Geographic Variation, Hybridization, and Taxonomy of New World Butorides Herons
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Geographicvariation, hybridization,and taxonomyof New WorldButorides herons FloydE. Hayes devallias specifically distinct. Departmentof LifeSciences In a criticalreanalysis of specimensfrom the Panamaniancontact zone Universityofthe West Indies betweenB. virescensand 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 species. This conclusion was accepted by the (email:[email protected]) AmericanOrnithologists' Union (1993,1998), which resplit Butorides 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 Heron (B. virescens) of icanceof suchindividuals, the followingreview is offeredto synthesizecur- North America,Central America, and the WestIndies; the gray-necked rentknowledge and to stimulatefurther study of the geographicvariation, StriatedHeron (B. striatus)of SouthAmerica and the Old World; and the hybridization,taxonomy, 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 notrepresent 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 the centerof 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 birds 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 Striated Heron (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 foundevidence 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. virescensand 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. Caribbean islands. 82 c 80 '• 78 o Figure4. A typicalnominate juvenile Green Heron (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 bird shows streaking on the brownishneck and sentedby numbers (see Fig. 1), of Butoridesspecimensin Panama. 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