Riskoffeatherdamageexplainsfaultbaroccurrenceinamigrant hawk,theSwainson’shawkButeoswainsoni
Jose´ H. Sarasola andRoger Jovani
.
Faultbarsarecommonstress-inducedfeatherabnormalitiesthatcouldproducefeather damage thus reducing flightperformance.For that reason, ithasbeenhypothesized that birdsmayhaveevolvedadaptive strategiesthat reducethecostsoffaultbars(the
‘faultbarallocation hypothesis’).Anuntestedprediction ofthishypothesisisthatfault barsinimportant feathersforflight(wingandtail)shouldbelessabundantwherethey
producemorefeatherdamage.Wetestedsuchapredictionusingmoultedwingandtail
feathers of the long-distance migrant Swainson’s hawk Buteo swainsoni in its Argentinean winteringquarters.Werecorded theoccurrence offaultbarsofdifferent strengths (light,medium and strong) and thedamage (lostofaportion ofthevane)
produced bythem. Theoccurrence offault barswasveryvariable, withstrong ones beingrarethroughoutandlightandmediumfaultbarsbeingmorefrequentinthetail than inthewing.Riskoffeather damage wassimilarlyhighand lowacross feather
groupsforstrongandlightfaultbars,respectively,andhigherinthewingthan inthe tailformediumstrength.Theoccurrenceoffaultbarsofdifferentstrengthsondifferent
feather groups was negatively correlated with their propensity to produce feather damage. Atlowdamage risk(B5%),theoccurrence offaultbarswashighlyvariable depending onthefeather group, but above 5%offeather damage theoccurrence of
faultbarswashighlyreducedthroughout.Ourresultssupports the‘faultbarallocation hypothesis’ofnatural selectionreducingfaultbaroccurrencewherefaultbarsaremore risky,but further suggestthat selectionpressure couldberelaxedinother instances,
leavingthewayfreeforother mechanisms toshapefaultbaroccurrence.
J.H.Sarasola(correspondence)andR.Jovani,DepartmentofAppliedBiology,Estacio´n
Biolo´gica deDon˜ana (CSIC),Avda.deMar´ıa Luisa s/n,Pabello´ndelPeru´,41013
Sevilla,Espan˜a.E-mail:
Fault barsarecommon abnormalitiesinbirds’feathers (Riddle 1908), that look like narrow and translucent bands arranged approximatelyperpendiculartothe feather rachis. They are produced by some barbules being slimmer or completely absent as a result of a variable time lag on the deposition of keratin during feathergrow(Murphy etal.1989,PrumandWilliamson
2001).Although mechanisms promotingfault bars are still poorlyunderstood, nutritionalconditions(Slagsvold
1982, Machmer et al. 1992) and stress episodes (i.e. escapefrompredators;KingandMurphy 1984,Negroet al. 1994) are some of the most commonly evocated causesforfaultbarformation.
Irrespective ofthemechanisms promotingfault bars, they could produce potential flight costs because of partial feather damage orevencomplete feather break- age(Slagsvold1982,Machmer etal.1992).Contraryto feathers lostduring moult, whichareimmediately replaced bynewones,damaged orbroken feathers are notreplaceduntilthenextnormalmoultoftheplumage. Thus, feather damage resulting from fault bars may reducewing-tailsurfaceareaforlongtimeperiods.Allof thisisrelevantforbirdfitnessbecausewingload(body weight/wingarea)is crucialforflightperformance (Pennycuick1989), birdsbeingforcedtoreducetheir weight during even slight reductions of wing area
(Swaddle and Witter 1997, Lind and Jakobson2001, Senaretal.2002).Moreover, experimental reductions of wingareaareknown toincreasetheenergeticdemands ofbirds,loweringtheirreproductive success(Mauckand Grubb 1995,Velando2002).Similarly,sharplosesoftail areahavebeenreported tocompromise manoeuvrability during flight(Fisher 1959,Muelleretal.1981).
Jovani and Blas (2004)proposed that birds should
haveevolvedadaptivestrategiesforreducingthecostsof fault bars (‘fault bar allocation hypothesis’). For im- portantflightfeatherssuchaswingandtailfeathers,an untested prediction ofthis hypothesis isthat the occurrence of fault bars should be lowest in those feathers wherethe riskoffeather damage due to fault bars ishighest. Inother words, different probabilityof featherdamagebyfaultbarsinimportant flightfeathers shouldbe behindtheobservedvariabilityinfaultbar occurrenceamongfeathers(e.g.KingandMurphy 1984, Machmer et al. 1992, Serrano and Jovani 2005). An indirect evidence of it was found studying feathers growingsimultaneouslyin whitestorkCiconiaciconia chicks,wherefaultbarswerefound tobelessabundant onthosefeathersthoughttohave morestrength requirementsduringflight,andthuspossiblya higher probabilityoffeatherdamagingduetofaultbars(Jovani andBlas2004).However,adirecttestofthisprediction islacking.
TheSwainson’shawkButeoswainsonibreedsinNorth
Americaandspendstheborealwinterinsouthern South America,mainlyin thecentralprovincesofArgentina (Englandetal.1997),performing amigratory tripofca.
10,000 km each way (Fuller et al. 1998). During migrationtheymainlyperformsoaringflight benefiting from thermals (Smith 1980). Soaring flight has low energydemands,butimposesahighflexionstressonthe distalprimaryfeathersthatcurveupforreducingtheair drag(Cone1962,Tucker1993). Moreover, theyshow diverse flight foraging displays, ranging from hunting smallvertebratesby directflight,hoveringorperching (Englandetal.1997), topreyinguponinvertebrates captured intheairwhilesoaring inthermals (Jaramillo
1993, Rudolphand Fisher 1993, Sarasola and Negro
2005).Inthat way,afeather damage duetoafaultbar could impose an importantflight constrain. Although mostdetaileddataexists onthetimingofmoultof Swainson’s hawks for the breeding season (Schmutz
1992),hawksalsomoult during thewinteringseasonin Argentina (Goldstein etal.1999,Bechard andWeiden- saul2005).Boththehighflightrequirements andmoult featuresthusmaketheSwainson’s hawkaninteresting study model forthe analysis ofthe adaptive nature of faultbardistribution patterns inbirds.Moreover,by studying moulted feathers webenefited from a more similarageofeachfeatherstudied, andthusourresults had not thebias ofdifferent agesoffeathers ascould happen whenstudyingfeathers onwild-caught birds.In
any case, the age of feather has been found not correlated with feather damage due to fault bars ina studywithcranes(Jovani etal.unpubl. data).
Here, we tested the prediction of the ‘‘fault bar allocation hypothesis’’ by studying the occurrence of faultbars,thepropensity offaultbarsproducing feather damage, and the correlationbetween both variables in theSwainson’shawk. Occurrence isusedthroughoutto resumeinasamewordboth prevalence(i.e.,percentage offeatherswithfaultbars)andabundance(i.e.,number offaultbars).Theroleofthetailinflightperformance refers to stability, balance, and turning (Thomas
1996a,b),sotheintensityoftheinduceddragsupported bytailfeathersisexpectedtobelowerthanthatofouter wing feathersinvolvedinflying activitieslikesoaring. Withinthewing,flightrequirementsarelowerin the innermost than in the outermostwing feathers, espe- ciallyforgliding(Tucker 1991).Thus, according tothe
‘fault bar allocation hypothesis’, weexpected that: 1) faultbarsoccurrence intheSwainson’shawkshouldbe loweronwingthan ontailfeathersandwithinthewing lowerindistal than inproximal feathers, 2)theriskof feather damage duetofault bars must belowerinthe groupoffeatherswithlowerflightrequirements(e.g.,the tail)than those supporting more physicalstressduring the flight such as the distal wing feathers, and 3) a negativerelationshipbetweenriskoffeatherdamageand prevalenceandabundanceoffaultbars.
Methods
Feathercollectionandanalyses
Swainson’shawksareterritorial duringthebreeding season, but they became gregarious in the winter quarters wheretheyroostandhuntinflocksofhundreds oreventhousands(England etal.1997).Webenefited fromtheselargeaggregations collecting801 moulted feathers from 2001 to 2003 from the ground of 12 different roosts incentral Argentina (La Pampa, Co´r- doba andBuenosAiresprovinces).
Inthelaboratory,we usedamuseumreferenceskinto classify wingfeathersinthreegroups:Pdist(distal primaries): from the outermost to the sixth primary (N=124 feathers), Pprox (proximal primaries): pri- mariesfifthtofirst(N=101feathers),andS-T:second- ary and tertial feathers (N=289 feathers). Primary feathers were separated in two groups because their distinct morphology (distal ones being more asym- metric) and function during flight(seeabove). Wedid auniquegroupwithsecondariesandtertialsbecausethe smallnumber ofpossibletertials collectedand because theyshowagradual changeonmorphologyfromouter secondariestoinnertertials,beingdifficulttodifferenti- ate.Tailfeatherswere classifiedinthreegroupsalso considering their distinct morphology: Rdist (distal
Fig.1.Agradientof faultbarstrengthsfoundonthestudied feathers. Arrows indicate fault bars. The asterisk indicates a breakoffeather barbules becauseofastrong faultbar.
rectrices):theexternalandmoreasymmetric feathersof the tail (N=41 feathers), Rprox (proximal rectrices): internal rectrices (from rectrices two to five;N=204 feathers),andRcen(centralrectrices): themoresym- metric,centralpairoftailfeathers (N=42feathers).
Each feather wasinspected forthe presenceoffault
barsby changingtheangleof lightincidenceandalso holdingthefeathersagainstthesky.Wecategorizedeach faultbaraslight(absenceofsomebarbules producing a visiblediscontinuity onthestructure ofthefeather;N=
407), medium(anarrow, i.e.B1mm,translucentline acrossthefeatherN=254),orstrong(]1mm,translu- cent line across the feather N=63; see Fig. 1 for examples,and Fig.3forsamplesizesdetailed foreach feathergroup).Inextremeinstances,faultbarsproduced thecutofaportion ofthevanefromitsposition upto the distal edge of the feather (Fig. 1). We recorded whether or not each fault bar produced breaking of feather barbules.
Wecalculated theprevalenceandabundanceforfault
barsandtheriskof featherdamageduetofaultbars. Prevalenceoffaultbarswascalculatedasthepercentage offeathersthathavefaultbars,whileabundanceoffault bars wascalculated asthe mean number offault bars
found oneachfeather. Weestimated theriskoffeather damage due to fault bars as the ratio between the number offaultbarsproducing featherdamage andthe totalnumberoffaultbars.Alltheseparameters were estimatedforeachof thefeathergroupsandfaultbar categories.
We alsocalculatedtheprevalenceandabundanceof feather damage duetofaultbarsandlooked for differencesbetweenfeathergroups.Prevalenceoffeather damagewasthepercentageoffeatherswithdamagedue tofaultbarswhile abundanceoffeatherdamagewas estimatedasthenumberof faultbarsoneachfeather producing feather damage.
Theuseofmoultedfeathersprecludedustodowithin bird comparisons of the occurrence of fault bars in different feathers. Rather, we included in the same analysesfeathersfromdifferentbirds.Thisisapotential sourceof noisebecausefeathersfromindividualswith differentage,sex,andhistoryaregrouped. However,for species suchasraptors, agivenindividualusestohave differentfeathergenerations,andthusfeathersgrown at differenttimeandof differentage.Inthisway,using moulted feathers inourstudycaseisagoodalternative that allowthestudyofalargesamplesize,whilebeing studyingfeathersofsimilarage(allarerecentlymoulted feathers).
Becausethenon-parametric nature ofthestudied variables(percentages, andcounts greatlyrightskewed) weusedChi-square, Mann-WhitneyandKruskal-Wallis two-tailed tests.
Results
Prevalenceandabundanceoffaultbars
Theprevalenceoflightfaultbarswashigherontailthan on wing feathers, and variable within wing feathers (Table1;Fig.2a).Theprevalenceofmedium faultbars wasalsovariable, higheronthetailthan onthewing, and variable within tail feathers (Table 1; Fig. 2a). However,strongfaultbarsoccurred atalowprevalence throughout and we did not find differences among
Table1. Differencesinfaultbarprevalenceandabundance forwingandtailgroupsoffeathers.Chi-square (P-value)forprevalence, andKruskal Wallis(for2groups)andUMann-Whitney(fortwogroups)forabundancecomparisons areshown.Wedidnotfind differencesintheprevalenceandabundanceofstrong faultbarsamong feathergroups(seetext).
Abundance
Fig.2. (a)Prevalence(%9SDoffeatherswithfaultbars),and (b) abundance(mean995% CI of number of fault bars per feather)according tofeathergroup.
feather groups (x2=6.63, P=0.25; Fig. 2a). For the abundanceoflightand medium fault bars wefound a similarpicturethanforprevalence(Table1, Fig.2b),and thenumberofstrongfaultbarswasevenly distributed (Kruskal Wallisx2=6.56,P=0.26;Fig.2b).
Riskofdamageaccordingtofaultbarstrengthand feathergroup
Theprobability ofafaultbarproducing thebreaking of feather barbules was greatly dependent on fault bar strength (x2=78.26, PB0.001), increasing from light (twobreakings outof407faultbars;0.5%),tomedium (22/254;8.7%),andstrong faultbars(17/63;27.0%).
Lightfaultbarsconstitutedalowriskofbreaking of barbules inallfeather groups (x2=1.93, P=0.86), and strongfaultbarsrepresented ahigherandsimilarriskof breaking throughout(x2=1.19,P=0.95;Fig.3).How- ever,faultbarsof mediumintensitydifferedontheir propensity ofbreaking offeather barbules (x2=28.22, PB0.001), being higher on the wing than on the tail feathers (x2=14.79, PB0.001), differing among
Fig. 3. Percentage (%9SD) of fault bars producing feather damage.Samplessizes(numberoffaultbars)forfeathergroups from left to right were: Light: 94, 14, 81, 42, 145, and 31; Medium:15, 23, 82, 32, 70, and32;Strong:4, 4, 24, 7, 20, and4.
wing(x2=7.81, P=0.02), but not among tail feathers
(x2=0.97,P=0.62;seeFig.3).
Faultbarprevalenceandabundanceinrelationto theriskofdamage
The risk of feather damage due to fault bars was negatively correlated with the prevalence (Spearman r=—0.68,N=18,PB0.01)andtheabundance(Spear- manr=—0.77,N=18,PB0.001)offault barsamong feather groups (Fig. 4).At a low risk of breaking of featherbarbules(B5%)faultbarsoccurredatavariable prevalenceandabundance,butwhentheriskofbreaking exceededthe5%,theoccurrenceoffaultbarswasalways reducedatlowlevelsforallfeathergroupsandfaultbar strengths (Fig.4).
Fig.4. Relationshipbetweenprevalenceoffaultbarsproducing breaking ofbarbules (riskoffeatherdamage duetofaultbars) and prevalence (%9SD of feathers with fault bars) and abundance(mean995%CIofnumberoffaultbarsperfeather) offaultbars.
Prevalenceandabundanceoffeatherdamage
The percentage offeathers with at least one break on their barbules was similarly low (between 3 and 6%) across feather groups (x2=1.45, P=0.92; Fig. 5).The abundanceofsuchbreakswasalsoequallylow(between
0.030 and 0.065 breaks per feather) among feather groups (Kruskal-Wallisx2=1.46,P=0.92;Fig.5).
Discussion
Oursisthefirststudylinkingtheoccurrenceoffaultbars and thedamage theyproduce onthefeathers ofbirds. Three main results may be noted: (a) light fault bars almostneverproducedfeatherdamagein anygroupof feathers, (b)strong faultbarsconstitutedasimilarhigh riskoffeatherdamagethroughout,and(c)mediumfault bars showed a variable risk of feather damage, from being as risky as strong fault bars on distal primary feathers, decreasingonproximal wingfeathers, tobeing asinnocuous aslightfaultbarsonthetail.Theseresults support thehigherstrengthrequirementsofwingvs.tail, and in some cases that of distal vs. proximal wing featherssupposedin previoustestsofthe‘faultbar allocation hypothesis’ (Jovani and Blas 2004,Serrano andJovani2005).However,theyalsorevealtheirregular pattern fortheprevalenceandabundance of faultbars throughdifferentfeatherand faultbar categories.For example, prevalence and abundanceoflight fault bars washigherinthemostdistalwingfeatherswhilemedium fault bars were evenly distributed in the wing with similar fault bar abundances among wing feathers. Suchresultshencedonotcompletelysupport our prediction oflowerprevalence and abundanceoffault bars inthose feathers withhighest flyingrequirements, andsuggestthat moreworkisneededhere.
Fig.5. Prevalence(%9SDoffeatherswithfaultbars)and abundance(mean995%CIofnumberoffaultbarsperfeather) offeather damage byfault bars along wingand tail feathers. Notethesamescalethan Fig.2forcomparison.
Medium faultbarsweremoreriskyinthemostdistal wingfeatherswhencomparedwithinnerwing andtail feathers.However,lightfaultbarswereinoffensiveinall feathertracts,whilestrongfaultbarsshowedahighrisk offeatherdamage through.
The relationship between the pattern of prevalence
andabundance offaultbarswasinaccordancewiththeir riskiness.Thatis,atalowriskoffeatherdamage(B5%: lightfault bars throughout,and medium fault bars on thetail)theoccurrenceoffaultbarswashighlyvariable, butabovea5%threshold (strongfaultbarsthroughout, and medium fault bars onthewing)theoccurrence of fault bars was low in all cases. Thus, more or less innocuousfaultbarswere highlyvariableontheir occurrence,butrisky faultbarsnever reachedthehigh occurrenceofunriskyfaultbarsinanygroupoffeathers. Thiscouldeasilyexplainwhytheoccurrenceoffeathers with some damage by fault bars was very low, and similar,among group feathers.
Itiscurioushowdespiteallthepreviousresultsmatch
onlypartially withour initial predictions, therelation- shipbetweenfaultbaroccurrenceandfaultbarrisk of damagewassoclear(Fig.4).Weenvisiontwoimportant factors to explain this pattern. First, a horizontal ordering because ofsimplephysical reasons with light faultbarsmainlyontheleft,mediumin thecentreand strong onesattheright. Second, averticaldistribution shaped by natural selection that tend to minimize in general those fault bars that produce more feather damage,thatis,mediumandstrongones.However,light faultbarswere notsoshapedbynatural selection,and greatly differed among feather groups. Note moreover, the interesting deviations of some points from this general pattern.For instance, there wasavertical shift of two points of medium strength fault bars, that occurred atthesamelevelthat themorefrequent light fault bars, but accordingly, theywerealsolowriskyas therestoflightfaultbars.
Inthisway,thesimilarlowoccurrenceofstrongfault
barsacrossfeathergroups,andthe higheroccurrenceof mediumfaultbarsonthetailthan onthewingfeathers could beexplained bythedamage probabilityofthese fault bars upon feathers. Accordingly, fault bars have beenfoundtobemoreprevalent onthetailthan onthe wingfeathersforotherthreeButeospecies,butasimilar occurrence betweenthetailand thewingwasfound in thesamestudyforanowlspecies(Strixvaria,Hawfield
1986).Theprevalenceoffaultbarshasbeenalsofound tobelowerinthewingthanonthetailfeathersofother raptor speciessuchastheAmerican kestrelFalco sparverius (Negro et al. 1994, Bortolloti et al. 2002) andtheospreyPandionhaliaetus(Machmer etal.1992), andpasserinespeciessuchasthewhite-crownedsparrow Zonotrichialeucophrys(KingandMurphy 1984)andthe Barn Swallow Hirundo rustica (Serrano and Jovani
2005). On the contrary, fault bars are common and
equallyabundant onwingandbodyfeathersofthenon- flyingostrichesStruthiocamelus(Duerden 1909). This suggests that the flight style and morphologyof the speciescouldgreatlyshapetheriskoffeatherdamageby faultbars,andthusitsoccurrencepattern acrossfeather groups, beinganissuethat requiresfurther comparative studies(Serrano andJovani 2005).
Thelowriskinessoflightfaultbarsthroughoutwing- tailfeathers couldnot explain, however,whytheywere morecommon inthetailthan onthewingfeathers.Itis thought that faultbarsareproduced bythecontraction of the musculaturearound the feather follicleduring featherformation (Murphy etal.1989).Forlowering(or evenprecludingitsformation)thestrength offaultbars, naturalselection couldhavethusoperatedthrough mechanisms aimed to reduce the contractionstrength ofthemusculatureduring stressfulepisodes.Therefore, feather folliclewing musculature in the study species wouldbe morerelaxedthanonthetailduringastress episodebecauseoftherisk ofproducingfaultbarsof mediumandstrongstrength.This physiologicalinertia couldbeanexplanation ofwhylightfaultbarswerealso morecommon onthetailthan onthewingfeathers.
The results presented here, together with previous indirectevidencereportedelsewhere, suggestthatthe probabilityofformationoffault bars islowered inan adaptiveway inthosefeatherswithmorestrength requirementsduringflight,accordingtothe‘fault bar allocation hypothesis’(Jovani andBlas2004).However, thefingerprintof thenaturalselectioncouldnotbefound in thosefeatherswherefaultbarsaremoreinoffensive, leavingthewayfree forothermechanismstooperate. Furtherstudiesoftheexternalfactorsandthephysiolo- gical mechanismsthatproducefaultbarsofdifferent strengths areneededtoreachadeeperunderstandingof the power of the ‘fault bar allocation hypothesis’ to explainfaultbaroccurrence,anditsintimaterelationship withtheevolutionaryhistoryofbirdflight.
Acknowledgements— Wewish to thank to M. Santilla´n, M. Galmes,V.Salvador, M.Reyes,A.Travaini andJ.J.Negrofor help collecting feathers. We thank A. Lanusse, S. Salva, M. Alliaga,J.Montoyaand locallandowners fortheirhospitality andlogisticsupport during fieldworkinArgentina. D.Serrano andE.Ursu´amadeusefulsuggestionstoapreviousdraftofthis manuscript.Wealsoappreciate thesuggestionsand comments madebyJohan Lindandoneanonymousreviewerthat greatly improvedthismanuscript.Thisstudywassupportedbyagrant fromtheWildlifeConservationSociety(USA).Whiledoingthis research,JHSwassupported byascholarship fromConsejo Nacional deInvestigacionesCient´ıficasyTe´cnicasdeArgentina (CONICET)andRJwassupportedbyagrantfromTelefo´nica Mo´vilesS.A.
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(Received27April 2005,revised7September 2005,accepted
13September 2005.)