worldwide.Thefirstorganizedefforttofocusdirectlyonclimateissueswas asymposium,BirdMigrationinRelationtoGlobalChange,heldatthe UniversityofConstance,Germany,inMarch2003.Mostrecently,acongressonBirdMigrationandGlobalChangetookplaceatAlgeciras,Spain, inMarch2007.
Since2006,theUnitedNationsEnvironmentProgrammehassupportedWorldMigratoryBirdDay(WMBD),aglobalinitiativefocusedon migratorybirdsandtheirconservation.ThesecretariatoftheAfrican–EurasianMigratoryWaterbirdAgreementinitiatedWMBD,whichnowinvolvesseveralothercooperatingorganizations.Actuallya2-dayeventon thesecondweekendinMay,eachannualprogramemphasizesaparticular issuerelatingtomigratorybirds.In2009WMBDfocusedonman-made structuresthatposedangerstomigratingbirds.
PatternsofResponsetoClimateChange
Changingclimatesarealteringmigratorybehaviorinseveralways.These include(1)changeintherelativenumbersofmigrantandresidentindividualsinlocalpopulations,(2)changeinthedistancebetweenareasoccupied atdifferentstagesintheannualcycle,(3)changeinthedirectionofmigratorymovements,and(4)changeintimingandspeedofmigratorymovements.Someoftheseresponsesappeartobesimplephenotypicresponses ofindividualstoalteredenvironmentalconditions,whereasothersare genotypicormicroevolutionaryresponses.
Changesintheproportionofmigrantsandresidentscanoccurifamelioratedclimaticconditionsduringthenonbreedingseasoninaparticularlocationfavormoreindividuals’becomingpermanentresidents.Or,if weatherconditionsandresourceavailabilitypatternsbecomemorestrongly seasonal,anincreaseinthemigratoryfractionofthepopulationcanoccur. Sincepatternsofglobalclimaticchangevaryfromregiontoregion,bothof thesechangesareexpected.
Withclimaticwarming,favorablehabitatsforbreedingbymanyspeciesmayappearathigherlatitudes.Thismayleadtolongermigrationsby individualscolonizingsuchareas,iftheareassuitableduringthenonbreedingseasonremainthesame.Or,asinthecaseofleapfrogmigration patterns,individualsbreedingfartherpolewardmaytendtomovetononbreedingareasevenfartherfromtheirbreedingareas.Lengthenedmigra-
tionscouldbeaseverechallengeifstopoversitesalsobecamelessfrequent orlessfavorable.Ontheotherhand,ameliorationofclimateinnonbreedingareasmightallownonbreedingbirdstoremainclosertotheirbreeding ranges.
Sinceclimaticchangesareunevenacrosslargecontinentalmasses, changesinthedirectionofmigratorymovementsarelikelytooccur.Regionssuitableforoccupationduringbreedingornonbreedingperiods mightappearathigherlatitudesbutatlongitudesdifferentfrompreviously suitableregions.Likewise,previouslyfavorablebreedingornonbreeding areasmightshiftinlongitude.
Withclimaticchange,theseasonsfavorableforoccupationofthe breedingrangeoroccupationofthenonbreedingareasmaylengthenor shorten,leadingtoaneedforchangesinthemigrationschedule.Opposing selectionpressuresatdifferentseasonsmaysloworpreventadjustmentsin timingofmigration,andlifehistoryfeaturesmayfalloutofsynchronywith seasonalprogression.Especiallyforlong-distancemigrants,suchasEurasianbirdswinteringinsub-SaharanAfricaorNearcticmigrantswintering inthetropics,correlationsbetweenhabitatconditionsonbreedingand nonbreedingrangesmaybeveryweakorevenlacking.Endogenousfactors andphotoperiodicresponsesthatarenotdirectlytiedtotemperatureare whatcontroltheannualcyclesofmostofthesespecies.Asaresult,many birdsmayfailtoadjustmigrationschedulestotakeadvantageofoptimal conditionsforbreeding.
Acentralissueofclimaticchangeforallmigratoryspeciesistheirdegreeofvulnerabilitytoextinction.Migratorybirdshavedealtsuccessfully withmajorclimaticshiftsduringperiodsofPleistoceneglaciation,althoughsomeextinctionsalmostcertainlydidoccur.Now,however,climate ischangingmorerapidlythanduringthePleistocene.Theextensivetransformationandfragmentationofhabitatsbyhumanactivitiesmayconstrain theabilityofmanyspeciestorespondtogeographicalshiftsintheiroptimal habitat.Forbirdsandotherlandanimalsthatoccupyhabitatsdefinedby plantcommunitystructure,theabilityofplantspeciestomoveinresponse tochangingclimatemaybecritical.Inaddition,eventhoughbirdsare highlymobile,strongsitefidelityandthemechanismbywhichsitefidelity isestablishedinjuvenilesmightalsolimitthecapacityofmigratorybirdsto occupynewregionsofsuitablehabitat.
Short-andlong-distancemigrantsmightalsobedifferentiallyvulnerabletoextinctioninthefaceofchangingclimates.Short-distancemigrants tendtoresponddirectlytoimmediateweatherconditionswithinthelocal environment.Inasmuchastheweatherconditionsontheirnonbreeding
rangesarelikelytobecorrelatedwiththoseontheirnearbybreeding ranges,appropriatechangesinmigratoryschedulesarequitelikely.Longdistancemigrantsmustrespondtoenvironmentalcuesthatarecorrelated withchangesontheirdistantbreedingornonbreedingareasandalong theirmigrationroutes.Howquicklytheycanrespondisuncertain.
Inthefollowingchaptersweshallfirstexplorethenatureoftheclimaticandenvironmentalchangesthatareoccurring.Weshallthenexamine responsesbybirdsofvariousworldregionsandecologicalgroups.Finally, weshallevaluatethecapacityofmigratorybirdstoadapttothechangesin climatethatareforeseen.
Summary
Migratorybirdsthroughouttheworldarefacingclimaticchange.Over26 percentofallbirdspeciesshowsomepatternofmigration.Majorsystems ofmigrationbylandandfreshwaterbirdsexistinNorthAmerica,Eurasia, andthecontinentsoftheSouthernHemisphere.IntheNorthernHemispheretheseincludepatternsofshort-distance,intracontinentalmovement andlong-distancetemperate–tropicalorintercontinentalmovements. Seabirdsatalllatitudesshowlatitudinalmovementsbetweenbreedingand nonbreedingareasornonbreedingdispersaltodistantoceanareas.Thecapacityofthesediversemigrantstoadjusttochangingclimatesandfood supplieswilldeterminewhetherornotmigrationwillremainamajorphenomenonofavianecology.
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