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Berthold 2002 Bird migration the present view of evolution, control, and further development as global warming progresses

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Berthold 2002 Bird migration the present view of evolution, control, and further development as global warming progresses
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  48 (3) :291¡« 301 , 2002  Acta Zoologica Sinica ¡¡¡¡¡¡¡¡¡¡ ¡¡ BIRD MIGRATION: THE PRESENT VIEW OF EVOLUTION, CONTROL ,AND FURTHER DEVELOPMENT AS GLOBAL WARMING PROGRESSES Peter BERTHOLD ( Max Planck Research Centre for Ornithology , Vogelwarte Radolf zell , Schloss Moeggingen , D 2 78315  Radolf zell , Germany) Abstract ¡¡ Studies during recent decades have shown that bird migration in general is to a great extent under direct geneticcontrol. There is evidence indicating the existence of an innate migratory drive as well as genetic control of  (i) the onset ,duration and end of the migration period , (ii) the amount of migratory activity , a genetically prescribed parameter thatdetermines the distance over which the bird flies , (iii) the migration directions and (iv) physiological parameters , inparticular fat deposition during the migratory period , but also those determining which individuals will migrate and whichwill not in forms that are obligatorily partial migrants. A two 2 way selection experiment has shown that within only a fewgenerations partial migrants can be converted by selection into pure migrants or nonmigrants. Selection for a newmigration direction , leading to new winter quarters , can occur equally rapidly in free 2 living birds. At least in the speciesmost closely studied so far (the blackcap ,  Sylvia atricapilla) ,  the attributes¡ migrant¡–or¡ nonmigrant¡–are inherited inconjunction with the characteristic amount of migratory activity (as a time program) , and the former are evidentlycontrolled by a threshold mechanism. A new bird 2 migration theory postulates that even in forms that at present areexclusively migratory , selection for lower levels of migratory activity can cause a threshold to be crossed , below whichnonmigrants appear. Accordingly , conversion of a population from migratory to nonmigratory can occur by selection witha transition through partial migration. This intermediate stage is prevalent among the recent bird species; it appeared earlyin the evolution of organisms and in the case of birds was probably acquired from ancestral , pre 2 avian forms. Modelcalculations indicate that with strictly directed selection , migratory birds can be converted to sedentary in about 40 yearsand conversely. This explains the changes in migratory behaviour currently observable in so many bird species in the courseof global warming; in particular , migratoriness is decreasing in various respects at higher latitudes. The new theory alsoenables us to predict the ways in which avifaunas are likely to be restructured if warming continues; these scenarios arebriefly outlined. Key words ¡¡ Birds , Migration , Evolution , Genetics , Global warming Received 24 September , 2001 ; revised 25 March , 2002 Brief introduction to the author ¡¡ Dr. Peter Berthold , Professor. Main research interests: avian migration , population dynamics , behavioural andpopulation genetics ( mainly related to avian migration) , experimental evolutionary studies , further annual rhythms , breeding biology ,changes of bird life in relation to global warming. E 2 mail : berthold @vowa.ornithol. mpg. de 1 ¡¡  Introduction Our institute , the¡ Vogelwarte Radolfzell¡–, wasestablished 100 years ago , in 1901. It was then called¡ Vogelwarte Rossitten¡– and was situated in EastPrussia , far to the northeast of its present site. Thefirst¡ ornithological 2 biological observation station¡–inthe world , it was brought to life as a result of proposals made at the First InternationalOrnithological Congress , held in 1884 in Vienna.From the outset it was directed primarily towards thestudy of bird migration , and throughout its history itbecame progressively more focused on that area of research. It was only for a short period that the work relied on pure field observations. Beginning as earlyas 1903 , data were obtained from a broadly basedringing system , an approach that had recently beendeveloped in pioneer studies by the Danish teacher C.C. M. Mortensen. By the time that the end of World War 2 brought activities at the originalinstitute to a halt , about a million birds had beenringed. Since the institute was re 2 established atRadolfzell in 1946 , the number has risen by about4 1 5 million. The insights obtained from recoveries of   ringed birds have been presented in many srcinal andreview articles , in particular in two publicationsentitled  Atlanten des Vogelzugs  (see , e. g. , Zink and Bairlein 1995) . As early as the 1920s the founderof our institute ¡“Prof. Dr. Johannes Thienemannbegan front 2 line research in the area of orientation.At first this involved retention and displacementexperiments , using the swallows (  Hirundo rustica) and then mainly white storks (  Ciconia ciconia) ;  thework was then continually extended by hiscollaborators. Research on the physiology and controlof bird migration , initiated during the 1920s inCanada by Rowan (1925) , was taken up in Rossittenduring the 1930s and then adopted as a major projectin Radolfzell after the war. To summarize : workersat our institute have been investigating all the basicaspects of bird migration almost uninterruptedly for acentury.Because Thienemann¡fls successors over the yearshave progressively built on and added to a vastresource of experience , mostly accumulated in ourown institute , we have been able to develop a numberof new approaches. Most prominent in recent yearsare the introduction of satellite tracking of migratorybirds in Europe in 1990 (Berthold  et al. ,  2000) andthe incorporation of experimental genetics andexperimental evolution research into the study of birdmigration during the 1970s. The last two researchareas, in particular , have produced novel insights ,hypotheses and theories , which will now beconsidered in detail. 2 ¡¡ Circannual rhythms and theirimplications¡¡¡¡  For species that migrate late in the year , forinstance not until the first snow falls , it has long beenpostulated that they are induced to depart byexogenous signals generated directly by environmentalfactors , such as cold temperatures or lack of food (e.g. , Farner , 1955) . In the case of species thatmigrate very early 2 such as the marsh warbler(  Acrocephalus palustris )  which leaves higherlatitudes during summer , beginning in mid 2 July ¡“thepossibility that their departure is caused byendogenous factors was raised centuries ago , by vonPernau (1702) . As he put it , they are¡ pulled awayat the right time by a hidden force¡–. Ideas aboutendogenous initiation and control of migrationmultiplied from the 1950s on (Aschoff 1955) , andafter evidence to this effect had been obtained formammals, in particular , beginning in the 1960sresearchers in our institute produced the firstdemonstrations of endogenous annual rhythms(circannual rhythms , i. e. biological clocks or internalcalendars related to the calendar year ; e. g. Gwinner ,1967 , 1986 ; Berthold  et al. ,  1971 ; Berthold ,1988) .These circannual rhythms are cyclic changes inphysiological body functions that are controlled by thecentral nervous system , though it is not yet knownexactly how they are produced. They have now beendemonstrated in about 20 species of migratory birds ,belonging to various systematic groups and distributedover five continents. In these birds they govern theperiodicity of migratory activity , rhythms in bodyweight (especially fat deposition) , food preferences ,the basic features of orientation behaviour , andprobably many fundamental physiological andendocrinological periodicities. As an example , Fig. 1shows four circannual body rhythms of a gardenwarbler (  Sylvia borin) .  Circannual rhythms of birdspersist under constant experimental conditions , andin this situation it becomes evident that their periodsare normally less than 12 months , usually of the orderof about 8 2 10 months. Under natural conditions , thisdeviation of circannual rhythms from the calendaryear is counteracted by external synchronizers ¡“especially photoperiodicity ¡“which adjust the rhythmsto the calendar year , matching them to theappropriate biological seasons.It was an intriguing question whetherendogenous circannual rhythms can trigger migratoryprocesses even when uninfluenced by environmentalfactors , which often appear to affect migratory eventsespecially in late 2 migrating species. There is indeedconvincing evidence that they can in some cases: forinstance , the initiation of the first departure from thebreeding grounds by young individuals , migrating for 292 ¶fl ¡¡¡¡˛¡¡¡¡§ ¡¡¡¡–¤ 48 ¡¡¡¡¡¡  Fig. 1 ¡¡ Endogenous annual periodicity ( circannualrhythms) of ( from top to bottom) testes length,migratory restlessness (  Zugunruhe ,  as measured bynocturnal locomotor activity,¡ hopping¡–) , body massand moult of a male garden warbler  ( Sylvia borin) The hand 2 raised bird hatched at the end of May and wastransferred to constant conditions (a light 2 dark regime of 10¡ˆ14hrs) in June (arrow) and kept there for 10 years; results aredepicted from the first three study years.RM : return migration ¡¡ OM : outward migration period(modified from Berthold , 2001) the first time , of a number of songbirds. Acomparative study in 19 species of European leaf andreed warblers , small thrushes and some othersshowed a very high correlation coefficient of 0 1 967between onset dates of migratory activity(restlessness , see below) in hand 2 raised caged birdsand the actual migration of conspecifics belonging tothe same populations in the wild. This finding impliesthat the endogenous mechanisms triggering migratoryactivity in captive individuals also initiate actualmigration , and that environmental factors in thesecases are of minor , if any , importance for theinitiation of departure. The same is likely to apply tobirds flying to the breeding grounds from winterquarters near the equator , because in equatorialregions there are very few regular seasonal changesthat could act as migration 2 eliciting factors(Berthold , 1996) .The discovery of circannual rhythms and thedemonstration that they can directly initiatemigratory events made it seem highly probable thatbird migration could to a considerable extent be underdirect genetic control. Similar hypotheses hadpreviously been put forward by Rowan (1931) , Nice(1933) , Lack  ( 1943/ 44) and others. So as toproceed along this line of thought , it was necessary toopen a new area of research : experimental bird 2 migration genetics. This was not an easy task. Butafter a long search we finally struck gold with onespecies: the blackcap (  Sylvia atricapilla) .  Thisspecies is widely distributed , ranging from the CapeVerde islands over other Atlantic islands such as theAzores and Madeira and through almost the wholeEuropean continent as far as western Siberia ; itincludes sedentary , migratory and partially migrantpopulations, and its migration routes have variousdirections and distances. Furthermore , it is commoneverywhere , can readily be kept in cages and aviaries ,and now ¡“after more than 5 years of pilotexperiments ¡“can be bred quite well in our institute.During the last 25 years or so we have hand 2 rearedover 3 000 individuals from various populations , anda good 1 600 have been bred in our aviaries in thecourse of crossing and selection experiments. We havealso achieved similar breeding results , thoughinvolving fewer individuals , with the commonredstart  ( Phoenicurus phoenicurus )  and black redstart  ( P. ochruros)  (Berthold , 2001) . Thesetwo species cross 2 breed regularly in the field and canalso be crossed in aviaries , producing fertile hybridsand thus enabling genetic studies beyond the speciesboundary. Because these warblers and redstarts innature migrate by night , their migratory activity (orits absence , in the case of non 2 migrating individuals)can easily be meausred quantitatively as nocturnalmigratory restlessness  ( Zugunruhe )  in automaticregistration cages (Fig. 2) . 3 ¡¡  Results of genetic and experimentalevolutionary studies¡¡¡¡  Since 1980 blackcaps and redstarts have beentested for genetic mechanisms directly underlying thefollowing characteristics and events associated withmigration : (i) the presence of a¡ migratory drive¡–(instinct in the sense of Konrad Lorenz) , i. e. thespontaneous , inherited appearance of migratory 3923 ˘Peter Berthold : Evolution , control and development of bird migration¡¡¡¡  Fig. 2 ¡¡Left : male (left) and female (right) blackcap (  Sylvia atricapilla) Right : a blackcap exhibiting nocturnal migratory restlessness (  Zugunruhe ,  migratory activity , captured on video by infrared light , invisible to the bird; from Berthold  et al. ,  2000) . activity in hybrids produced by experimental cross 2 breeding of migratory and sedentary birds; (ii) thespecies 2  and population 2 specific onset , duration andend of the first outward migration (Fig. 3) ; (iii) theamount of migratory activity produced in the firstoutward migration period ¡“which , according to thevector navigation hypothesis (see below) , in the caseof young birds migrating for the first time , with noprior experience , represents a time program thatdetermines the migration distance and hence thelocation of the winter quarters; ( iv) directionalpreferences for migration , including both themaintenance of a particular direction and changes of direction (which often occur as sharp turns , flying incurves etc. ) in the course of migration ; (v) increasesin body weight during the migration period , such thatthe bird can become as much as twice as heavy ,primarily owing to fat deposition but also by proteinaccumulation etc. ; ( vi ) the determination of migrants and nonmigrants in partially migrantpopulations; ( vii ) the phenotypic and genotypicvariability and the heritability of individualcharacters, in particular the amount of migratoryactivity and of partially migrant behaviour ; (viii) thetiming of migration in accordance with otherprocesses having annual periodicity , namely juveniledevelopment , moulting and breeding. For all of thesecharacteristics and events it has been possible todemonstrate direct genetic control. Furthermore ,considerable phenotypic and additive geneticvariability has been found , which together withintermediate heritability values (around 0 1 4) suggesta high potential for rapid selective andmicroevolutionary processes. Two examples follow.Common redstarts are long 2 distance migrants ,travelling about 5 000 km from central Europe tooverwinter in central Africa , south of the Sahara ; tocover this distance takes from mid 2 August to mid 2 December. Black redstarts start from the same areabut migrate over the much shorter distance of about1 000 km to winter quarters in the Mediterraneanregion , from October/ November to December.Correspondingly , hand 2 reared individuals of the twospecies kept in registration cages exhibit migratoryrestlessness that differs in duration and appears whenthe birds have reached different ages. When aviarybirds of the two species are cross 2 bred and the hand 2 reared hybrids are tested in the same way , the time of onset and the duration of their migratory activity areintermediate. This result shows that these parametersare under direct genetic control , which is evidentlyneither monogenic nor dominant , but polygenic ; thatis , these are evidently quantitative genetic characters(Fig. 3) .The second example relates to partial migration.This is a form of the regular yearly migratory periodin which only part of the population leaves thebreeding grounds , while the other birds stay there for 492 ¶fl ¡¡¡¡˛¡¡¡¡§ ¡¡¡¡–¤ 48 ¡¡¡¡¡¡  Fig. 3 ¡¡ Migratory restlessness of black redstarts ( Phoenicurus ochruros ,  top) , common redstarts ( Phoenicurus phoenicurus ,  bottom) and their hybrids( middle) ; mean values and standard errors of timecourse of migratory activity in hand 2 raised individuals 0 : date of hatching of the experimental groups (from Berthold et al. ,  1995) the winter. Regarding control of obligatory partialmigration (i. e. , occurring regularly every year) , twocompletely disparate hypotheses have been proposed.According to the ¡ behavioural 2 constitutionalhypothesis¡–, weaker individuals are displaced duringpost 2 breeding skirmishes over food etc. , and areeventually forced to migrate , whereas the strongerwinners of these disputes can remain resident. The¡ genetic hypothesis¡– states that the decision as towhether an individual will later be a migrant or anonmigrant is already made in the egg by thecombination of parental genes. We have carried outthe first experimental tests of these hypotheses , withbirds from a partially migrant blackcap population insouthern France that is composed of 75 % migrantsand 25 % nonmigrants. A two 2 way selectionexperiment (Fig. 4) yielded a strong selectionresponse even in the F1 generation. Continuedselection gave very striking results: it was possible toobtain a fully migratory population after just 3generations and an (almost) resident one after only 5 2 6 generations. Thus partial migration in the blackcaphas an extremly high evolutionary potential towardsfull migratoriness or sedentariness.Such rapid changes on a genetic basis also occurin nature. The central European blackcaps had alwaysmigrated exclusively southward , to winter quarters inthe Mediterranean region and in Africa , until about40 years ago ; then some birds began to migrate in anew , northwestern direction and to overwinter on theBritish Isles. We tested the directional preferences of birds taken from these new winter quarters and thenraised their progeny in aviaries and tested them in the Fig. 4 ¡¡ Results of a two 2 way selection experimenton partially migratory blackcaps ( Sylvia atricapilla) ,  from southern France Nonmigrants were bred to the F6 generation , migrants up tothe F3 generation. Solid lines: results of selection experimentsBroken lines: mathematical functions with best fit. 267 :number of hand 2 raised birds of parental stock. Other numbersindicate how many individuals have been bred in differentgenerations (modified from Berthold , 2001) . same way. Although this novel migratory behaviourhas just arisen in recent decades , the new migrationdirection is already genetically fixed : it is inherited bythe descendants , and accordingly has very recentlybeen brought about by rapid selection. Evidently aconsiderable number of selection pressures havecontributed to its quick development , such as ashorter migration distance , ideal winter quarters in amild climate with only slight intraspecificcompetition , an ample food supply and relativelyshort days in winter , which cause a timely return tothe breeding grounds and hence optimal choice of habitat , early gonadal development and breeding andprobably also assortative pairing with partners fromthe same overwintering area (Berthold , 2001) . 4 ¡¡  A novel comprehensive theory of avian migration¡¡ ¡¡  In addition to the results of the two 2 wayselection experiment with blackcaps describedabove ¡“that the distinction between migrants andnonmigrants is genetically determined , and thatpartial migration can rapidly evolve into puremigratory or sedentary behaviour by selection ¡“wealso gained two other important insights. Whenmigratory birds mate with one another (or sedentary 5923 ˘Peter Berthold : Evolution , control and development of bird migration¡¡¡¡
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