Stonechats: Effects of Genetic and Environmental Factors
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The Auk 116(3):589-603, 1999 TIMING OF POSTJUVENAL MOLT IN AFRICAN (SAXICOLA TORQUATA AXILLARIS ) AND EUROPEAN (SAXICOLA TORQUATA RUBICOLA) STONECHATS: EFFECTS OF GENETIC AND ENVIRONMENTAL FACTORS BARBARA HELM AND EBERHARD GWINNER • ResearchCenter for Ornithologyof theMax-Planck Society, D-82346 Andechs,Germany ABSTRACT.--Dataon the timing of postjuvenalbody molt of 322 Stonechats(Saxicola tor- quata)were examinedby multifactorialresidual maximum-likelihood analysis for effectsof externaland geneticfactors. The Stonechats, which belonged to theEuropean subspecies (S. t. rubicola),the Africansubspecies (S. t. axillaris),and their F• and F2 hybrids, were exposed to differentphotoperiodic conditions. The birds differedconspicuously in the timing and durationof their postjuvenalmolt. Thesedifferences were significantlyrelated to photo- periodicconditions, genotypic group, and the interactionof thesefactors. European birds generallymolted earlier and fasterthan African birds, and hybridsshowed intermediate patterns.Both subspecies started molt earlier under shorter photoperiods but tooklonger to completeit. African Stonechatshad a weakerresponse to photoperiodthan the European subspecies.In addition,molt timing was negativelyrelated to hatchingdate, with chicks hatchedlate in the seasonmolting at a youngerage than those hatched earlier. Finally, the two subspeciesdiffered in theirmolt timing when exposed to shortdays at an earlyage. The earliera Europeanbird was placedunder shortdays, the earlierit molted;the reversewas true in AfricanStonechats. From a smalldata base, we estimatedheritabilities (h 2) through full-sib analysesand offspring-parentregressions. In both subspecies,the timing of molt showedhigh geneticvariation, especially at its onset.Within photoperiodicgroups, h 2 at molt onsetwas closeto unity but decreasedduring the courseof molt. The two Stonechat subspeciestimed their postjuvenal molts in a qualitativelysimilar manner but showeddif- ferencesthat mayreflect differences in the selectionpressures of their respectiveenviron- ments. Received26 March 1998, accepted6 November1998. WE ANALYZED POSTJUVENALMOLT in Stone- al. 1993, Klaassen1995) may favor slower re- chats(Saxicola torquata) from two differentpop- placementof feathers.Ideally, molt shouldnot ulations to assess the influence of environmen- overlapwith energeticallyexpensive activities, tal factorson the timing of molt (seeGwinner suchas somaticgrowth and migration.Behav- et al. 1983, Gwinner and Neut•er 1985).Postju- ioral observationssuggest that costsof molt are venal molt is the first molt in Stonechats and high in Stonechats,which catchtheir prey in correspondsto thefirst prebasic molt in the ter- short, preciseflights (K6nig 1996,Flinks and minology of Humphrey and Parkes (1959). Kolb 1997).To preservemaneuverability and to Here,we are concernedonly with body molt, reduce daily energy expenditure,Stonechats althoughStonechats previously were found to should maximize the time taken for molt with- renew at leastsome of their secondariesduring in the range allowedby externalconditions. postjuvenalmolt (Gwinner et al. 1983).An ap- The two subspeciesof Stonechatsconsidered propriately timed molt is an important com- here live in very differentenvironments. Cen- ponent of the overall seasonalorganization tral EuropeanStonechats (S. t. rubicolaL. 1776) (Gwinner 1986, Jenni and Winkler 1994). The are short-distancemigrants that winter in the deterioration of climatic conditions toward the Mediterraneanregion (Cramp 1988,Glutz von end of breedingexerts high pressureto accel- Blotzheimand Bauer1988). They produce two eratemolt, whichmay be opposedby energetic or threeclutches of aboutfive eggseach season. limitations (Jenniand Winkler 1994).Molt costs Whennew younghatch, siblings from previous that dependon the rate of molt (Lindstr6met clutchesare driven out of the territory, where they molt in aggregationsof juveniles(K6nig Addresscorrespondence to this author.E-mail: 1996).Young from late clutchesmust prepare [email protected] for migrationmuch faster than their oldersib- 589 590 HELMAND GWINNER [Auk, Vol. 116 TABLE1. Classificationof Stonechatsused in the presentstudy according to factorsrelevant to the analyses. Shownare absolutenumbers, as well as row and columntotals. Origin indicatesthe numberof chickstaken from nestsor raisedin captivity.For all genotypicgroups, the numberof familiesand the averagefamily size are given. Family data Origin No.of Family Field Captivity sibgroups size (•) Total Africa 130 35 52 3.0 165 Central Europe 107 14 31 3.8 121 F• hybrid -- 22 5 4.4 22 F2 hybrid -- 14 5 7.0 14 Total 237 85 93 3.5 322 lings.The so-called "calendar effect" relates an tions: African birds originatedfrom the Lake Nak- earlierand quicker molt to a laterhatching date uru region in Kenya(0ø14'S, 36ø00'E), and European (Berthold1993, Jenni and Winkler 1994,K6nig birds originated from Lower Austria (48ø14'N, 1996).Late-hatched young may reducethe ex- 16ø22'E).We transportednestlings to Andechs,Ger- many,and hand-rearedthem asdescribed elsewhere tent of their flight-feathermolt, and sometimes (Gwinner et al. 1987). The remaining birds hatched they start migration while still molting (H. in captivity (Table1). Birds of both subspeciesand Flinks pers.comm.). Time pressureto complete their hybridsof the F• generationsuccessfully bred molt is high for CentralEuropean Stonechats. in aviaries,and hatchlingswere fed eitherby their The seasonsof the year are reliablyreflected in parentsor by humans(Gwinner et al. 1987).The var- the photoperiodiccycle, and the birds' molt re- iation in rearing conditionscaused small differences spondsto day length(Gwinner et al. 1983). in molt duration (Helm 1997). After fledging, all The AfricanStonechats (S. t. axillarisShelley birds fed on a standard diet and lived either individ- 1884)in our studyoriginated from Kenya,close ually in cages,or in groupsof four to eightbirds in to the equator,where the photoperiodis nearly aviaries (K6nig 1996). We accumulatedthe data for this study from dif- constant. The climate undergoes seasonal ferentexperiments. About 20%of the datahave been changesbetween a dry seasonand two rainy previouslypublished, as indicated in Table2. African seasons(Brown and Britton 1980). Birds main- Stonechats contributed more than one-half of the tain pair territories year-round (Dittami and data,European Stonechats more than one-third,and Gwinner 1985). African Stonechatsinitiate the two hybridgenerations together made up 11.2% breedingquickly with thebeginning of thefirst of the birds (Table 1). Chicks collected in the field rainy season.The birds anticipatethe approx- usuallycame in completesibling groups. The degree imatetiming of rainfallto preparetheir repro- of relatednessof the birds was high. Owing to dif- ductive systems,relying on an endogenous ferencesin clutchsize (Gwinner et al. 1995),African circannual rhythm (Gwinner and Dittami birds had, on average,fewer siblingsthan European 1990).They usually produce only oneclutch of birds.Because chicks bred in captivityoften had sib- three eggsper year, and the young staywithin lings from more than one brood, the familiesof hy- brids were relatively large. Chicks from the Euro- parental territories for severalmonths before pean populationhatched between April and July, dispersing.Hence, they can go through postju- and those from East Africa in April. The hatching venalmolt withoutobvious external time pres- dates of birds breeding in constantconditions are sure. Nevertheless, African Stonechatsare also distributed throughoutthe year. In this study, we responsiveto photoperiodicchanges (Gwinner give the dateas the serialnumber of the day in the et al. 1983, Gwinner and Dittami 1985). Here, year (Julian date, from 1 to 365). In comparisonsof we study differencesin molt betweenthe two the two sexes,we found no differencesand therefore subspecies,using an analysisof hybrids,envi- pooled all birds (Helm 1997). ronmental manipulation of photoperiod,and Photoperiodicconditions.--In all experiments,light exploratoryquantitative genetics. intensitywas about400 lux during daytimeand 0.01 lux at night. The birds were assignedto 11 photo- METHODS periodic groups (Table 2) that experiencedone or more of the followingthree typesof light-dark (LD) Studyarea and birds.--Most Stonechats in thisstudy regimes.The first was constantequatorial photope- were collected between 1982 and 1990 in two loca- riod. In early studies,we used LD 12.8:11.2hours, July 1999] Molt in Stonechats 591 TABLE2. Photoperiodictreatments of all Stonechats,classified by genotypicgroup (data are numberof in- dividuals).Photoperiod (PP; see Fig. 1) is givenas the light: dark (LD) subdivisionsof the 24-h day.Day x refersto x daysafter a bird hatched.Groups are numberedby type of photoperiodictreatment followed by a serialnumber Treatment types include unchanged (1), shortened(2), and extended(3) PP.The single birdsin groups1-2 and 2-2 were only consideredin comprehensiveanalyses. In groups3-3 and 3-4, some data were eliminatedafter days160 to 180because of experimentalcomplications. Group African European F• F2 Descriptionof photoperiodictreatment 1-1 88 • 3 17 b 14 LD 12.25:11.75from hatching;control condition for Afri- can Stonechats. 1-2 1 23 • Naturally fluctuatingEuropean PP from hatching;control conditionfor EuropeanStonechats. 2-1 57 4 EuropeanPP, changed to LD 12.25:11.75after day 2 to 20. 2-2 6 1 EuropeanPP, changed to LD 14.5:9.5,then LD 12.25:11.75 between days 6 to 18. 2-3 16 EuropeanPP,