LESTER L. SHORT - University of New Mexico

TAXONOMIC ASPECTS OF AVIAN HYBRIDIZATION

LESTER L. SHORT

INSTANCEoSf natural hybridizationfrequentlyposedifficult taxonomic
problems.A decisionregardingthe taxonomicstatusof hybridizingforms
shouldbe baseduponan evaluationof all availablebiologicalinformation
concerningtheir relationship.The paucityof availableinformationusually
hamperstaxonomiststreatinghybridizationamongthe highervertebrates.
Alsoit is difficult or impossibleto subjectsuchanimals,and particularly
animalpopulationst,o experimentasl tudy. Neverthelesstaxonomistsmust
renderdecisionsthat reflect currentavailableknowledge.I offer the follow-
ing discussionand definitionsin the hopeof facilitatingsuchdecisionsby
taxonomists,especiallythose working with terrestrial vertebrates. Ex-
amplesformingthe frameworkfor the discussionare drawn from the litera-
ture of avianhybridization,includingsomework of my own.

The widely divergent treatment of hybridizing forms evident in the
recent ornithologicalliterature demonstratesthe need for some guide-
lines. At one extremethis divergenttreatmentappearsto reflect a hold-
over of typologicalthinking (e.g. Godfrey, 1966; Sutton, 1967), while
the oppositeextreme(e.g. Phillipset al., 1964; West, 1962) resultsfrom
an overly strict interpretationof the biologicalspeciesdefinitionin that
differentspeciesdo not interbreed,and henceinterbreedingformsmustbe
conspecific.The resultsof suchdivergenttreatmentare taxonomicover-
splittingin the formercase,andoverlumpingin the latter.

I definehybridizationas the interbreedingof individualsof morphologi-
cally and presumablygeneticallydistinct populations,regardlessof the
taxonomicstatusof suchp' opulations(seenAnderson,1949: 61; Short,
1965: 360). This definitionof hybridization,essentiallythat of Mayr
(1963: 110), does not differ fundamentally from its broader genetic
meaning,in which it can evenincludeinterbreedingof genotypicallydif-
ferent individuals;I have simplyrestrictedits usage. Includedare both
sympatricandallopatrichybridization;the latter includessecondaryinter-
gradation,but not primary intergradation(see Mayr, 1963: 368 369).
Secondaryintergradation is distinguished,occasionallywith difficulty,
from primary intergradationby the increasedvariability of populations
within comparedwith thoseoutsidethe area of intergradation,contrasted
with the normalvariability of populationswithin and outsidean area of
primary intergradation.I define "hybrids" as the productsof sympatric
and allopatric hybridization (including secondaryintergradation), and
"intergrades"as the productsof primary intergradation. The practiceof
defining hybridization on the basis of the taxa involved shouldbe dis-

84 The Auk, 86: 84-105. January, 1969

Jan., 1969] TaxonomicAspectsof Avian Hybridization 85

couragedb, ecauseit requiresa taxonomicdecisionbefore"hybridization"
can even be employed(see Mayr, 1963: 111; Bigelow, 1965), and it
unnecessarilylimits the use of this word. I cannot see the need for en-
cumberingtaxonomicallya term that is fundamentallynontaxonomic.

How distinctmust populationsbe to warrant consideringtheir inter-
breedingas "hybridization"?The populationsought to be sufficiently
distinctto permit recognitionof a hybrid as such. I suggesthat their
distinctnesosughtto approache,qual,or exceedthelevelof distinctnesosf
subspecies.Criteria for subspeciesare a purely taxonomicmatter, ir-
relevantto thispoint,the ideabeingthat thepopulationsareaboutasdis-
tinct, or moreso, than subspeciesF. or example,designationof an area
of primary intergradationdependson the distinctnessof the populations
involved.Thesepopulationsare usuallyconsidereads subspeciebs,ut that
dependson the criteria of the taxonomistviewing the situation. I have
designatedas intergradesabovethe productsof interbreedingthroughan
area of primary intergradation,but this doesnot precludethe possibility
of true hybridizationoccurringbetweenindividualsof the sametwo popu-
lations. Wanderingbirds.from one populationoccasionallymay traverse
the area of intergradationand mate with birds of the other population,
producingtrue hybrids. Of courseonemight obtainindividualsof the two
populationsand mate them undercaptiveconditionsto produceoffspring
that are F• hybrids,not intergrades.

While the framework for the considerationof hybridization presented
herein is novel, mostof the basic ideasare drawn, someinsensibly,from
the vastliteratureon speciationand populationgenetics.Especiallyhelp-
ful have been the importantworks of Mayr (1942a, 1963), Anderson
(1949, 1953), Dobzhansky(1951), Stebbins(1959), and Sibley (1957,
1959, 1961).

The biologicalinformation forming the basis for a taxonomicdecision
in casesof hybridizationshouldideally include: 1) knowledgeof the oc-
currenceand/or lack of occurrenceand the placeof occurrenceof hybrids;
2) knowledgeof the distributionand habitats of the parental forms and
hybrids;3) knowledgeof the relativefrequencieosf hybrid and parental
phenotypesin the areaof hybridization;4) knowledgeof the type of cross-
ing (F•, F2,backcrosst)hat is occurring;5) knowledgeof the occurrence
and the extentof any introgression(introgressionis geneflow resulting
from hybridization;for discussioannd definitionseeShort, 1965: 360);
and 6) knowledgeof the populationdynamics(especiallyof the spatial
distributionof individualsand the populationstructure) of the formsin-
volved. Althoughfull data are oftenunavailablefor someof theseitems,
theremay existinferentialbasesfor their considerationpermittinga tenta-

86 LESTERL. SaORT [Auk, Vol. 86

tive taxonomicevaluation.Other (behaviorial,genetic,ecological)data
are of coursedesirable;their availability greatly aids in the taxonomic
evaluationa, ndstrengthentshe decisionthat is ultimatelyreached.

Hybrids producedunder captive conditionsare not discussedin this
paper. I agreewith Mayr (1963: 112) that "the mere possibilityof
hybridizationin captivity provesnothingas far as speciesstatusis con-
cerned." Artificially inducedhybridizationprovesonly the existenceof
considerablegeneticsimilarity and compatibility. Of coursestudiesof
hybridizationundercaptiveexperimentaclonditionsarea usefuladjunctto
the studyof naturalhybridizationin thoseanimalsthat canbe maintained
and bred successfullya,nd in sufficientnumbers.

The severacl ategorieosf naturalhybridizationare discusseidn the order
fromrarehybridizationto frequenthybridizationand morecomplexinter-

actions.

INFREQUENT AND RARE HYBRXDIZATION

Singleinstancesof hybridizationare generallyof limited taxonomicuse-
fulness,although they may provide cluesto relationship. Many inter-
genericarian hybrids involvingtaxonomicallyvalid generahave been
recorded.Otherpurportedcasesinvolvespeciesof overlysplit generathat
shouldbe congenericf,or examplehybridsamongspeciesof North Amer-
icanhummingbird"genera"(Shortand Phillips,1966). Recentlyreported
uniqueintergenerichybridsincludea flycatcherContopussordidulusX
Empidonaxtraillii (Shortand Burleigh,1965), a woodwarblerDendroica
strfataX Seiurusnoveboracens(isShort and Robbins,1967) and a hum-
mingbird CynanthuslatirostrisX Euge'nesfulgens (Short and Phillips,
1966). In the first two of thesecasesthe occurrencoef the hybrid tends
to corroborategenerallyacceptedgenericrelationshipsand the current
adjacentplacement(A.O.U., 1957) of the generainvolved. Study of the
third caseled to the suggestedcloserelationshipbetweenCynanthusand
Eugenesw, hichpreviouslyhad not beenconsideredcloselyrelated. Thus,
lone hybrids betweenspeciesrepresentingvalid generamay corroborate
relationshipsdeterminedby otherstudies,or they may suggestcomparisons
not previouslyconsideredT. he occurrenceof severaluniqueintergeneric
hybridsinvolvingdifferentspeciesof the sametwo generais morestrongly
suggestiveof genericrelationship.I have elsewhere(Short and Phillips,
1966) notedas inconsistenwt ith modernphylogeneticand evolutionary
conceptsthe suggestion(Banksand Johnson1, 961: 26) that intergeneric
hybridizationmightoccurmorefrequentlythanintragenerihcybridization.
Aside from servinga taxonomicfunction, thesehybrids may help to
elucidatethe patternof evolutionof a group,or they may aid in establish-
ingthepatternsof evolutionof a trait or traitswithina group..

Jan., 1969] TaxonomicAspectso! Avian Hybridization 87

Unique intragenerichybridsvary in their taxonomicsignificance.Gen-
erally they are of little significancien groupsin whichhybridsare un-
common.Miller (1955) suggestedthe evolutionaryhistory of certain
North Americanwoodpeckerosf the genusDendrocopoms ainly on the
basisof a singlehybridD. scalarisx D. villosus.Recently,I havefound
(Short, MS) numbersof hybridsof D. scalarisx D. nuttallii, and two
hybridsof D. nuttalliix D. pubescensT.he newevidencefromthesespeci-

mens necessitates considerable modification of Miller's views. Hence a

loneintragenerichybridis of limiteduseas a basisfor inferringrelation-
ships. The amountof hybridizationmay suggesot r corroboraterelation-
shipsin generasuchas the duck genusAnas,that exhibit,for reasons
elucidatedby Sibley (1957), a greatdeal of hybridization(Johnsgard,
1960). Importanthereis the rare occurrencoer absenceof hybridsbe-
tween certain species,contrastedwith more frequently occurringwild
hybridsamongotherspeciesof a genus.One must,of coursee, valuate
theseoccurrenceosnth'ebasisof theopportunityof thevariousspecietso
hybridize,i.e. the degreeof sympatryof potentiallyhybridizingspecies.
More distantrelationshipis impliedif few,one,or no hybridsoccur,while
closerrelationshipis suggestebdy occurrencoef crosseisnvolvingseveral
to manyhybrids (Johnsgard,1960).

Hybridsmorefrequenthantheuniqueor rarehybrid,yet lessfrequent
than sporadic,i.e. up to 10 or so, may assumegreatertaxonomicsignifi-
cance.If the speciesrepresentdifferentgeneracloserelationshipis sug-
gested,possiblyincludingthe mergingof the genera. Recently reported
casesin thiscategoryare th'oseof North Americanhybrid sparrowsJunco

byemallsx Zonotrichiaalbicollis(Shortand Simon,1965) and humming-
birdsArchilochuaslexandrxi Calyptecostae(Shortand Phillips,1966).
When consideredwith otherkinds of evidencein the light of instancesof
intergenerichybridizationbetweencertainof theseand otherspeciesof the
generamentioned,their h'ybridizationsuggestsgenericmerger. Another
interestingcaseis the hybridizationbetweenthe ducksMerguscucullatus
and Bucephalaclangula (Cockrum, 1952; in the U.S. National Museum
is an additionalsubadulthybridmale). Althoughthesespeciesrepresent
different generasometimesplacedin different "subfamilies"(A.O.U.,
1957), behavioralstudy (Johnsgard,1961a) has indicatedtheir close
relationshipT. he existencoef severahl ybridsand a morphologiccaol m-
parisonof the two speciesa,nd especiallycomparisoonf the femalesl,ead
me to concurwith Johnsgard.The divergencein morphologicaslpecializa-
tionsamongthe ducks (subfamilyAnatinae) seemsto.belie their funda-
mentallycloserelationshipandtherecentnesosf theiroriginand adaptive
radiation (Delacourand Mayr, 1945).

88 LtsrER L. SaORT [Auk, Vol. 86

Existenceof severalintragenerichybrids betweentwo speciesmay be
evidencefor their relativelycloserelationship.In groupswherehybridiza-
tion is rare, severalhybridsbetweensympatricspeciesmay suggesthat
they formerlycompriseda superspecieas,superspeciebseinga "groupof
entirely or essentiallyallopatrictaxa that were once racesof a single
speciesbut which now have achieved speciesstatus" (Amadon, 1966).
This may be obvious,as in the caseof the meadowlarksSturnella magna
and S. neglecta(Lanyon, 1966), whichhave recentlycomebroadlyinto
sympatry. A few hybrids have occurredat scatteredpoints within the
area of sympatry. Sympatricintragenerichybridstend to occurwhereone
speciesis at th'eborderof its range,encouragedby restrictedmate choice
wherethat speciesis uncommon.Suchspecialcircumstanceussuallyare
involvedbecausesympatricspeciesnormallyare effectivelyreproductively
isolated,whichis partly responsiblefor their sympatry. The woodpeckers
Dendrocoposnuttallii and D. pubescensare broadly sympatricin Cali-
fornia,whichcomprisesalmostall of nuttallii'srange. Two of their three
knownhybrids (Short,MS) are from the southernextremeof the range
of D. pubescenisn California; the locality of the third hybrid (Ridgway,
1887) is unknown. Southof the range of pubescensn,uttallii replaces
pubescensin riparian vegetationlike that occupiedby the latter where
they are sympatric. These two woodpeckersprobably formerly comprised
a superspeciesT.he formerexistenceof a superspeciemsay be lessobvious,
as perhapsin the broadly sympatricand sporadicallyhybridizing tanagers
Thraupis palmarum and T. virens (Haverschmidt, 1966). Likewise in a
groupproneto hybridizelike the grouse(Tetraoninae; seeSibley, 1957),
evenmorenumeroushybridsin sympatrymay suggesthe formerexistence
of superspeciesA. n exampleis the caseof the Eurasiancapercaillie(Tetrao
urogallus)and black grouse(T. tetrix), which interact ecologicallyand
hybridizeoccasionallyin their extensivearea of sympatry,althougheach

todaycomprisaesuperspecoiefistsownwithrelatedAsianspecie(sShort,

1967). Existenceof a superspeciemsay be directly inferred from the oc-
currenceof two to severalhybrids in a contact betweenallopatric con-
genericspecies.An exampleis the caseof two hybrids of the tanagers
Piranga ludovicianaand P. olivacea,respectivelyo, f westernand eastern
North American(Tordoff, 1950; Mengel, 1963).

When discussingth'enumbersof natural hybrids, one must recognize
the vastly greateropportunityto recoverhybridsof waterfowland other
gamebirdsc,omparedwith otherbirds. Fully half thepopulationof certain
gamespeciesmay be killed yearly by hunters,and hencea considerable
portionof obvioushybrids (mainly males) are recovered.The discussion
hereinis mainlyconcernedwith nongamespecies.

Jan., 1969] Taxonomic Aspectsof Avian Hybridization 89

ZONES OF OVERLAP AND HYBRIDIZATION

The term "hybrid zone"has beenlooselyusedto designateany area of
secondarycontactin which h'ybridsoccur. I restrict a "hybrid zone" (see
below) to includean area of hybridizationwhereonly hybridsoccur,as
distinguishefdrom a "zoneof overlapand hybridization."I prefer to in-
clude both of thesecategorieswithin the categoryof secondaryinter-
gradation, thus retaining the latter general term for all situations in-
volving hybridizationand backcrossingbetweenallopatric forms in a
secondarycontact. A zone of overlap and hybridization is an area of
secondaryintergradationoccupiedby numeroushybridsand both parental
formsaswell. The parentalphenotypesmustoccurin numberssufficient
to precludetheir representingextremehybridphenotypes(recombinants).
I arbitrarilyconsidetrhe occurrencoef bothparentalphenotypeisn num-
bersgreaterthan 5 per cent,taken together,of the populationin the zone
of overlap and hybridizationas indicating that at least someparental
phenotypesnormallycomprisea part of this population.The useof 5
per cent has no functionother than to insurethe presenceof parental
phenotypes;if theseotherwisecan be demonstratedt,heir per cent oc-
currenceis unimportant. A zone of overlap and hybridization has two
taxonomically significant features. The existence therein of numerous
parental phenotypesshouldresult from impedimentsto gene flow, pre-
sumablythe action of partial isolatingmechanisms(Short, 1965: 418;
Bigelow,1965). Immigrationof parentalphenotypefsromoutsidethe zone
alsomaytakeplacewhenhybridsareat a selectivedisadvantageS. econdly,
phenotypeosf theparentalformsaresympatricin thiszone;theirreactions
can includeprimary (F •) hybridization,competition,and reinforcement
or breakdownof isolatingmechanisms(Table 1.)

Other than the indirectevidencefor partial isolatingmechanismpsro-
videdby the existenceof a zoneof overlapandhybridization,thereappears
to be no logical basis for decidingthe taxonomicstatus of the interacting
forms. The amountof hybridizationand the relativeproportionof hybrids
and parentalphenotypesin. the zonedo not provideuseful,nonarbitrary
criteria for a taxonomicdecision,for both may be influencedby factors
suchas the sizeof the contact; henceneither necessarilyis correlatedwith
the efficacyof partial isolatingmechanismsT. he presenceandmaintenance
of parentalphenotypeisn the zoneof overlapandhybridizationappearsto
constitutea nonarbitrary basis for a taxonomicdecision. The taxonomic
valueof this phenomenoins enhancedby the direct relationshipit seems
to havewith functioningreproductiveisolatingmechanisms.

I restrictthe term "semispecies("Amadon,1966) to thoseformsactually
or potentially capableof forming a zone of overlap and hybridization.

90 LESTER L. SHORT [Auk, Vol. 86

TABLE 1
A CLASSIFICATION OF So!viE HYBRIDIZING FOR!VrS

Forms involved Distribution Interactions

Subspecieso, r subspecies contiguouslyor strictly primary intergradation,or
groupsof polytypic species allopatric potentially capable of so
doing

subspecies,or subspecies strictly allopatric hybridize in hybrid zone
groups of polytypic species (or potentially capable of
so doing)

......... TAXONO!V[IC SPECIES BORDER .........

semispecies (allospecies) basically allopatric, but form zone o/ overlap and
some sympatry evident hybridization (or potenti-
or possible ally capable of so doing);
competition; reinforcement
of isolating mechanisms

allospeciesof a basically allopatric; some rare, inconsequential or no
superspecies
or no sympatry hybridization; effective

isolating mechanisms,rarely

breaking down; competition

if sympatric

related, but not Sympatric or allopatric rare or no hybridization;
allospecific species effective isolating mech-
anisms, very rarely break-

ing down; competition or
not when sympatric

These semispeciestruly combineattributes of populationsat both the
specieslevel,suchas morphologicadl istinctnessand reproductiveisolation,
even if only partial, and at the intraspecieslevel, such as the ability to
interbreed. On the basis of suggestiveevidencefor existenceof partial
isolating mechanismsin situations involving semispeciesI, recommend
that semispeciebse consideredtaxonomicallyas species(Table 1). Semi-
speciesare one type of allospeciescomprisinga superspecies(Amadon,
1966). Other superspecieasre comprisedof allospeciesthat infrequently
or never hybridize in areas of secondary contact or limited areas of
sympatry, or of allospeciesthat are completelyallopatric. All types of
allospeciesare geographicallyrepresentativeforms taxonomically con-
sideredspecies(Areadon,1966).

Zonesof overlapand hybridizationmay be stable,but often they may
changedynamically,either on a broad level or locally. The frequencyof
suchchangessuggestsa needfor continuinganalysisand reexaminationof
all zonesof overlap and hybridization. In his discussionof the invasion
by the woodpeckerDendrocopossyriacusof the European range of its
closerelativeD. •najor from southeasterEnuropeearlierin this century,
Bauer (1958) showsthat hybridization occurredin a narrow belt along

Jan., 1969] TaxonomicAspectsof Avian Hybridization 91

the advancingzoneof overlapand hybridizationas D. syriacusexpanded
itsrangeto thenorthwest.As the frontpasseda givenarea,hybridization
diminishedt,henceaseda,nd th'etwo speciescameto existin sympatry.
Hybridizationin thiscaseseemsdueto restrictedmatechoiceamongim-
migrantsof D. syriacus;interbreedindgiminisheassits populationreaches
sufficientsize to make availableenoug•hconspecifimc ates. A similar
caseinvolvesthe titmiceParuscaeruleusand P. cyanus(Vaurie, 1957).
Late in the pastcenturythe AsianP. cyanusmassivelyinvadedthe east
EuropeanandwestAsianrangeof thelargelyEuropeanP. caeruleusS. ince
the 1880stherangeof P. cyanushasrecededsomewhabt,ut a vastareaof
overlapstill exists.Hybridization,apparentlywidespreaduringtheperiod
of expansionh,asdiminishedandnowtakesplaceonlyin th'enorthernpart
of thezoneof overlap(Vaurie, 1957). In thesetwocasesthe zoneof over-
lap and hybridizationhas becomegreatly restricted,while the former
regionwherethe zoneoccurredis nowan areaof sympatry,or simplya
zone of overlap. The gulls Larus argentatusand L. hyperboreusare
sympatric without interbreedingover much of the northern Holarctic
region.Icelandwasformerlyoccupiedby L. hyp.erboreubsu, t not by L.
argentatus.The recentarrival of tke latter as a breedingbird in Iceland
has resultedin widespreadhybridization(A. Ingolfssonp, ers. comm.),
despitetheir sympatryelsewhereA. ll theseexamplesare specialcasesof
zonesof overlapand hybridization--they might alsobe consideredcases
of combinationosf situationsa, s shownbelow. As the formsinvolvedare
basicallyreproductivelyisolated,they are species.I wouldnot designate
themassemispecietsh;eyareformersemispecieasb,leto coexisot verlarge
areaswithout interbreeding.

Large, broad zonesof overlapand hybridizationare not infrequent.
(The adjectives"large,""small,"and "moderate"hereinpertain to the
overallextentor area of the zone,while "broad" and "narrow" describe
thewidthof thezoneat a specifiedpoint,or theaveragewidthof thezone
if nopointis specified.)The warblersVermivorapinusand V. chrysoptera
extensivelyoverlap and hybridize in eastern North America. Parental
phenotypeaspparentlyaremaintainedthroughoutthe zoneof overlapand
hybridizationa,ndintro.gressiosntakingplace(Short,1963). In thesame
regiontheducksAnasplatyrhynchoasndA. rubripesbroadlyoverlapand
hybridizew, ithparentapl henotypersemainingmorecommonthanhybrids
(Johnsgard1, 961b, 1967). Likewise,the kingfishersCeyx erithacusand

C. rufidorsusinterbreedandoverlapovera largeareaincludingmostof

Sumatra,Borneo,and the Malay Peninsula(Sims,1959). In mostcases

of thistypetheparentalphenotypeasndrecombinantcsloselyresembling
themcomprisea majorityof the populationin muchof the areaof over-

92 LESTERL. SUORT [Auk, Vol. 86

lap andhybridization.Theseformsaresemispeciethsat I believeshouldbe
accordedthe taxonomicstatusof species.

Moderatezonesof overlapand hybridizationexist in many areas. In
central North America broad zonesof overlap and hybridization exist
betweenthegrosbeakPsheucticusludovicianuasndP. melanocephaluasnd
the buntingsPasserinacs'aneandP. amoena.Althoughbroad,the zones
are limitedin extentbecausethesespeciesinhabitonly river valleyswithin
the vast central Great Plains area the zoneencompassesT.he maximum
width of the grosbeakzoneis about 200 miles (West, 1962; ShOrt,un-
publisheddata from recentlycollectedNebraskaspecimens).Within this
zoneboth parentaland hybrid phenotypesare encounteredS. lightintro-
gressionis detectableimmediatelyadjacent to the zone of overlap and
hybridization. The buntingsoverlapand hybridizealong river valleys
in the sameregion(Sibleyand Sh'ort,1959a), but the zoneis broader,
about400 mileswide. The easternsemispeciePs., cyanea,recentlyhas
bred sporadicallyfar within the range of P. amoena in California,
Arizona,and Utah (somecitationsare in Sibleyand Short, 1959a). Sig-
nificantly, most pioneerbirds were apparentlyphenotypicallypure P.
cyanea,and someindividualssecuredmates of their own speciesdespite
beingfar beyondthe speciesn' ormalrange.

Examplesof small and narrow zonesof overlap and hybridization are
providedby semispecieosf gulls (Larus glaucescenasnd L. argentatus)
and chickadees(Parus atricapillusand P. carolinensis).The gulls meet
in southernAlaska wherethey sometimesoccurin mixed colonies.In at
least somecoloniesconsiderablheybridizationoccurs,altho.ughparental
phenotypesapparently are maintaineddespiteinterbreeding(Williamson
and Peyton, 1963). The rangesof the chickadeesare sporadicallycon-
tiguousfrom the MississippiRiver to the Atlantic Coastof North'America.
Knowncontactsmainlyinvolveno overlap(Tanner, 1952; Brewer,1963),
but slightoverlapwith limited hybridizationoccursin southernIllinois.

HYBRID ZONES

A hybrid zoneis an area occupiedby a hybrid populationconnecting
twoparentalgenepools(Short, 1965andabove). The parentalphenotypes
togethercompriselessthan 5 per centof the hybrid zonepopulation,i.e.
they occursoinfrequentlythat they might be due to chancerecombination.
Occasionalimmigrantsor wanderersfrom the range of one or the other
parental form couldaccountfor someoccurrenceof parentalphenotypes,
which are virtually or actually lacking from mosthybrid zones. Parental
phenotypesdo not occur togetherin the hybrid zone; rather, each is
limited usuallyto that edgeof the hybrid zoneappropriatefor its adjacent

Jan., 1969] Taxonomic Aspects of Avian Hybridization 93

distribution. Each border of the zone is marked by an increasein the
parental phenotypeof the immediatelyadjacent form, and a decrease
in hybrid and introgressanitndividuals.

A significantfeatureof the hybrid zoneis that the hybrid population
simultaneouslyboth connectsand separatesthe parental populations;
the latter henceare completelyallopatric(Table 1). The opportunityfor
primary (F l') hybridizationis lackingbecauseindividualsof the parental
formsare no longerdirectly in contact,as they werewhen the secondary
contactinitially was established.Reproductiveisolatingmechanismsare
nonexistenotr ineffectual.Analysisof populationsadjacentto the hybrid
zone usuallywill uncoverevidenceof introgression.The geneticcontact
betweenthe hybrid populationand the parental form insuresoccurrence
of introgressionw, hetheror not it is detectable.Formsconnectedby a
hybrid zoneare consideredconspecificbecauseof lack or ineffectiveness
of reproductiveisolatingmechanisms.

Oneisoftenreluctantto acceptconspecificityof two formsinterbreeding
in a hybrid zonewhen one or both of the formsare polytypic. As free
interbreedingand lack of reproductiveisolationdemandconspecificityof
the formsinvolvedu, seof the term"subspeciegsroups"is helpful(Table
1). Subspeciecsan vary tremendouslyin their degreeof morphological
divergence.Also, they may be entirely allopatric, broadly connected
throughareasof primary intergradationinvolvinggradualor step-clines,
or connectedthroughhybrid zones.A groupof slightlyor moderately
differentiatedclinal races may comprisea subspeciesgroup having a
hybrid zonewith anothersuch'group,as in the flickers (Short, 1965)
discussedbelow. Of coursepolytypic specieswith numerousracesmay
havesubspeciegsroupse.venwithouthybridzones.

The often considerablemorphologicadl ifferencesbetweenformsinter-
breedingin a hybrid zoneis no reasonfor reluctanceto mergethem, for
the morphologicafel aturesby whichthey differ haveno significancwe ith
regardto their reproductiveisolation.Why shouldwe questionthe con-
specificityof two morphologicallydivergentforms,when individualsof
theseformsaccepteachotherasconspecific?When this acceptanceresults
in the formationof a hybrid zone,the hybrid populationbecomesa con-
nectinglink betweenthe parentalforms,whichare therebybufferedfrom
directcontact. The formationof an intermediaryhybridpopulationattests
to the succesosf the interbreedingt,he lack of reproductiveisolationof the
parentalforms,and the insignificancoef the morphologicadlifferences
betweentheparentalformswith regardto reproductiviesolation.Because
the evolutionof reproductiveisolatingmechanismiss related only in-
directly to morphologicadl ivergencet,he level of morphologicadl istinct-

94 LESXE• L. Sao•x [Auk, Vol. 86

nessof interbreedingsubspeciegsroupscanapproache, qual,or occasionally
exceedthat achievedby semispecies.

The best test for the existenceof reproductiveisolatingmechanisms
is a largecontactbetweentwo populations.When a largecontactensues,
thereis a directthreat of geneticswampingof the parentalgenepools
throughhybridizationand introgressionA. ny existingincipientisolating
mechanismsshouldbe enhancedunderthesecircumstancesS.ibley(1961)
discussesthe reinforcementof isolatingmechanismsthrough selection
actingagainsthybrids; reinforcemenits a part of Brown and Wilson's
(1956) "characterdisplacement."

Large and broad avian hybrid zonesare evidenton mostof the con-
tinents. The morphologicallydivergent auratus and caJer subspecies
groupsof the North Americanflicker (Colaptesauratus) form a vast mid-
continentaland northwesternhybrid zone (Short, 1965). Introgression
affects parental populationsas far from the zone as the east and west
coastsof North America. Likewise two forms of grackle (Quiscalus
quiscula)form a largehybrid zoneover muchof easternNorth America
(Chapman, 1940; Huntington, 1952). The flowerpeckersPardalotus
melanocephalusand P. "uropygialis" form such a zone in northern
Australia (Salomonsen1, 961); for severalother fine examplesof this
phenomenosneeKeast (1961). Th'eformsinvolvedin thesehybrid zones
areobviouslyconspecifica,snoisolatingmechanismesxistand theparental
genepoolsare firmly connected.This doesnot mean that these forms
necessarilywill merge completely. Very often, as in most continental
hybrid zones,the large parental populationsoccupyan immensearea,
allowingforisolationby distanceandgradualfiltering(seebelow)of intro-
gressedgenesaway from the hybrid zones. Just as variable clinesoften
are evident in casesof primary intergradation,the differential sorting
action of natural selectionwill fashion variable clines for introgressant
genesand genecombinationsI.n the flickersmentionedabove,some
featuresby whichthe hybridizingformsdiffer are sharplydinal within the
hybridzone(e.g.nuchalpatch; seeShort,1965), whileothers(e.g. shaft
color) are step-dinalwithin the hybridzone; theseclinesassumea gradual
slopeoutsidethe hybrid zone.

Narrownessof a hybrid zone is not related necessarilyto any basic
geneticincompatibilityof two hybridizingforms. Narrower zonesoccur
when differencesbetweenthe parental formsare few. Geneticgradients
for variouscharactersacrossand beyondthe hybrid zoneinevitably will
vary with the selectiveforcesregulatinggeneflow, as notedabove. This

differentialactionof naturalselectionoperatingwith respecto introgres-

sionout of a hybrid zone,which Bigelow(1965) ignoredin his contrary

Jan., 1969] Taxonomlc Aspectsof Avian Hybridization 95

view of the narrow hybrid zone between Corvus coroneand C. "cornix"
(below), has no bearingon reproductiveisolationof the two forms; hence
it has no bearing on their taxonomicstatus. When individuals of two
forms can interbreed, and their hybrid offspring and successivehybrid
populationsare geneticallyconnectedwith both forms, then these forms
patently are not reproductivelyisolatedspecies.

There exist a few large or moderatelylarge but narrow hybrid zones,
notably the famous case of the Eurasian crows Corvus corone and C.
"cornix" (Meise, 1928; Vaurie, 1954). These crows form a narrow
hybrid zone acrosscentral and north•vesternEurope; little introgression
is apparent beyond the zone. A third crow (C. "orientalis") occupies
northeasternAsia and forms a broader hybrid zone with C. "cornix" in
West Siberia (Mayr, 1942a; Vaurie, 1954). Previousworkersdiscussing
this problem generally overlookedthe great morphologicalsimilarity of
thesecrowsin virtually all traits exceptthe onecolorfeatureby which they
differ strikingly, and upon which we have had to dependfor evidenceof
introgression.The narrownessof this hybrid zone simply may reflect
the dearthof morphologicaclharactersavailablefor analysis. Even with
free interbreedingthrougha hybrid zone selectionmay fashiona sharp
step-cline(seeabove) for certain genesand genecombinationsp, erhaps
including those responsiblefor the gray versusblack backs of the two
crows. Advantageousgenesand genesnot stronglydisadvantageoums ay
pass unimpededthrough the hybrid zone. For example, morph•ological
analysisof a hybrid zone in lizards (Cnemidophorus)of southwestern
North Americaindicatedvery limited introgressionb, ut useof biochemical
techniquesproved that introgressionwas actually extensive(Dessauer
et al., 1962). As the crowpopulationsare connectedby a totally hybrid
population,and there is no evidenceof isolating mechanismsacting to
limit the hybridization,we are obligedto considerthem conspecific.

A broad h'ybrid zone of moderateextent is found in the Great Plains
of North America between the galbula and bullockii groups of orioles

(Icterusgalbula) (SibleyandShort,1964). The zoneis roughly200 miles
wide, but as it occursprincipallyalongriver valleysits extent is limited.
Introgressionisevidentimmediatelyadjacentto thehybrid zone. A moder-
ate, narrowhybrid zoneis that connectingthe bicolorand atricristatus
groupsof titmice (Parus bicolor) in east-centralTexas (Dixon, 1955).
A true hybrid zone 25 to 50 milesin width occursin five areas.of contact
betweenthem. Marked introgressionis not evident.

Hybridization in small areas of secondarycontact inevitably places
lessstresson the parental populationsthan occursin situationsi.nvolving

massivecontact. Small hybrid zoneshenceposemore difficult problems

96 LESTERL. SHORT [Auk, Vol. 86

for the taxonomistthan do large hybrid zones. A small hybrid zone in-
volving extensivehybridization but little introgressionmay result from
chancetopographicaol r habitat conditions,restrictedmate choice,or other
special local factors. Thus a small contact may not test effectively the
parental forms' capacity to interbreed. The existenceof a small hybrid
zone demonstratesthat reproductiveisolating mechanismsare not fully
effective, but the presenceor absenceof partial isolating mechanisms
often remainsa moot point.

Small hybrid zonesmay persistindefinitely. This simply may reflect
their small size and limited genetic effects. There is a possibility that
minor geneticcontact may benefit populations,whether or not partial
isolatingmechanismesxist (Andersonand Stebbins,1954; Stebbins,1959;
Short, 1965). Counterselectionfrom this sourcecouldbalancethe effects
of weak isolatingmechanismsor even negate them (Stebbinsand Daly,
1961), producinga stalemate.

The problemof establishingthe presenceor absenceof isolatingmech-
anismsin situationsinvolvingsmallhybrid zonesis a seriousone. Never-
theless,whenno evidenceof isolatingmechanismsexistsin suchsituations
we are obliged tentatively to considerthe interbreedingforms as con-
specific. Converselythe demonstrationof isolatingmechanismsfunction-
ing in small contactsituationssupportsconsiderationof the interbreeding
formsas species.

A small,narrowhybrid zoneexistsin Colombiabetweenthe tanagers
Ramphocelusfiammigerusand R. "icteronotus"(Sibley, 1958). The
hybridzoneis but 10 or 12 mileswide,anda total changefrom one"pure"
parental populationto the other takes place within about 50 miles. A
hybrid zone about 16 miles acrossconnectsthe wrens Campylorhynchus
rufinuchahumilisand C. r. nigricaudatusin Chiapas,Mexico (Selander,
1964, 1965; seealso Short, 1966). The ColombiantoucansPteroglossus
torquatustorquatusand P. t. sanguineusform a hybrid zone only about
12 milesacross(Haffer, 1967). Finally a very smallhybrid zoneconnects
the birds-of-paradiseAstrapia stephaniaeand A. "mayeri" (Mayr and
Gilliard, 1952). Introgressionwas detectedin a populationof the latter
about 15 miles from the hybrid zone. Regardlessof the size of these
hybrid zones, they are indicative of free interbreeding; the forms in-
volvedthereforeshouldbe consideredconspecific.

HYBRID SWARMS

Hybrid swarmsare hybrid populationsof small to large size that are
nearly or actually geneticallyisolatedfrom populationsof the parental
forms. Their lack of contactwith the parental stock hampersevaluating
these populationstaxonomically. It is usually impossibleto establish

Jan., 1969] Taxonomic Aspectso! Avian Hybridization 97

whether the swarm resulted from breakdown of isolating mechanisms
underthe exigenciesof local selectiveforces,or whethersuchmechanisms
simply were lacking. On one hand the occurrenceof severalhybrid
swarmsinvolvingthe sametwo formssuggeststhat their isolatingmech-
anismsare weak and probably ineffective,or nonexistent.On the other
hand,populationswhichare partly sympatricor occuradjacentlywithout
interbreedingmay form a lone hybrid swarm; this would suggestlocal
breakdownof extant isolatingmechanismsas the causeof the hybrid

swarm.

Evidencefor the occurrenceof introgressionshouldbe soughtin popu-
lationsof the parental formsnearestthe hybrid swarm. The existence
of introgressionmay be usedas a basisfor consideringthe formsinvolved
asconspecificw,hilelackof introgressiownouldbe inconclusiveE. vidence
for the stabilizationof a singlehybrid swarm also would warrant con-
specificstatusfor the parentalforms. The basisfor this conclusionis
that stabilizationproves the parental genotypesa.re sufficientlycom-
patibleand alike to permit selectionof adaptivelysatisfactorygenotypes
fromtheir recombinantsa,t leastin oneregion. Isolatedsingleinstancesof
hybrid swarmsnot exhibitingstabilizationremain a problem. A hybrid
swarmdoesprovethattheformsinvolvedcaninterbreeda,t leastin certain
circumstancesL. ackingother concreteevidence,especiallysympatryelse-
where,for the presenceor absenceor reproductiveisolatingmechanisms,
I suggesthat the lone instanceof a hybrid swarmprovidesa tentative,
albeitweak,basisfor mergingthe formsin question.This actionis more
justifiablethanassigningeachof themfull speciestatus,becausexistence
of thehybridswarmprovesthat theyhavenotachievedcompletereproduc-
tive isolation,or at any rate had not at the time the swarmbecame

established.

Variousinsular populationsof the highly polymorphicPachycephala
pectoralisof the SouthwestPacific (approximately70 races) comprise
hybrid swarms(Mayr, 1942a; Galbraith, 1956). These apparentlyre-
sultedfromcolonizationof certainislandsby individualsrepresentingtwo
races. Most of thesehybrid swarmshave retained the high variability
typicalof hybridpopulations.Solomonse(n1950) regardedthe Iceland
populationof redpollsas a stabilizedhybridswarmof Carduelisflammea
x C. hornemannai,lthoughthe twospeciesare sympatricelsewhereO. ther
examples(Colaptesauratus,Pipilo erythrophthalmuso)f hybrid swarms

are discussed below.

COMBinATiOnS OF S•TUA•O•S

Variouscombinationosf situationssometimesoccurbetweenpopulations,
includingdifferent types of hybrid reactions. These combinationsoccur

98 LESTERL. SaORT [Auk, Vol. 86

becauseof geneticdifferencesamongvariouspopulationsof two forms,
reflectingthe simplefact that evolutiondoesnot proceedat the samerate
in all populationsof a species.The often difficult taxonomicproblems
presentedby thesecombinationsought to be solvedon the basisof the
extent of reproductiveisolation. Thus, existenceof a large hybrid zone
wouldmorethan offsetthe presenceelsewhereof a smallarea of overlap.
Converselya largearea of sympatrywithout interbreedingwouldoverride
existenceelsewhereof a hybrid swarmor a small hybrid zone. Neverthe-
lessinstancesoccur,like thoseof circular overlapdiscussedbelow, that
involveboth a large area of overlapand a large h'ybridzone. Each case
mustbe decidedafter carefulanalysisof every situationand an evaluation
of the sum of the reactions. Combinationsof situations are sufficiently
complicatedto merit discussinsgeveralexamples.

Miller's (1941) intensivestudy of the North Americanjuncosled him
to recognizea number of species,notwithstandinghybridizationamong
them. The easternJunco"byemalls"formsa large hybrid zonewith the
westernJ. "oreganus."Miller attemptedto showthat the hybrid popula-
tion is a stablehybrid race ("cismontanus")affiliated witk hyemalis.The
parental populationsmainly involved in the interbreedingare virtually

identical in those mensural characters that Miller used to demonstrate the

supposedstability of the hybrid race. A hybrid populationshouldnot
showincreasedvariability in featuresnot differing in the parental forms;
hencethe supposedstabilityof "cismontanusi"s spurious.Miller's analysis
of "cismontanus"was basedon only two samplesfrom the vast area
ascribed to that form. He admitted (1941: 342-343) that "actual"
hybrids cannot in fact be distinguishedfrom individualsrepresenting
"cismontanus." Critical examination (Sh'ort, MS) of large numbers of
northwesternjuncos, many taken after Miller's study, indicates that
"cismontanusa"ctuallyrepresentsa vasthybrid zonepopulation,and that
considerableintrogressionoccursbeyondthis zone. SinceMiller's (1941)
work, Dickinson (1953: 128-131) and Phillips (1961: 372 377) have
studiedthe interbreedingof hyemalisand oreganusthrough"cismontanus";
both concludedthat the interbreedingof theseformsshowsthem to be con-
specific.The SouthernRockyMountainJunco"caniceps"formsisolated
hybridswarmswith theracethurberiof J. "oreganus"in easternCalifornia
and Nevada, and a moderatehybrid zonewith mearnsiof J. "oreganus"
in Wyoming, Idaho, and Utah. Finally, although Junco "aikeni" of the
BlackHills regionis geographicalliysolatedfrom otherjuncopopulations,
severalhybrids (aikeni X mearnsi) were obtainedfrom the populationof
aikeni geographicallynearestto another junco (J. "oreganus"mearnsi).
I concurwith the suggestionosf Mayr (1942b), Dickinson(1953), and

Jan., 1969] Taxonomic Aspectsof Avian Hybridization 99

Phillips (1961; and in Phillips et al., 1964) that the oreganus,caniceps,
aikeni, and hyemalisgroups( = subspeciegsroups) of juncosshouldbe
consideredconspecific(: Junco byemalls). The genusJunco actually
appearsto be a superspecieas,view Mayr (1942b: 379) suggestedt;he
componenst peciesare J. byemalls,J. phaeonotusand J. vulcani. Theseare
probablycongeneriwc ith Zonotrichia(Short and Simon,1965).

Three morphologicallydivergent forms of North American flickers
(Coloptesauratus), consideredby someas separatespeciesh, ybridizein
North America (Short, 1965). The easternauratus and westerncafer
subspeciegsroupsform a largehybrid zoneacrossNorth America,as men-
tioned above. The third form (chrysoidesgroup) inhabits the south-
westerndesertregion,and it formsboth variableand semi-stabilizedhybrid
swarmswith the western cafer group in Arizona and Baja California
(Short, 1965; Short and Banks, 1965). In addition, it forms a single
tenuousnarrow hybrid zone with flickers of the ca•er group along one
river valley in Arizona. The existenceof the auratusx ca•er hybrid zone,
the narrowca•er• chrysoidehsybridzone,andevidencefor somestabiliza-
tion in a hybrid swarmof ca•er• chrysoidesjustify conspecificstatusof

these forms.

The towhee (Pipilo erythrophthalmus)of most of North America and
the towhee(P. "ocai") of the Mexicanhighlandsinteractin theefollowing
threeways(Sibley,1950,1954; SibleyandWest, 1958; Sibleyand Sibley,
1964): 1) they form semiconnectedv,ariablehybrid swarmsthat almost
constitutea hybrid zonein westernMexico; 2) they form two smallbut
moderatelybroad hybrid zonesin south-centraland easternMexico; and
3) they are sympatricwith little or no interbreedingin severalparts of
Oaxaca and one area in eastern Mexico. Introõressionis evident about
the hybrid zonesand is affecting populationsin at least one of the three
areas of sympatry. When hybridization and introgressionaffect major
populationsof two forms,thesecannotbe consideredfully reproductively
isolated. It is not fitting to designatetwo formsas specieswhen the extent
of their interbreedingand introgressionoutweighsthe extent of their
sympatry. Hence the towheesare bestconsideredas conspecific.

The bulbulsPycnonotuscaferand P. leucogenysbroadlyoverlapin India
and Pakistan. Scatteredhybrids occurin the overlap area near the western
endof caJer'srange,whiletwolocalhybridswarmsalsooccurin that region
(Bannu and Kohat in northwesternPakistan; Sibley and Short, 1959b).
These bulbuls are evidently speciesexhibiting occasionalbreakdown of
reproductiveisolation,probably only where one speciesis uncommonand

the choice of mates is therefore limited.

Chapin (1948) reporteddifferentreactionsbetweenthe Africanparadise

100 LESTERL. S•ORT [Auk, Vol. 86

flycatchersTerpsiphoneru/iventerand T. viridis. The subspecieTs. r.
nigricepsand T. v. viridis occursympatricallywithout interbreedingin
someareas,but in Gambia and PortugueseGuinea they formeda hybrid
zonethat apparentlyhasbecomea relativelystabilizedhybrid population,
designatedas T. r. ru/iventer. Other racesof thesespecies(T. r. somereni
and T. v. ferreti) form a zoneof overlapand hybridizationin Uganda,
wherethe hybridsappearlessnumerousthan individualsof the parental
types. It seemsbestto considertheseflycatchersas separatespecies..

The sparrowsPasser domesticusand P. hispaniolensisare broadly
sympatricin Spain, northwesternAfrica, the Balkans,Asia Minor, and
south-centraAl sia. Yet theyhaveformeda largestabilizedhybridpopula-
tion in Italy, P. d. italiae, considereda raceof P. domesticubsecauseof its
hybrid zone with that form (Meise, 1936; Vaurie, 1956, 1959), small
stabilizedand nonstabilizedhybrid swarmsin centralAlgeriaand southern
Tunisia, and a zone of overlap and hybridization in northern Algeria
(Meise, 1936). Althoughgeneticexchangebetweenthese forms indicates
that they comprisea singleevolutionaryunit, the great area over which
they coexistwithout interbreedingmeritsassigningspecificstatusto each.

Three taxa of sapsuckerso,nceregardedasseparatespeciesw, eremerged
in Sphyrapicusvarius followingstudiesby Howell (1952). The three
formsare: the northeasternS. "varius,"the RockyMountainS. "nuchalis,"
and the westernand northwesternS. "ruber." Two racesof ruber (ruber,
daggetti) hybridize with nuchalis. In one case (daggettix nuchalis) a
small,narrowzoneof overlapandhybridizationhasresulted,with parental
phenotypesas numerousas the hybrids. The othercase(ruber x nuchalis)
involvesa moderate,narrow zone of overlap and hybridization with few
hybrids. Sphyrapicus"nuchalis"and S. "varius" makecontactin Alberta,
but little is knownof their reactionsexceptthat hybridsare few and inter-
breeding apparently is limited. Sphyrapicus "ruber" and S. "varius"
overlapin British Columbia,wherehybridsrarely are produceddespite
the considerablextent of the area of overlap (Howell, 1952; Dickinson,
1953). Thus in every situationinvolvingthesesapsuckersgeneexchange
is limited; barriers seemto prevent free interbreeding.As no basis for
mergingthem exists,I considerthe sapsuckersS. varius,S. nuchalisand
S. ruberas speciesT. hesesemispeciecsomprisethe superspecieSs. varius.

CASES OF CIRCULAR OVERLAP

Thesesituationsare very complexand all known casesrequire further
study before even tentative taxonomic decisionscan be rendered. It is

crucial that all reactions between two forms be documented before under-

takinga full taxonomicevaluation.For exampleVaurieand Snow(1957)

Jan., 1969] TaxonomicAspectsof Avian Hybridization 101

mentionevidencoef interbreedinbgetweenthe terminapl opulationosf the
major and minorgroupsof Parusmajor; the terminalformsinvolvedin
thisfamouscaseof circulaor verlapthusdonotoverlapwithoutinterbreed-
ing,aspreviouslhyadbeensuppose(dMayr, 1942a: 182).

Cain(1955) andKeast(1961) discusseadcaseof circularoverlapin
Platycercupsarrotsof Australia.A terminalformPlatycercueslegans
eleganisntergradewsithP. e. adelaidaew,hichin turnhybridizewsiththe
otherterminalformflaveolusO. nehybridexistsbetweenflaveo.luasnd
P. e. eleganso; therwisetheseformscontactor overlapslightlywithout
interbreedingT.he cruxof the problemis a needfor analysisof the
adelaidaxe [laveoluisnterbreedingIf. a zoneof overlapandhybridization
existsJ, laveolushouldbe consideread separatespeciesT. his wouldnot
involvecirculaor verlap.Shouldfreeinterbreedinogccurthrougha hybrid

zone,circularoverlapwouldexist. I would then considerthe three forms

conspecifoicn the basisof the geneticexchangaemongthe populations
andrelativelysmalloverlapof Jlaveoluasndelegans.

The circularoverlapinvolvinggullsof the Larusargentatugsroupis
exceedingclyomplexd,espitetherecentworkof Smith(1966) andothers
(citedby Mayr, 1963: 508-510). Thereis notevenagreemenotnwhich
formscomprisethe complex.Mayr (1963) includesL. glaucoideisn the
argentatuscomplexb, ut excludesL. glaucescenasnd L. schistisagusR.e-
centstudies(Williamsonand Peyton,1963; Vaurie, 1965: 735) have
shownthatthelattertwoformshybridizewithandarecloselyrelatedto
L. argentatusS. tresemananndTimof•eff-Ressovs(k1y947) demonstrated
that gapsexistbetweenEurasianpopulationosf this complexthat pre-
viouslywerethought(Mayr, 1942a)to be continuousA.ny taxonomic
decisionin thiscasewill be prematureuntil thereis furtherdocumentation

of all existingreactions.

Othercasesof circularoverlapMayr (1963) citesalsoare fraughtwith
problemsT.heseespecialilnyvolvethecontinuitoyf linksin thecirclea, nd
theexactreactionof theterminapl opulationsW. hentruecirculaor verlap
is demonstrablIe,wouldconsidetrhe terminalformsas conspecififc
their geneticconnectioins continuoutshrougha' reasof primaryinter-
gradationandhybridzones.Major gapsin the continuityof the chain,
includingtheexistencoef zonesof overlapandhybridizationw,ouldneces-
sitatedesignationof severalspeciest;he situationwouldnot be oneof true
circularoverlap.

ACI•I•OWLEDGMEI•ITS

DeanAmadonW, alterBock,EugeneEisenmanna,nd ErnstMayr kindlyreadthis
manuscripta, nd I am grateful for their suggestionbsenefitingit. Of courseI take
full responsibilityfor the views expressedherein. Studiesof the author that con-
tributedto thisreporthavereceivedsupportfrom the ChapmanMemorialFund of

102 LESTERL. SHORT [Auk, Vol. 86

the American Museum of Natural History, the National ScienceFoundation (Grant
GB-5891), Adelphi University, and the U.S. Fish and Wildlife Service.

SUMMARY

Instancesof naturalhybridizationin birdsprovidethe frameworkfor a
discussionof guidelinesfor taxonomica]lyappraisingvarious phonomena
of hybridization. Hybridization and semispeciesare defined, and the
superspeciecsonceptis relatedto hybrid situations.

The occurrenceof uniquehybrids is generallyof little taxonomiccon-
sequence,but these may corroborateor suggestrelationships. Several
hybrids betweenspeciesof different generasuggestgenericrelationship,
while severalintragenerichybrids may suggestthe existenceof super-
speciesor formersuperspecies.

Zonesof overlap and hybridization occurbetweensemispeciesthat are
taxonomicallyspecies.The occurrenceof parentalphenotypeswith hybrids
indicates the operation of partial isolating mechanismsrestricting gene
exchange.In suchcasesthe parentalformsare actually sympatric.On
the otherhand,hybrid zonescontainonly hybrids,and they connectaswell
asseparateconspecifipcopulations(subspecieosr subspeciegsroups). The
fact that the hybrid populationboth'connectsand separatesthe allopatric
parental forms attests to their lack of reproductiveisolation. Unique
hybrid swarmsposeseriousproblemsthat are discussed.

Combinationosf relationshipsin, cludinghybridizationa, rediscusseadnd
examplesareprovided.In suchcasesthe extentof hybridizationandintro-
gressionis weighedagainst the degreeof reproductiveisolation and the
amountof sympatryexhibitedby the populationsinvolved. Instancesof
circular overlapare complicatedand they seemto require further study
beforethey can be taxonomicallyresolved.

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