TWO SUBSPECIES OF WARBLING VIREO DIFFER IN TH!:.!R RFPONSES TO COWBIRD EGC

SPENCER G. SEALY, Departmentof Zoology,University of Manitoba,Winnipeg, Manitoba R3T 2N2, Canada ALISON J. BANKS, Montana CooperativeWildlife Research Unit, Universityof Montana, Missoula,Montana 59812 JAMESON F. CHACE, Departmentof Environmental,Population, and Organismic Biology,University of Colorado,Boulder, Colorado 80309-0334

ABSTRACT: Usingreal cowbirdeggs, we experimentallyparasitized 41 nestsof the WarblingVireo (Vireo gilvus), three each in BritishColumbia and Colorado, five in Montana, and 30 in Manitoba, and recordedwhether the cowbirdeggs were acceptedor rejected.Cowbird eggs were acceptedat all neststested in British Columbia and Colorado, but both acceptanceand rejectionwere recordedin Montana.In Manitoba,all cowbirdeggs were rejected(29 by puncture-ejection,one bydesertion). The resultssuggest acceptance by a westernsubspecies of the Warbling Vireo, V. g. swainsonii,and rejectionby the easternsubspecies, V. g. gilvus. The geographicvariability in acceptance/rejectionagrees with suggestedtaxonomic differencesfor the WarblingVireo, i.e., that there are two speciesand that neither appearsto vary in responseto the presenceof cowbirdeggs in its nests.

Speciesof birdsthat sufferreduced reproductive success when parasitized by Brown-headedCowbirds (J•olothrus ater) shouldevolve strategies that reduceor eliminatethe costsof parasitism,especially because cowbird eggs are distinguishablefrom the eggsof most host species.Adaptations for rejectionof parasiticeggs from nests have evolved in somespecies, but most speciesaccept cowbirdeggs (Rothstein1975, 1990). Why only a few speciesreject cowbird eggs has been attributed to the durationof exposure to the selectivepressure of cowbirdparasitism (Rothstein 1975) or con- straintson the abilityof smallhosts to eject cowbirdeggs from the nest (Rohwerand Spaw 1988). Amongthe ejecterspecies, the largerones grasp and remove cowbirdeggs with their bills, whereasthe smallerspecies puncture-ejectthem (Rohwerand Spaw 1988). Recently,Sealy (1996) determinedexperimentally that the Warbling Vireo (Vireo gilvus)removes cowbird eggs from its nests.At 15 g, it is the smallestspecies in North America known to do so (Sealy 1996). This species'responses to cowbirdeggs also appear to varyacross its geographic range.A literaturesurvey of the frequencyof parasitismin variouspopula- tionsof WarblingVireos (table 2 of Sealy1996) revealedthat 0 to 11% of WarblingVireo nestswere parasitized in populationswithin and eastof the Central Great Plains, whereas 50 to 70% of nests were parasitizedin populationswest of the GreatPlains. Where experimental data are lacking, Friedmannet al. (1977) assumedthat a speciesaccepts cowbird eggs (i.e., is an accepter)if 20% or more of its nestsare recordedparasitized. Under thisdefinition, western populations of the WarblingVireo would be classified as accepters,whereas central and easternpopulations reject parasitism. Sealy's(1996) experimentson WarblingVireos in Manitobasupport the assessmentof birdsthere as rejecters.

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Fromthose data, Sealy (1996) concludedthat the apparentgeographic variationin responseto parasitismby WarblingVireos generally fits with data regardingthe durationof sympatrywith cowbirdsby easternversus westernpopulations (Mayfield 1965, Rothstein1994, but see Ward and Smith1998). Experimentson populationsacross the breedingrange of the WarblingVireo are requiredto confirmthe extentof responsesto cowbird eggs.Here we presentthe resultsof testson nestsof the WarblingVireo in BritishColumbia, Colorado, Montana, and Monitoba.

METHODS

We experimentallyparasitized Warbling Vireo nestsat four localities, describedby Wardand Smith (2000), Marviland Cruz (1989), MacKenzieet al. (1982), and Tewksburyet al. (1998). BritishColumbia: In 1994, David Ward parasitizedone nestabout 7 km WNW of Oliver (49ø 10' N, 119 ø 13' W) andtwo nests7 km E of OkanaganFalls (49 ø 21'N, 119ø 33' W), in the southernOkanagan Valley. Montana: Banks tested five nests in 1996, two alongthe BitterrootRiver (45 ø 56' N, 114 ø 08'W and46 ø 07' N, 114ø 10' W), threealong Rock and Lickcreeks (45 ø 09' N, 114ø 13' W) and McCoy Creek(45 ø 59' N, 114 ø 12' W). One of thesenests was naturally parasitized, receivingone cowbirdegg. Colorado:In 1994, Chacetested two nestsand recordedthe responseto a naturallylaid cowbirdegg in anothernest, all in BoulderCounty. One nestwas near Ward on the SawtoothSprings Ranch (40ø 07' N, 105ø 28' W), the otherswere on FlagstaffMountain (40 ø 00' N, 105ø 18' W). Manitoba:Sealy (1996) parasitized16 nestsand recorded the responseto naturalparasitism at one nestin 1992 and 1993 at Delta Marsh (50 ø 11'N, 98 ø 19'W). He tested13 additionalnests in 1996 and 1997, for a total of 30 nests at this site. We introducedone real cowbird egg into each nest during the layingstage or incubationstage. In all but one nest, cowbirdeggs were addedbefore 12:00 (respectivestandard times). A hostegg was not removedfrom nests becauseegg removal by cowbirdsis variable(Sealy 1992). Mostnests were inspectedthe day after parasitismand everyday afterthat untilthe cowbird eggdisappeared or hadremained in the nestfor at least5 days,when it was assumedto be accepted(Rothstein 1975, Sealy1996). We watchedall nests tested in Colorado and Montana and 17 nests in Manitoba for 30 to 60 min immediatelyfollowing parasitism to recordreactions of adultsto the parasit- ized clutches(see Sealy 1996 for descriptionsof behavioralresponses to experimentalparasitism in Manitoba).

RESULTS

Cowbirdeggs were accepted at all neststested in BritishColumbia (n = 3, D. Ward, pers. comm.) and in Colorado(n = 3). One of these nestswas naturallyparasitized, receiving one cowbirdegg. In Montana,cowbird eggs were acceptedat threenests (one experimentally parasitized during laying; two duringincubation, one clutchalready parasitized) and ejectedfrom two incubated clutches.

191 DIFFERING RESPONSES OF WARBLING VIREO TO COWBIRD EGGS

Reactionsof femalesupon their return to the parasitizednests differed. In Manitoba, at 16 nestswhere an adult returnedto the nest duringthe observation,the vireopunctured or brokethe cowbirdegg and ejectedit, generallywithin secondsof its returnto the nest(description of behaviorin Sealy 1996). At the three Coloradonests, vireos returned unagitated and settledon theireggs without probing or peckingthe cowbirdegg or behaving in anyother way that suggestedrecognition of the foreignegg. Vireos also resumedincubation at the fiveMontana nests with no signsof recognitionof the cowbirdeggs, although the cowbirdeggs were laterejected at two nests.

DISCUSSION

WarblingVireos' responsesat experimentallyparasitized nests varied. Cowbirdeggs were acceptedat all neststested in BritishColumbia and Coloradobut were rejectedfrom all neststested in Manitoba.In Montana, cowbirdeggs were acceptedat three and ejectedfrom two nests.Collec- tively, these resultsare subjectto two interpretations.The first is that acceptanceand rejectionbehavior in the WarblingVireo is variableand that thereforethis species joins a smallgroup of speciesin whichacceptance or rejectionis not near 100%, suchas the Yellow-breastedChat (Icteria virens, Burhansand Freeman 1997), Yellow-headedBlackbird (Xanthocephalus xanthocephalus,Dufty 1994), andCommon Grackle (Quiscalus quiscula, Peer and Bollinger 1997). The other interpretationis that despite the variableresponses to parasitism,Warbling Vireos do not conflictwith the generalizationof low variationin responsesbecause the WarblingVireo as traditionallyconstituted is actuallytwo siblingspecies (Johnson et al. 1988, Sibleyand Monroe 1990, Murrayet al. 1994). Sibleyand Monroe (1990) recognizedtwo speciesof WarblingVireo, Vireogilvus (Eastern Warbling-Vireo) and V. swainsonii(Western Warbling- Vireo). The speciesdiffer in morphologyand vocalizations(J. C. Barlowin Sibleyand Monroe 1990) and by about 3% sequencedivergence in their mitochondrialDNA (Murrayet al. 1994). At one point where their ranges meet,in north-centralAlberta, males singing the gilvussong-type and males singingthe swainsoniisong-type have been foundon adjacentterritories, butinterbreeding between the two hasnot beenrecorded (W. B. McGillivray and J. C. Barlowpers. comm.).Although the limitsof the rangesof the easternand westernsubspecies of the WarblingVireo are known only roughly(e.g. Sibley 1940, Worthen 1969, Browning1974, Voelkerand Rohwer 1998), we suspectthe two subspecies,or species,meet also in western Montana (seemaps in Voelker and Rohwer 1998) becauseboth acceptanceand rejectionof cowbirdeggs were recordedthere. Confirma- tion of the trendtoward acceptance of cowbirdeggs in a westernsubspecies and rejectionby the easternone, suggestedby our limiteddata, requires more testingof nestsacross the entire range of the WarblingVireo, especiallyof birdsfarther east and preferablywith the attendantspecies or subspeciesverified at each nest. The variable responseto parasitismsupports the contentionof two speciesof Warbling Vireo, with swainsonii occurringfrom the Rocky Mountainsto the PacificOcean and gilvus from the Rockiesto northeastern

192 DIFFERING RESPONSES OF WARBLING VIREO TO COWBIRD EGGS

North America.As cowbirdswere foundhistorically from the Rockiesto the easternslopes of the SierraNevada and CascadeRange (Rothstein 1994), swainsonii islikely to haveexperienced some parasitism before the cowbird's rangeextensions since 1900. Butthe criticalthing in gilvusbeing a rejecter isthat it likelyexperienced extensive parasitism in andadjacent to the Great Plains.Sealy (1996) assumedthat swainsonii,though smaller than gilvus (Ridgway1904), is large enoughnot to be physicallyconstrained from ejectingcowbird eggs. If not, and rejectionis morecostly, these individuals may be in equilibriumwith cowbirdparasitism. This seemsunlikely, how- ever,because the usualresult of parasitismon WarblingVireos is the lossof their entire brood (referencesin Sealy 1996). Therefore,there may be essentiallyno costto rejection,only a reductionin the net valueof not rejectingcowbird eggs.

ACKNOWLEDGMENTS

In Manitoba,the staff of the Universityof ManitobaField Station (Delta Marsh) providedimportant logistical support, and the officersof the Delta Waterfowland WetlandsResearch Station and PortageCountry Club permittedSealy to conduct someof the experimentson theirproperty. Karim Haddad, Celia McLaren, and Glen McMasterassisted with the fieldwork. Financialsupport was providedby the Natural Sciencesand EngineeringResearch Council of Canada.David Ward tested the nests in British Columbia. In Montana, JoshuaTewksbury and crew of the Bitterroot Riparian Bird Project (fundedby the United StatesForest ServiceIntermountain ResearchStation and the U.S. GeologicalSurvey Biological ResourcesDivision's BreedingBiology Research and MonitoringDatabase program) provided logistical supportand locatednests. Dan Cariveauand ToddOndick collected cowbird eggs, andJamie Luke and Lauren Golten experimentally parasitized the nests.Funding was providedfor the workin Coloradoby the BoulderCounty Nature Association and the Universityof Colorado.Comments by Tom Gardaliand Todd Underwoodon early draftsof the manuscriptand Anna Lindholmon the finaldraft are greatlyappreciated, asare those by the reviewers,Charles A. Drostand Stephen I. Rothstein.Mark Sogge providedhelpful editorial suggestions.

LITERATURE CITED

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Accepted 19 March 2000

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