Geographicvariation, hybridization,and taxonomyof New WorldButorides FloydE. Hayes devallias specifically distinct. Departmentof LifeSciences In a criticalreanalysis of specimensfrom the Panamaniancontact zone Universityofthe betweenB. virescens and B. striatus, Monroe and Browning (1992) concluded St.,A, ugustine thatinterbreeding between the two forms was limited and recommended that Trinidadand Tobago theybe recognizedas distinct . This conclusion was accepted by the (email:[email protected]) AmericanOrnithologists' Union (1993,1998), which resplit into threeseparate species. ABSTRACT Giventhe existence of individualsappearing intermediate between these •o eNew World B•torides herons comprise three distinct forms whose tax- formsin theareas where their ranges meet and the debated taxunomic signif- nomyhas been debated: the rufous-necked Green (B. virescens) of icanceof suchindividuals, the followingreview is offeredto synthesizecur- ,, and the WestIndies; the gray-necked rentknowledge and to stimulatefurther study of the geographicvariation, StriatedHeron (B. striatus)of SouthAmerica and the ; and the hybridization,, and identification of NewWorld Butorides herons. darkLava Heron (B. sunderalii) of theGal•pagos Islands. An extensiveanaly- sisby Payne(1974) concluded that they interbreed freely where their ranges MOLTSPLUMAGES, AND SOFT PARTS meetand should be considered conspecific. However, Monroe and Browning A soundanalysis of geographicvariation and taxonomy in birdsis premised (1992)alleged that Payhe's voucher specimens used as a hybridindex for B. uponcomparisons of individuals of likeage, which in turn requiresa thor- virescensand B. striatus included juveniles and did not represent a continuous oughunderstanding of molts and plumages. Although Butorides herons are series;they concluded that B. virescensand B. striatusrarely interbreed and widespreadand relatively common, the sequence of theirmolts and plumages werespecifically distinct. My reexaminationof Payhe'svoucher specimens remainssurprisingly poorly known. The followingdescriptions are based revealedthat all had attained adult neck coloration and represent a continu- uponB. virescens,thesequence of moltsand plumages of whichhas been ten- ousseries. My reanalysisof Payhe'sdata demonstrates increased variability tativelydescribed (Oberholser 1912, 1974, Bent 1926, Palmer 1962, Davis and andintermediacy in the contactzone between B. virescensand B. striatus, Kushlan1994) and is presumably similar in poorlyknown B. strmtus (Cramp implyingextensive hybridization there. However, the presence of apparently andSimmons 1977, Marchant and Higgins 1990) and B. sunderalii. pureB. virescensand B. striatusphenotypes within the contact zone suggests Juvenaland immatureplumages--The distinctive juvenal plumage of thatassortative mating does occur, supporting the treatment of thetwo forms youngbirds of thisgenus is acquiredbefore flight is attained.Juveniles are asdistinct species. Similarly, the persistenceof pureB. sunderaliiin a poten- immediatelydiaguosable by heavystreaking on thebrownish neck (usually tial hybridzone with B. striatusin the Gal•pagosIslands supports its treat- absentin thecenter of thewhitish throat) and upper breast, a brownishcast mentas a distinctspecies. Further information is providedon moltsand to theupperparts, and whitish underparts. Juveniles also possess distinctive plumagesof Butoridesherons. wingfeathers in whichthe coverts are more rounded, with broad, buffy mar- ginsand a buffytriangular spot at thetip of eachcovert that rapidly fades to INTRODUCTION whiteand wears away, and the remiges (primaries and secondaries) arebroad- Studiesof geographicvariation in birdsare crucial for testinghypotheses of ly tippedwith white.The tibial (thigh)feathers are often,but not always, phylogeneticrelationships and speciationprocesses among closely related markedwith darkerhorizontal bars. In juveniles,the bill is palerthan in species(Zink andRemsen 1986). Where zones of phenotypicintermediacy adults. occurbetween sister taxa, it is importantto distinguishbetween primary Thejuvenile appearance isgenerally retained during the first fall and early intergradation(elihal variation) and secondaryintergradation (hybridiza- winteruntil all bodyfeathers are gradually replaced during the first prebasic tion). Hybridization,a geneticphenomenon that is widespreadin molt,which begins a fewweeks after attaining flight and lasts for several (Grantand Grant 1992), is generallydefined as interbreeding between popu- monthsuntil the first-basicplumage is obtained.During this period,an lationsin secondarycontact (Sibley and Short 1964) and can be phenetically "immature"appearance is gradually acquired as the cap becomes more solid inferredby an increasein variabilityand intermediacy in concert (Schueler black, the chin and throat becomea clearerwhite, and the sidesof the neck andRising 1976). losetheir streaking, becoming more rufescent. However, the wing feathers do TheNew World herons of thegenus Butorides include three distinct forms: notmolt; thus, the distinctive juvenile appearance of triangular white spots at the rufous-neckedGreen Heron (B. virescens)of North America, Central thetip of eachcovert and white-tipped remiges is retained. America,and the West Indies; the gray-necked (B. striat,s) of Subadultplumage--The first prealternate molt, beginning in latewinter SouthAmerica and the Old World; and the dark LavaHeron (B. sunderalii) andending in spring,is poorly known but apparently partial, probably limit- of theGalfipagos Islands. ed to newhead and neck feathers, scapulars and some wing coverts, and Thethree forms were generally considered distinct species (e.g., Bock 1956, plumeson the back. The first alternate plumage, acquired by spring, is essen- Peters1931, Hellmayr and Conover 1948, Palmer 1962, Wetmore 1965, Slud tiallya "subadult"plumage resembling that of theadult except that the back 1967),although sometimes considered conspecific (Harterr 1920) or possibly plumesare shorter, the wingsretain some worn juvenal coverts, all juvenal conspecific(Eisenmann 1951, Parkes 1955), until Payne (1974) examined 837 flightfeathers (each tipped with white), the chin, throat, and underparts are NewWorld Butorides specimens and found evidence of extensivehybridiza- whiter,and the duskystripes of the lowerforeneck and upperbreast are tionbetween Green and Striated Herons where their ranges meet in southern broader.It is importantto notethat the sidesof the neckhave essentially Central America,the southernCaribbean islands, and coastalnorthern South acquiredadult coloration by this time but may be slightly brighter. America,and between Striated and LavaHerons in the GalfipagosIslands. Adultplumage--Definitive basic adult plumage is obtainedduring the Payne(1974, 1979) and Payne and Risley (1976) concluded that the three secondprebasic molt, which begins in latespring or earlysummer, when the formsrepresented a single, polymorphic species comprising six recognizable birdis aboutone year old, and is completed in thefall. The second prebasic subspecies.This conclusionwas partially acceptedby the American molt,which begins earlier than subsequent prebasic molts, is usuallycom- Ornithologists'Union ( 1976,1983), which lumped B. virescens and B. striatus plete,although molting of somejuvenal secondaries and primaries may be intoa singlespecies, the Green-backed Heron B. striatus, but regarded B. sun- furtherdelayed. Adults subsequently undergo a partialbut moreextensive

4 NORTH AMERICAN BIRDS Figure2. Geographicvaria- tion in neck colorationof New World Butoridesherens (excludingthe Gal•pagos Islands),based on a reanalysisof datain Payne (1974).Vertical hne = mean ( score;box = +1 S. D.; hori- zontalline = range.Sample sizesare given for each geographicalregion. An out- • Co!omb,a(37) lier of "3" for the Lesser • VenezuelaI Trinidad (19) Antilles is not illustrated. • Gumhas(29 Note the anomaloussitua- E•adorIPe• tion with baharnensisGreen Herens,whose scores place them closer to birds of the Figure1. Voucherspecimens from the National Museum of NaturalHistory southern Caribbeanrather usedby Payne (1974) to scoreneck coloration of Butoridesona hybrid index than the southeastern NECKCOLOR SCORE scale(from left to right)of 1-9 (seeTable 1 forfurther details). Photograph by United States or northern FloydE. Hayes. islands.

82 c 80 '• 78 o Figure4. A typicalnominate juvenile (B. v. virescens) from eastern NorthAmerica, photographed inJuly on Big John's Pond, at JamaicaBay N.W.R., Figure3. "FigureIr' of Payne(1974) illustrating variation in neckcolor, repre- Queens,New York, New York. This shows streaking on the brownishneck and sentedby numbers (see Fig. 1), of Butoridesspecimensin . Circled upperbreast, a brownishcast to the upperparts,with whitish underpads. Wing numbersrepresent winter birds (both residents and northern migrants) col- covedsare more rounded than those of theadults and bear broad, bufPj margins lectedduring October to April;uncircled numbers represent probable breed- anda bufPjtriangular spot at thetip of eachcovert. In juveniles of allButorides ingresidents collected from May to September.Reprinted by perrnissior/ of herens,the bill is palerthan in adults. Photograph byA. Morris/Birds asArt. RobertB. Payne and the British Ornithologists' Club.

Figure5. Thisjuvenile/immature nominate Striated Heron (B. s. striatus)from Figure6. Fourimmature specimens of StriatedHeron (on the left; all from Argentinashows markings similar to thoseof similar-agedGreen Heron, but Brazil)and five immature specimens of GreenHeron (on the right; from it is a palerbird, with more washed-out gray-brown tones to the neck CentralAmerica and North America on the right).Note the grayertones of streaking,lacking the chestnut or rufescenttones of theGreen Heron in neck colorationin Striated Heron and more rufescenttones in Green Heron Rgure4. Photographby T.J. Ulrich/VIREO. Photographby FloydE. Hayes.

VOLUME 5 6 (2OO2), NUMBER ! 5 GalfipagosIslands (Harris 1973) and have strayed to CocosIsland (Slud 1967), Bonaire(Voous 1986), Tobago (Payne 1974), and as far north as Costa Rica in CentralAmerica (Stiles and Skutch1989) and St.Vincent in the LesserAntilles (Bond1964, Payne 1974).

Contactzone between Green and Striated Herons--In his comprehensive analys•sof geographicvariation within B. virescensand B. striatus,Payne (1974)compared neck coloration of all specimensthat had "completed most or all of theirpostjuvcnilc moult" with a voucherseries of ninespecimens in theNational Museum of NaturalHistory (USNM; Washington, D.C.) repre- sentinga"smoothly graded series of ninecolors from grey through brown and maroon"(Fig. 1, Table 1 ). In a criticalreanalysis ofPayne's voucher specimens, Monroeand Browning (1992) incorrectly stated that"two of thevoucher spec- imens(5 and6) ... arewhitish below as in thetvpical juvenal plumage:' In fact, specimen5 is an adultlacking evidence of juvenalplumage. However, speci- mens0, 7,and 8 eachretain some juvenal wing feathers (white spots at thetips Figure7. A typicaladult Green Heron photographed at SharkValley, of thegreater primary coverts and, in spccimen8, broadwhite tips to some EvergladesNational Park, Florida. Photograph by A. Morris/Birdsas Art. remiges).Furthermore, specimen 6 possessestraces of barringon the tibial feathers(a conditionnot foundin "allwhite-bellied juvenal virescens," contra prealternatemolt from latewinter to earlyspring and a completeprebasic Monroeand Browning1992) and a palerbelly, both features representing moltfrom late summer to fall.In definitivealternate plumage, the back plumes tracesof juvenalplumage. Although these three subadult voucher specimens of bothsexes are longer than during definitive basic plumage. havenot yetacquired definitive adult plumage, each (including specimen 6) Adultsare readilydistinguished from juveniles and iramaturesby their clearlycompleted most of itspost-juvenile molt and possesses adult neck col- darkerbill, solidrufous sides to the neck,greenish-gray upperparts lac 'king oration,although neck coloration of specimen6 may be slightly brighter than browntones, longer back plumes, more pointed scapulars and wing coverts, whenadult (Monroe and Browning 1992). narrowerbuffy margins to thewing coverts (which lack white spots), narrow- Payne(1974) considered the specificidentity of birdsscored 4-6 asarbi- er whitetips of flightfeathers restricted to theinner primaries and secondar- trary,but Monroe and Browning (1992) alleged that "there is a definitebreak ies,and darker brown-gray underparts. [inneck coloration] between voucher specimens 5 and 6." However, this break The sexesare indistinguishableby plumage,but femalesaverage smaller is not apparentin Fig.1, in whichfacial coloration of specimen5 resembles thanmales. When breeding, the bill andIoral area become glossier black, and specimen6 more than specimen 4, whereasneck coloration resembles speci- theiris and legs become more orange, especially in themale. men4 morethan specimen 6. Payne(1974) summarized neck coloration scores for 19geographic regions GEOGRAPHICVARIATION (excludingthe Galfipagos Islands) in his"Figure I;' "Figureii;' and"Table 2.' Butoridesvirescens--The Green Heron is representedby fourcurrently rec- Monroeand Browning(1992) werepuzzled by the discrepancybetween ognizedsubspecies (Payne 1974, 1979, Hancock and Kushlan 1984, Davis and Payne's(1974) "Figure I;' in whichthe 72 Panamanianspecimens included 14 Kushlan1994), though as manyas 18 wereonce recognized (Oberhoiser scoredas 5 andnone scored as 4, and"Figure II," whichincluded 11 scored as 1912).Nominate B. v.virescens, a rufous-necked form, occurs from centraland 4, fivescored as 5, andone scored as 4-5. However,Payne stated in thelegend eastern North America southward to Panama and in the southern Caribbean of"FigureI" that"'5' in Fig.1 = '4' or '57 buthis rationale for lureping scores is[andseast to Tobago.Body size averages larger in northernpopulations than 4 and5 as"5" remainspuzzling. A reanalysisof Payne'sdata on geographical in southernpopulations. Voous (1986) and Steadman et al.(1997) argued that variationis presented in Fig.2. In myreanalysis, I have included only specimens a generalCaribbean race should be recognizedas B. v.maculatus, which aver- collectedfrom May to September,most of whichwere probably breeding resi- agessmaller and slightlypaler-necked than easternNorth AmericanB. v. dents, from , Central America, Panama, Bahamas, Greater Antilles, virescens,but considerableoverlap occurs with CentralAmerican populations of B. v.virescens (Payne 1974). A largerand paler-necked form, B. v.anthonyi, occursin westernNorth America.A distinctivepurplish-necked form, B. v. frazari,inhabits southern Baja California (south of about27 ø 20' N latitude). The smallestand palest form, B. v.bahamensis, occurs in theBahama Islands. The NorthAmerican populations of B. v.virescens and B. v.anthonyi migrate southwardas far asEcuador (Ridgely and Greenfield 2001 }, Colombia(Hilty and Brown1986), Venezuela (Meyer de Schauenseeand Phelps1978), Trinidad(ffrench 1973), Guyana (Snyder 1966), and Suriname (Hayerschmidt 1968),where they occur primarily during winter and only rarely during sum- mer(Hilty and Brown 1986). The other forms are relatively sedentary. Butoridesstriatus• is inhabitedby the nominaterace of the StriatedHeron B. s. striatus(other racesoccur in the Old World), which is typicallygray-necked, although a smallproportion of browner-neckedindi- vidualsoccurs throughout its range. The South American populations tend to bevariable, with no distinct geographical trends in plumageor morphometric Figure8. •n adultGreen Heron of the raceanthonyifrom western North America,photographed at LakeCunningham Park, San Jose, California, in variation(Pavne 1974). Although B. s. striatus is not known to undergolong- January2001. Photographby PeterLaTourrette. distancemigratory movements, individuals are apparentlyresident in the

6 NORTH AMERICAN BIRDS New World Butorides Herons

LesserAntilles, southern Caribbean, Venezuela, and Trinidadsamples. Only I winteringB. virescensare deleted from the Colombia sample. Specimens col- lectedduring October to Aprilin theseregions were excluded because they includedNearctic migrants in additionto residents.Southern Baja California specimensindicated by an"X" in Payne's(1974) "Figure [" weredescribed as having"acolour darker purple (less brown, more greyish) than colour 9"; for the purposesof analysis,I have scored these specimens as 10.I havelimited southernCaribbean specimens toAruba, Los Roques, and Margarita, and have treatedTobago specimens separately. Although all but onespecimen from Tobagowere collected during October to April,only one appears similar in sizeand neck color to nominateB. virescens,but it wascollected during the NorthAmerican breeding season; because these specimens appear to represent residents,all areherein included. Specimens from Espada, Venezuela, and Guajira,Colombia, were included within the southernCaribbean sample by Payne(1974), but thesecoastal localities are situated on thecontinent and are hereinincluded within the respective country's sample. For South American Figure9. A typicaladult Stdated Heron photographed at Manaus, Brazil. countriesexcluding Colombia and Venezuela (with whichthe few Trinidad Photographby Andrew Whittaker/VIREO. specimenswere lumped), I scoredthe few specimens lumped as 3-4 in "Table II" of Payne(1974) as 3.5, which slightly reduces variability. Trinidad,where it nestedin mangroveswith reported "to be more round- Paynealso provided a detailed map of scoresin Panama(see Fig. 3), where edon the average" than those of B.striatus. Junge and Mees (1958) stated that typicalB. virescensoccurred in westernPanama in contrastwith highly vari- B.striatus was the most common species in Tobagobut collected only a spec- able,intermediate populations in central Panama. Based on the increased vah- imenof B.virescens. Specimens from Tobago were taken approximately a cen- abilityand prevalence of intermediatespecimens with neck coloration rang- tury ago,when the populationappeared to be intermediatebetween B. ing from 4-6 in centralPanama, southern Caribbean islands (Aruba, Los virescensand B. striatus and included specimens of both species (scores rang- Roques,Margarita and Tobago),and coastalnorthern South America ingfrom 3-8; Fig. 2). However,the resident population today is comprised (Colombiaand Venezuela), Payne concluded that secondary intergradation predominantly of rufous-neckedB. virescens scoring higher on theindex. B. (hybridization)occurred between B. v.virescens and B. s. striatus. striatusoccurs in smallnumbers, possibly as a seasonalvisitor, but the pres- Voous(1986) pointedout that the populationsof Bumridesin the enceof intermediateindividuals implies that hybridization occurs (Hayes NetherlandsAntilles (Aruba, Bonaire, and Curaqao)were less variable than unpubl.data). These observations suggest a recent historical shift toward typ- Payhe'slimited sample suggested. Of 123records comprising 16 specimen, icalB. virescensin Tobago. Payhe's sample of B.$triatus from Trinidad includ- eightphotographic, and 95 sight records (enigmatically these numbers do not edonly four specimens, with scores ranging from 1 to 3. However,the current add up), all but eightwere rufous-necked B. virescens,including some Trinidadpopulation appears much more variable than Payhe's limited sample migrantsfrom North America (at leastthree of 16specimens). Only one was suggested,with a smallproportion of intermediateindividuals and B. virescens an adultB. striatus,seen in Bonaire.Individuals of intermediateappearance, occurringonly rarely, apparently as a wintermigrant (Hayes, unpubl. data). whichcomprised only 6% of thesample, induded two from Aruba and five Monroeand Browning (1992) argued that specimens scored 1-5 represent- fromBonaire. However, the intermediate individuals likely comprised more ed B. $triatu$,whereas those scored 6-9 representedB. virescens. Although than6% of theresident population, as 19% of thespecimens, along with an specimensscored 1-4 occur naturally throughout the South American range unestimatednumber of photographicand sight records, were represented by of B. striatusand specimensscored 6-9 occurnaturally within the North Nearcftcmigrants of B.virescens. Americanrange of B. virescens,specimens scored as 5 in southernCentral Monroeand Browning (1992) stated that B. striatus is the resident form on America,the southern Caribbean, and coastal northern South America appear Trinidad,whereas B. virescensisresident on nearbyTobago. However, the sit- tobe truly intermediate. Outside the zone of overlapbetween the two species, uationon these islands is complex. AccoMing to Belcherand Smooker (1934), specimensscored as 5 occuronly in the pale-neckedBahamian population B. striatuswas common on bothislands but wasknown to nestonly in (29%of Payhe'ssample; n=21), which represents anexception to dinaIvaria- Trinidad;they did not find B. virescens onTobago but reported it to berare in tionand obviously does not represent B. striatus. Monroeand Browning (1992) argued that neck coloration of theresident Table1: Voucher specimens (see Fig. 1) from the NationalMuseum populationof B.striatus in Panamawas no more variable (scores ranging from of NaturalHistory used by Payne(1974) to scoreneck coloration 2-5) thanthe populationsof B. striatusthroughout South America (scores of Butorideson a hybridindex scale. rangingfrom 1-4) and wasmerely shifted up by onelevel in the index. However,even when specimens obtained from Octoberto April (which Catalogue Score Neckcolor number Locality includeNorth American migrants at theupper end of thescale and two pos- l gray 263848 Venezuela:Culata sibleresidents scored at thelower end of thescale) are excluded, variability in 2 gray,tinge brown 423096 Panama:Charco del Toro centralPanama still exceeds that of anyother population (Fig. 2). Monroe and Browningalso pointed out that no mixed breeding pairs had ever been report- 3 gray,wash brown 444948 Panama:Rio Indio ed,but failed to notethat relatively few ornithologists reside within the region 4 brownishgray 400113 Panama:Pese, Herrera whereintermediates occur and that visiting and resident ornithologists invari- 5 grayishbrown 448634 Panama:La Jagua ablystudied other species of birds.They further speculated that the two 6 grayishred-brown 368472 Colombia:Guajira Panamanianand one Colombian specimens scored as 6 weremore likely 7 reddish brown 206343 Panama: Rio Indio vagrantsfrom the LesserAntilles than extremevariants of B. virescens,but 8 purplishbrown 316840 CocosIsland giventhe extremedistance and east-westdirection of movementfrom the 9 darkpurplish brown 468699 Panama:Almirante LesserAntilles, such long-distance vagrancy from populations thought to be relativelysedentary seems highly unlikely.

VOLUME 5 6 (ZOO2}, NUMBER 1 7 Basedon theirreanalysis of Panamanianspedmens, Monroe and Browning (1992)concluded: "there is no substantial evidence that there is interbreeding, andcertainly if it occursat all,it mustbe at a lowlevel." However, biologists havelong recognized that hybridization can be inferred by an increase in vari- abilityand intermediacy in concert (Schueler and Rising 1976). The data in Fig.2 clearlyillustrate the increased variability and high proportion of inter- mediate individuals within the zone of contact between B. virescensand B. striatus,even when spedmens of presumedNearctic migrants (plus residents) areremoved from the sample. Such a patternstrongly implies extensive sec- ondaryintergradation between the two forms in a relativelynarrow hybrid zone between 8ø and 10ø N latitude. Extensivehybridization between taxa often has been interpreted asevidence forlack of reproductiveisolation, requiring that the taxa be consideredcon- specificaccording to the traditionalbiological species concept (BSC; Mayr 1970).However, the degree of hybridizationmust be considered, inasmuch as morethan 10% of birdspedes retain the ability to interbreedand produce viableoffspring with other closely related species, including non-sisters (Grant andGrant 1992). Johnson et al. (1999)proposed a new comprehensive bio- logicspedes concept (CBSC) applicable to birds:"An avian species isa system of populationsrepresenting anessentially monophylefic, genetically cohesive, andgenealogically concordant lineage of individualsthat share a commonfer- tilisafionsystem through time and space, represent anindependent evolution- arytrajectory, and demonstrate essential but not necessarily complete reproduc- tiveisolation from other such systems" (emphasis added). Assuming pheno- typeis correlatedwith genotype, the presence of"pure" phenotypes within a hybridzone, even when hybridization isextensive, provides evidence of assor- tativemating; in thiscase, the two taxa demonstrate essential reproductive iso- Figures10 and11. Two adult Striated Herons illustrating variability of neckcol- lationand should be considered specifically distinct. In contrast,when allindi- orationscores ranging from 1 (birdon left; taken at Pointe-a-Pierre,Trinidad, 7 vidualswithin a hybridzone are intermediate, free interbreeding isinferred October2000) to 4 (birdon right; taken at Pointe-a-Pierre,Trinidad, 11 August andthe two taxa should be regarded as conspecific. 2001).The bird on the right could be mistaken for a GreenHeron because of its In NewWorld Butorides, the seemingly continuous variation in neckcolor somewhatrufous neck coloration, but the distinctly grayer hindneck is character- frompurplish-brown in the north to grayin thesouth strongly implies poly- isticof manybrown-necked Striated Herons. The possibility of hybrid origin can- geniccontrol of thedeposition of grayeumelanin and rufous phaeomelanin notbe ruled out in this case. Photographs byFloyd E. Hayes. pigmentsin thedistal barbules of neckfeathers (Schodde et al. 1980).Each currentlyrecognized subspedes appears todiffer in thecombination ofalldes codingfor neck coloration, with intermediate combinau.'ons occurring where theranges of B.virescens and B. striatus meet. The presence of apparentlypure B. virescensand B. striatusphenotypes within the contact zone suggests that althoughthe two forms frequently hybridize, assortative mating does occur. However,the samplesizes of museumspedmens are small, and it remains uncertainwhether both forms actually breed within the hybrid zone. In Tobago,a large body of recentlycollected data clearly indicates the pres- enceof mostly"pure" phenotypes in an apparenthybrid zone (neck scores rangefrom 1 to 8;Hayes unpubl. data), providing further evidence that assor- tativemating occurs. Thus, the two forms appear to maintainessential repro- ductiveisolation and should be regarded as spedfically distinct. However, fur- thercolorimetric, morphological, behavioral, and genetic studies are needed to shedlight on the extent of geneflow and the stability of thehybrid zone. Basedon current taxonomy, B.s. striatusis thought to be more closely relat- ed to Old World taxa as well as to B. sunderaliithan to B. virescens.Thus, the twoforms are not consideredsister taxa, although they retain the ability to hybridize.Amadon (1953) speculated that North American B. virescenswas derivedfrom Asia, whereasSouth AmericanB. striatuswas derivedfrom .Although this hypothesis may explain the current zone of secondary contactbetween the two taxa, it is unsupportedby fossilor geneticevidence. Butoridessunderalii The LavaHeron is endemicto the Galfipagos Islands,where its dark gray coloration enables it to blendin witha backdrop Figure12. A typicaladult (B. sunderall/) from the Galfipagos of exposedlava. Harris (1973) pointed out that B. striatus was also resident on Islands,Ecuador. Photograph by S. Bahrt/VIREO. the islandsand that somespedmens were intermediate between the two forms.Payne (1974) examined 50 specimensfrom the Gal•tpagos Islands, whichvaried from blackish to palegray as in B.striatus. The majority of the

8 •N•ORTH AMER1CAN BIRDS New World BurGrides Herons specimenspossessed blackish (26%) or dark-gray,nearly blackish under- Acknowledgments parts(34%), with a slightlypaler throat (white-edged leathering, slight I thankD. B.McNair, W. Murphy, and J. V. Remsen,Jr. for reviewing the man- streakingor simplygrayer). Twelve (24%) specimens with dark gray under- uscript.My examination of museum specimens in the U.S. National Museum partshad a lightergray belly, blackish to darkgray sides of the neckand of NaturalHistory (USNM) and the American Museum of NaturalHistory cheeks,and a throatstreaked with whitish edges. Four (8%) specimenswith (AMNH)was financed by a Studyand Travel Grant from the University of the grayunderparts darker than B. striatuswere also gray on thesides of the WestIndies, St. Augustine. For their assistance I thank C. Angle(USNM), M neckand cheeks, and streaked white and gray-brown on the upperbreast Foster(USNM), J. Weicker(AMNH), P. Rasmussen(USNM), and P Sweet with whitishfeathers forming a midline.Finally, four (8%) palespecimens (AMNH).I alsothank J. Eitniear, A Kratter,R. Payne,and K. Voousfor pro- withlight gray underparts resembled B.striatus from the mainland but were vidingpertinent literature. streakedwhite and gray-brown on the neckwith no morethan a traceot rufouson theupper breast. Literature Cited Becausethe paler Galfipagos birds differed from those of the mainland, Amadon,D. 1953.Avian systematics and evolution in theGulf of Guinea,the Payne(1974) regarded them as local residents rather than vagrants and con- I. G. Correiacollection. Bulletin of theAmerican Museum of Natural H•story sideredthe high proportion of intermediatebirds as evidence for free inter- 100: 393-452. breedingbetween derived populations of B. sundevalliand morerecent AmericanOrnithologists' Union. 1976. Thirty-third supplement to theAOU colonistsof B. striatusfrom the mainland.Hancock and Kushlan(1984), Check-list. Auk 93: 875-879. however,regarded the variable forms of B.sunderalii as a situationof color AmericanOrnithologists' Union. 1983. Check-list ofNorth American B•rds, 6th polymorphismwithin the taxon rather than intergradation between B. sun- ed.American Ornithlogists' Union, Washington, D.C. devalhand B. striatus. Given the persistence of apparently pure phenotypes AmericanOrnithologists' Union. 1993. Thirty-ninth supplement to theAOU of B sunderaliiwithin a potentialhybrid zone, the two forms do not appear Check-list. Auk 110: 675-682. to beinterbreeding freely; thus, they may be considered specifically distinct. AmericanOrnithologists' Union. 1998. Check-list ofNorth American Birds, 7th However,further studies of colorimetry,morphology, behavior, and genetics ed.American Ornithlogists' Union, Washington, D.C. areneeded to resolvethe taxGnomic status of B. sunderaliiadequately. Belcher,C., andG. D. Smooker.1934. Birds of theColony of Trinidadand Tobago.Ibis 1934: 572-595. LIMITS OF IDENTIFICATION Bent,A. C. 1926.Life historiesof North Americanmarsh birds. UnitedStates Identificationof B. virescensand B. striatusis problematicin southern National Museum Bulletin 135: 1-490. CentralAmerica, on southernCaribbean islands, and in coastalnorthern Bock,W. I. 1956.A genericreview of thefamily Ardeidae (Aves). American SouthAmerica. Wetmore (1965) claimed that adultB. striatuspossessed a Museum Novitates 1779. lighter,more grayish-green back than B. virescens,but specimens'back col- Bond,I. 1964.Ninth Supplementto theCheck-list of Birdsof theWest In&es orationoverlaps .greatly between the two species. Thus, neck coloration (1956).Academy of NaturalSciences, Philadelphia. remainsthe solecriterion for identificationduring all ages.Juvenile and Cramp,S., and K. E. L. Simmons,eds. 1977. Handbook of theBirds of , immaturespecimens are often separable by the more rufous brown tones of theMiddle East and North Africa. The Birds of the Western Palearctic Vol 1 B wrescensand grayer brown tones of B.striatus (Figs. 4, 5,6). However,cau- Ostrichto Ducks. Oxford University Press, Oxford. tionis warranted, especially in thefield, because some juvenile and immature Davis,W. E., lr., and J.A. Kushlan.1994. Green Heron Butoridesvirescens B•rds specimensfrom NorthAmerican and SouthAmerican populations are so ofNorth America 129:1-23. similarthat theycannot be safelydistinguished. Because inexperienced Eisenmann, E. 1952.Annotated list of birdsof Barro ColoradoIsland, Panama observersoften misidentifybrown-necked immature B. striatusas B. Canal Zone. Smithsonian Miscellaneous Collections117: 1-62. v•rescens,accurate ageing of individualbirds is crucialto correctidentifica- Eisenmann,E. 1955. The species of MiddleAmerican birds. Transactions ofthe tion to species. LinnaeanSociety of New York 7. Neckcoloration in subadultand adult B. virescenstypically ranges from ffrench,R. 1973.A Guideto theBirds of Trinidadand Tobago.HarrowGod reddish-brownto purplish-brown with no hint of gray(Fig. 7); Green Herons Books,Newton Square, Pennsylvania. of therace anthonyi from the western United States are typically paler than Grant,E R.,and B. R. Grant. 1992. Hybridization of birdspecies. Science 256 easternbirds of thenominate form (Fig. 8). Becauseeach neck feather of B. 193-197. wrescensis gray basally, a displacedtuft of feathersmay reveal some gray Hancock,J., and J. Kushlan.1984. The HeronsHandbook. Harper & Row tones,so caution is warrantedin assessingneck colors. Neck coloration in Publishers,New York. adultB. striatustypically ranges from gray to brownishgray, with narrow Harris,M. E 1973.The Galfipagos avifauna. Condor 75: 265-278. rufousstreaking anteriorly (Fig. 9). In browner-neckedadults, which also Hartert,E. 1920.Gattung Butorides Blyth. Die VOgel der palaearktischen Fauna maybe mistakenby inexperiencedobservers for B. virescens,the brownis 2:1249-1251.Friedl•inder, Berlin. usually(though not always) more intense toward the front of theneck, where Hayerschmidt,E 1968. Birds of Surinam.Oliver & Boyd,Edinburgh, United it mergeswith the rufousstreaking, whereas the sidesof the neckbecome Kingdom. grayerposteriorly (Figs. 10, 11). Hindneck coloration of adultsoften provides Hellmayr,C. E.,and B. Conover. 1948. Catalogue of birdsof theAmericas and thebest clue for identification:in B. virescens,it is always rufous without any theadjacent islands. Field Museum of Natural Histor)4 Zoological Ser•es 13, hintof gray,whereas in B.striatus it isusually gray, but may be brownish gray. 1(2): 1-434. Trulyintermediate individuals obviously cannot be identified. Hilty,S. L., and W. C. Brown. 1986. A Guideto the Birds of Colombia. Princeton In the Galipagos,most individuals of B. sunderaliiare readilydistin- UniversityPress, Princeton, New Jersey. guishedfrom B. striatus by their consistently darker plumage at all ages(Fig. Johnson,N. K., J.V. RemsenJr., and C. Cicero.1999. Resolution of the debate 12) Onlythe palest individuals with a contrastingblack cap and streaking on over speciesconcepts in ornithology:a new comprehensivebiologic theforeneck can be considered to represent B. striatus,although if thesebirds speciesconcept. Pp. 1470-1482in Adams,N.J., and R. H. Slotow,eds actuallyrepresent alight morph of B.sunderalii, assuggested byHancock and Proceedingsof the 22ndInternational Ornithological Congress, Durban Kushlan(1984), B. striatus may not even occur in theGal•pagos. BirdLifeSouth Africa, Johannesburg. Junge,G C A, andG F Mees1958 The avifauna of Trinidadand Tobago

VOLUME 56 (2002), NUMBER l 9 ZoologischeVerhandlungen 37:1-172. Zink,R. M., andJ. V. Remsen,Jr. 1986. Evolutionary processes and patterns Marchant,S., and P. J. Higgins, eds. 1990. Handbook of Australian, New of geographicvariation in birds.Current Ornithology 4: 1-69. Zealand& AntarcticBirds. Vol. l:Ratites to Ducks.Oxford University Press,Melbourne. --Received25 February2001; accepted 13 December 2001. Mayr, E. 1970.Populations, Species and Evolution.Harvard University Press,Cambridge, Massachusetts. Monroe,B. L., Jr.,and M. R. Browning.1992. A re-analysisof Butorides. Bulletinof theBritish Ornithologists' Club 112: 81-85. Meyerde Schauensee, R., andW. H. Phelps.1978. A Guideto theBirds of Venezuela.Princeton University Press, Princeton, New Jersey. Oberholser,H. C. 1912.A revisionof thesubspecies of the GreenHeron (Butoridesvirescens [Linnaeus]). Proceedings of the UnitedStates National Museum 42: 529-577. Oberholser,H. C. 1974.The Bird Life of Texas,vol. 2. Universityof Texas Press,Austin, Texas. Palmer,R. S. (ed.). 1962.Handbook of NorthAmerican B•rds.Vol. 1. Loonsthrough Flamingos. Yale University Press,New Haven. Parkes,K. C. 1955.Systematic notes on NorthAmerican birds.I. The heronsand ibises(Ciconiiformes). Annals AviSyg Version 5 of theCarnegie Museum 33: 287-291. As SimpleAs YouWish ~ As PowerfulAs YouNeed Payne,R. B. 1974.Species limits and variation of theNew Seeour web site for greatnew features! World Green Herons Butorides virescens and Striated PalmSupport! Clements' 2001Taxonomy! Subspecies Support! HeronsB. striatus.Bulletin of theBritish Ornithologists' Club 94: 81-88. New!Over 2,000 Formal Places, Unlimited Locations! Payne,R. B. 1979.Family Ardeidae. Pp. 193-243in Mayr, TheLeader ~ Alwaysa GiantStep Ahead of AllThe liest! E , and G. W. Cottrell,eds. Check-list of Birdsof the AviSysis a full-featuredworldwide database and reporting system for seriousb•rd- World.,Vol. 1. 2nd ed. Harvard UniversityPress, ers.It can be enhancedby addingthe OfficialShawneen Finnegan Worldwide Na- Cambridge. tionChecklist Add-On (as providedin BirdAreaby SantaBarbara Software Prod- Payne,R. B.,and C. J.Risley. 1976. Systematics and evolu- ucts),providing the mostauthoritative, highest quality, and most up-to-date check- tionaryrelationships among the herons(Ardeidae). listsof the254 nations of theworld. The checklists are tightly and seamlessly inte- M•scellaneousPublications in Zoology,University of gratedwith AviSys, providing beautiful screen and printed output with seen, seen- M•chigan150. in-nation,and endemic markers, and instant worldwide nation-by-nation range query Peters,J. L. 1931.Check-list of Birdsof theWorld, Vol. 1. forany species. AviSys produces seen reports, checklists and hit lists of anynat•on HarvardUniversity Press, Cambridge, Massachusetts. or state,any group of nationsor states,and all ABARegions and Areas. Rldgely,R. S., and P. J. Greenfield.2001. The Birdsof Youcan instantly reduce the on-screen list from a worldor NAlist to thecheckhst of Ecuador.Cornell UniversityPress, Ithaca, New York. anynation, state, county, wildlife refuge, etc., whether provided by AviSysor cre- Schodde,R., I. J. Mason, M. L. Dudzinski, and J. L. McKean. 1980. Variation in the Striated Heron atedby you. Dealwith only the birdsyou need. Rotate through modes instantly. Butoridesstriatus in Australasia. Emu 80: 203-212. OneAviSys user wrote: "OUTSTANDING! I have been a computerprofessional forover 30 Schueler,E W., and J. D. Rising.1976. Phenetic evidence of yearsand must congratulate you on generatingstate-of-the-art, convenient to use,effi- naturalhybridization. Systematic Zoology 25: 283-289. cientand reliable software." Another says: "AVISYS IS A BLAST!"Precisely our objecWe Slbley,C. G.,and L. L. Short,Jr. 1964. Hybridization in the -• UnlimitedNumber of Lists:all major geographic lists automatically updated orioles of the Great Plains. Condor 66: 130-150. Example:assigning a sighting toyour yard also updates your City, County, State, Slud, E 1967. The birds of CocosIsland [Costa Rica]. Nation,Continent, worldwide ABA Area, worldwide ABA Region, and Life lists Bulletinofthe American Museum ofNatural History • FullABA N.A. Checklist, Clements World Checklist, and Official Tony White 134 261-296. State/ProvinceChecklists, all fully integrated with screen and report facilibes Snyder,D.E. 1966. The birds ofGuyana. Peabody Museum, • TheFastest, Easiest and Most Flexible sighting entry--just click on the birds Salem,Massachusetts. AviSyshas absolutely unmatched search facilities, including World Band Codes• Steadman,D.W., R. L. Norton, M.R. Browning, andW.J. • UnlimitedReporting and Listing by date range, season, geography, species, Arendt. 1997. The birds of St. Kitts, LesserAntilles. ,behavior, sex, nesting status, heard-only, photographed, key-words, etc CaribbeanJournal of Science33: 1-20. CensusSpreadsheets for population, sighting, CBC, and ornithology studies Stiles,F.G., and A. F. Skutch. 1989. AGuide tothe birds of • Free!NABA Butterfly, Dragonfly, Reptile/Amphibian, and Mammal data sets• BirdBaseusers -- askfor ourfree comprehensive data conversion facility. CostaRica. Cornell University Press, Ithaca, New York. Voous,K. H. 1986. Striatedor Green Herons in the South Visitour web site at: www.avisys.net Caribbean islands? Annalen des Naturhistorischen Ordersor info,call 1-800-354-7755 ~ 24 hours~ MC/VlSA Museumzu Wien (B) 88/89: 101-106. AviSys5.0 for Windows 95/98/XP/NT/2000 ~ $99.95 ~ S&H$4.00 Wetmore,A. 1965.The birdsof the Republicof Panama. NationChecklist Add-On (BirdArea) ~ $59.95~ (S&H$4.00 if orderedseparately) Part1.--Tinamidae (tinamous) to Rynchopidae(skim- 60 daymoney back ~ PerceptiveSystems, PO Box 369, Placitas, NM 87043 mers). SmithsonianMiscellaneous Collections 150: 1- Fastas a Falcon~ Powerfulas an Eagle~ Friendlyas a Chickadee 483

10 NORTH AMERICAN BIRDS