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Home , Cuckoo, Hoatzin

THE AUK A QUARTERLY JOURNAL OF

Vo•,. 90 J^NU^R¾1973 NO. 1

THE RELATIONSHIPS OF THE

CH^R•,ESG. Sm•,E¾^N•) JON E. AHLQmST

THE taxonomicposition of the Hoatzin (Opisthocomushoazin) of South Americahas long been one of the mostdebated problems in avian system- atics. It hasusually been placed in the ,but someauthors have allied it with the African (Cuculiformes: Musophagidae), the pigeons(), or the rails (Rallidae), and othershave placedit in its own monotypicorder. The Hoatzin occursin riparianvegetation alongthe streamsof the Orinocoand Amazonriver drainagesin northern SouthAmerica. It is a slenderbird, about 25 inchesin lengthand generally brownishin color, somewhatresembling a chachalaca(). The smallhead bears a ragged,bristly, reddish-browncrest and the bare facial skin is bright blue. As is apparentfrom the Frontispiece,the Hoatzin is actuallymost like the smallerGuira (G. guira) in coloration;in fact, the two speciesare remarkablysimilar, except in size. This paperreviews the taxonomichistory of the Hoatzinand presents newevidence from a studyof the -whiteproteins indicating that the resemblanceto Guira is due to a closerelationship, not merelyto con- vergenceor coincidence.The correctposition of the Hoatzinis as a withinthe neotropicalsubfamily (Cuculidae) and its dosest relativesare Guira and Crotophaga.The longassociation with the Gal- liformes,we believe,was based upon little morethan the prejudiceestab- lishedby the originaldescription as Phasianushoazin (Mi•ller 1776) and the inabilityof subsequentworkers to interprettheir anatomicaldata. The link to the Musophagidaeseems to be basedupon the commonpossession of a numberof anatomicalcharacters (see Verheyen 1956), the vegetarian diets, poor flying ability, generalproportions, and similar size. In the turacosand in the Hoatzin the younghave wing clawsand clamberabout near the nest before they can fly.

THE TAXONOMIC HISTORY OF THE HOATZIN In generalappearance a museumspecimen of the Hoatzin resemblesa galliformbird andit is not surprisingthat M[iller (1776) originallynamed 1 The Auk 90: 1-13. January 1973 The tlOATZIN(lower tigure) and the GUIBACUCKOo dr•twn to the samesize. Insert shows relative sizes of the sanac . From an original acrylic painting by Jon Ahlquist. SIBLEY AND AllLQUIST [Auk, Vol. 90

TABLE 1 SOME OPINIONS ON THE SYSTEMATIC POSITION 01* OPISTHO½OMUS

Evidence of relation- Placed in Galliformes Placed in separateorder Placed near Musophagidae ship to Cuculidae

Miiller, 1776 Linnaeus, 1788 Illiger, 1811 Cuvier, 1817 L'Herminier, 18372 Nitzsch, 1829, 1840 Carus, 1868 Huxley, 18682 Gray, 1844-49, Garrod, 1879• 1869-71 Reichenow, 1882 Sclater, 1880• Cabanis, 1847ø Newton, 18842 Fiirbringer, 18884 Elliot, 1885a Goodchild, 1886 Sharpe, 18915 (secondary coverts) Gadow, 1892, 18934 Beddard, 1898• Pycraft, 1895 (pterylography) Shufeldt, 1904 Stresemann, Beebe, 1909 1927-34, 1959ø (behavior) Lemmrich, 1931 (sclerotic ossicles) Wetmore, 1930, 1934, 1940, 1951, 1960 Peters, 1934 Berlioz, 1950 Mayr and Amadon, Barnikol, 1953 Verheyen, 1956 1951 Storer, 1960 Verheyen, 1961 Sibley and Ahlquist, 1972

• Between Galliformes and Cuculiformes. a Between Galliformes and Columbiformes. a Next to Ga]]iformes. a Noted ties to Musophagidae. • Between Co]umbae and Ralli, following Seebohm (1888, 1890, 1895) who believed that Opistlto- comushad gruiform affinities. o Cabanis was apparently the first to note the similarity in co]or and pattern between Opistltocomus and Guira. it Phasianushoazin. Apparently this was enoughto establisha bias in favor of galliformaffinities, and in most classificationsto the presenttime, the Hoatzin has beenplaced in or near the Galliformes. Someof the opinions concerningthe classificationof the Hoatzin are summarizedin Table 1. The first suggestionthat the Hoatzin might be related to the Muso- phagidaewas that of Nitzsch who found similaritiesin the carotid arteries (1829) and pterylography(1840). In both papersNitzsch placedOpistho- comuswith Colius and the Musophagidaein the "" Amphibolae. January 1973] Hoatzin Relationships 3

Gray (1844-49, 1869-71) followeda similarscheme, placing these three groups,along with the Bucerotidaeand severalpasserine families, in the Conirostres. Huxley (1867) first examinedonly an incompleteskull and the feet of a Hoatzin. He found that the slendervomer bifurcates anteriorly in an unusualway but he consideredthe palate to be schizognathus,as in the Galliformes,rather than desmognathous,as are the .Huxley con- sideredthe tarsometatarsusto be galliform and assignedOpisthocomus to a "specialsubdivision," Heteromorphae, in his large "suborder"Schizog- nathae. He placed the Heteromorphaebetween the ("Ptero- clomorphae")and the pigeons("Peristeromorphae"), not far from the galliforms ("Alectoromorphae").Huxley soonobtained a completeskele- ton and (1868) decidedthat in the majority of its osteologicalcharacters the Hoatzin resembles the Galliformes and the Columbiformes and that where it differs from these it is either unique or like the Musophagidae. He concludedthat Opisthocomusis a highly modified form derived from "Gallo-columbine"stock, worthy of distinctionin his monotypic"Hetero- morphae." Huxley's prestigeand influencewere great and these pronouncements must have had a strong effect upon subsequentworkers. The galliform affinities of the Hoatzin seemedconfirmed, but an alliance to the cuckoos, via the turacos,had beenleft open. This basicconcept has prevailedto the presentday, in spite of evidenceto the contrary. Garrod (1879) was amongthe first to follow Huxley's lead by placing the galliform and cuculiformbirds togetherin his Galliformes(which also included the ratites, , rails, ,and ). Garrod expressedhis beliefthat Opisthocomusis an intermediateform, linking the Cuculidaeand Musophagidaeto the typical galliforms.He thus emphasized that Opisthocomushas a relationshipto the cuckoos,although he retained the supposedlink to the Galliformesas well. Otherswho adoptedOpisthocomus as the evidencefor a galliform-cuculi- form allianceincluded Ftirbringer (1888) who placedthe family Opistho- comidaenext to the Gallidae in the "gens" Galli, but also noted a dozen characterslinking Opisthocomuswith the Musophagidae. Gadow (1892: 229) also consideredOpisthocomus to be a connection betweenthe cuckoosand the galliformsbut (1893) he placed the Opistho- comiin the Galliformes.Like Ftirbringer,Gadow consistently emphasized the allianceto the cuckoos.Gadow (1893: 177) noted the followingchar- acters in comparisonsamong the three groups: I. Opisthocomusagrees with the Galliformesand differs from the Cucu- liformes in: 1) downy precocialyoung, not blind at hatching, 2) 4 SlBLEYAND AI-ILQUIST [Auk, Vol. 90

schizognathouspalate, 3) fusion of many dorsalvertebrae, 4) , 5) foot structure, 6) large crop. II. Opisthocomusagrees with the Cuculiformesand differs from the Galliformesin: 1) deep temporal fossae,2) short mandibularproc- esses,3) lack of basipterygoidprocesses, 4) lack of spina interna, 5) metasternalconfiguration, 6) large procoracoid,7) spottedeggs, 8) 10 rectrices.

III. Opisthocomusdiffers from both Galliformes and Cuculiformesin: 1) lacking apteria on the sidesof the neck, 2) arrangementof the adult downs,3) numberof cervicalvertebrae, 4) small thoracichae- mapophyses,5) shapeof the liver, 6) other specializedcharacteristics, for example,sternum, stomach, crop.

Becausevirtually all subsequentclassifications, including those of Strese- mann (1927-34), Wetmore(1930, 1934, 1940, 1951, 1960), Peters(1934) and Mayr and Areadon (1951) are basedupon Gadow and FiJrbringer,it is easy to understandwhy the Hoatzin has been placed so consistentlyin the Galliformes. The anatomical evidenceindicating a relationship to the Cuculidae includesa few additionalpoints. Pycraft (1895: 362) found a general resemblancein the pterylographyof the spinal tract amongOpisthocomus, Tauraco, and Centropusbut declinedto offer an opinion on their relation- ships. Lemmrich (1931) studied the bony platelets of the scleroticring in severalgroups of birds. He found that all of the 10 speciesof galliforms he examinedhad 13 to 15 plates but that Opisthocomushas only 12. Lemmrich noted (p. 534) that such a differenceis "very remarkable" becausethere tends to be little variation within a group. Among the few other birds with 12 plates is . The skull musculatureand its innervationin Opisthocomuswere studied by Barnikol (1953). He comparedthe Hoatzin with representativespecies of galliforms,turacos, and pigeonsand summarizedthe study in a table (p. 520) listing 40 anatomicalcharacters. Of these Opisthocomusshared 8 with the Galliformes,9 with the Columbidaeand 13 with the Muso- phagidae. Barnikol concludedthat Opisthocomusis an isolatedspecies deservingrecognition as the soleoccupant of the Opisthocomae. Verheyen (1956) defined 66 osteologicalcharacters of the Hoatzin and found that 50 of them are shared with the Musophagidae. This led him to combinethe two groupsas the orderMusophagiformes, placed near the Galliformesand the "Anhimiformes."However, in his later (1961) clas- sification Verheyen included Opisthocomusnext to the Cracidae in his Galliformes,"owing to new information" (p. 21). January 1973] Hoatzin Relationships 5

Some especiallyconvincing sets of data arguing against a galliform alliancefor Opisthocomushave appearedin recentyears. In 1959 Hudson, Lanzillotti, and Edwardsfound 13 aspectsof the hind limb musculature in which the Hoatzin differs from all galliformsand in 1964 Hudsonand Lanzillottilisted 21 importantdifferences between the pectoralmusculature of Opisthocomusand that of the galliforms. They concluded(p. 111) that "Opisthocomushas either been erroneously associated with the Galliformes, or has divergedso far from the original ancestralcondition, that there is little or no justification for retaining it in that order? Becauseof his long interestin the Opisthocomusproblem, the opinionof E. Stresemannis of particular significance.In his classificationsof 1934 and 1959 he followedFiirbringer, placing the Hoatzin with the Galliformes and attributing resemblancesto the tufacosto convergence.But in recent studiesof pterylosisand molt patternsE. Stresemann(1965) and E. and V. Stresemann(1966) discoveredthat Opisthocomusdiffers from all galli- formsin four ways: 1) the nestlingplumage lacks flight ;2) the first flight feathersgrow to nearly adult sizeand are not moltedbefore the reachesmaturity; 3) the two outer secondariesare not shorter than the adjacentones and all developat the sametime; and, 4) the primaries are replacedin a continuous,stepwise process, not in the galliformsequence. Thesefindings prompted E. Stresemann(1965: 64) to conclude(transl.): "If Opisthocomusis related to the gallinaceousbirds it is only by way of Adam and Eve."

L•FE H•STOR¾ A•r) BEHAVIORAL EvH)E•CE Accountsof the natural history of the Hoatzin have been publishedby Goeldi (1886), Quelch (1890), Beebe(1909), Chubb (1916) and Young (1929). A popular article by Grimmer (1962) is one of the most in- formative,including excellent photographs, and Sick (1964) presenteda summary. From these accountsa fairly complete life history can be constructed. The Hoatzin livesin the permanentlyflooded forests along the banks of someof the larger rivers and their tributaries in northeasternSouth America.The birdsfeed primarily, and perhapsexclusively, on the , ,and fruits of certain marshplants, includingthe generaMon- trichardia() and ,the arborealWhite . Grim- mer foundthem feeding only on plants; othershave recordedsmall (, crabs) as part of the diet. When feedingthe birds fill the large, specializedcrop, the resultantweight of which makesbalance difficult and the bird oftenrests the sternalarea on a branchto supportthe heavy crop. A specialepidermal callosity has evolvedin the area of contact. 6 SIBLE¾AND AIILQIJIST [Auk, Vol. 90

Hoatzinsare usuallydescribed as clumsy,poor fliers, their feet not well adaptedfor graspingbranches and their weak wingsrestricting flight to shortdistances. The wingsare usedin clamberingabout in the vegetation and the weak flight is correlatedwith the reductionin the sternumand pectoral musculatureassociated with the exceptionallylarge crop. When not breedingthe Hoatzin forms flocks of up to 40 individuals which break up into smallergroups of 2 to 6 birds when nesting. The discovery (Grimmer 1962) that the Hoatzin forms communal nesting associationsis of particular interestbecause the three speciesof anis (Cro- tophaga) and the Guira Cuckooalso build communalnests (Young 1929, Skutch 1935, 1966, Chapman1938, Davis 1940a,b). With the first rains the membersof a nestinggroup build a flat nest of looselyorganized dry twigs in branchesoverhanging the water. Mating seemsto be indiscriminateand may be polygamous,and all membersof the group participate in incubationand care of the young. The 2 to 5 buff-coloredeggs have pink, brown, or bluish spots,and incubationtakes about 28 days. The younghave two successivecoats of down and are fed from the cropsof the adults, apparentlyon plant material. The young remain in the nest for a considerabletime but soonbegin to undertakeclimbing excursions nearby, using the well-knownwing , as well as the feet and . When frightenedthe young I-Ioatzin drops into the water and either dives or swimsinto the vegetationand hides. Later it climbs back to the nest. The youngof the Groove-billed(Crotophaga sulcirostris) and Smooth- billed (C. ) Anis have a similarpattern of behavioraccording to Skutch (1959, 1966). The flightlessani youngsteralso uses the beak, feet, and wings (which are clawless)in clamberingabout near the nest. When frightened it either moves away through the branchesor drops to the groundand hidesin the vegetation,later returningto the nest. Both Beebe (1909) and Skutch (1959: 294) commentedupon the behavioralsim- ilarities between the Hoatzin and the anis.

THE EGG-WHITE PROTEIN EVIDENCE Egg white of the Hoatzin was first receivedin 1961 throughthe kindness of J. Lear Grimmer and WoodbridgeWilliams. Additional material came from Ram E. Singhin 1972. It wasobvious from the first comparisonsusing starch gel electrophoresis (Sibley and Ahlquist, 1972, but see Sibley 1970 for a descriptionof the technique)that Opisthocomusis not a galliform and is probably a cuckoo, but the resolvingpower of the starch-geltechnique was insufficientto provide an unequivocalanswer. A study of the literature on the natural historyof the I-Ioatzinand of museumskins caused us to feel fairly certain January 1973] Hoatzin Relationships 7

• PENELOPEPURPURASCENS

CRAXBLUISlENBACHI I

OPISTHOCOHUSHOAZIN

GUIRA GUIRA

CROTOPHAGAANI

COCCYZUSAI4ERIC^HUS

GEOCOCCYXCALIFORNIANUS

• I40ROCOCCYXERYTHROPYGUS PH 3.0 PH 10,0

Figure 1. The egg-white proteins of Opisthocomftscompared with those of some cuckoosand cracidsusing the techniqueof isoelectricfocusing in acrylamide gel. The similaritiesamong Oplsthocomfts,Gfdra, and Crotophagaare greater than betweenany one of theseand the other generacompared. The patternswere produced using an Ampholine range of pH 3-10. that the neotropicalcrotophagine cuckoos (Guira, Crotophaga)were the mostlikely relatives.In 1972 we felt confidentenough to suggestsuch an alliance,but proofstill seemedelusive. During 1972 we beganwork with the electrophoretictechnique of isoelectricfocusing in acrylamidegel (IFAG) (see$ibley and Frelin 1972,for a descriptionof the technique), whichhas muchgreater resolving power than that of starchgel electro- phoresis.Using the IFAG techniquewe comparedthe egg-whiteproteins of Opisthocomuswith thoseof Guira, Crotophaga,other cuckoos,various galliforms, pigeons,and many other groups of nonpasserinebirds. It quickly becamedear that the patterns producedby Hoatzin egg white were most similar to those of the Crotophaginae.We now feel confident in stating,without qualification,that Opisthocomusis a dose relative of Guira, which it resemblesin coloration,although it is a much largerbird. If the two had beenmore alike in size,or if the Hoatzin had possessedthe typicalcuculiform foot, we feel confidentthat this relation- 8 $IBLE¾^ND AHLQUXST [Auk, Vol. 90

PEHELOPEPURPURASCEHS

CRAXBLUMENBACHII

OPISTHOCOMUSHOAZIN

GUIRAGUIRA

CROTOPHAGAANI

COCCYZUSAHERICANUS

GEOCOCCYXCALIFORNIANUS

: I•OROCOCCYXERYTHROPYGUS ell 6,0 eH q,O

Figure 2. The same speciesas in Figure 1 but using an Ampholine range of pH 6-4. shipwould long ago have been recognized. The egg-whitedata confirmthe relationship,but ample additional evidencefrom the plumage, , life history, and behaviorsupports this conclusion. A detailed descriptionof the IFAG patterns follows. Figures I and 2 depict those egg-white proteins that have isoelectric points betweenpH 3 and 10. Becauseno egg-whiteproteins are known to be isoelectricbelow pH 3 and only lysozymemay be isoelectricabove pH 10, nearly all componentsare resolvedin these patterns. In these figuresthe eggwhite of Opistkocomusis comparedto that of severalspecies that have been suspectedof being related to it. The pattern of Opistko- comusdiffers from thoseof the curassowCrax and the Penelopein nearly all aspects. In the cracids the ovalbuminsand most other corn~ ponentsare isoelectricnear pH 5. This producesa "short" pattern with many bandsclose together. In Opistkocomusthe ovalbuminsare isoelectric nearpH 4 and many components,mainly the ovotransferrins,are isoelectric at pH 6 or above. Crax and Penelopehave a prominent lysozymenear pH 10 that Opistkocomuslacks. In thosefeatures where the Opistkocomuspattern differs from thoseof the cracidsit is like thoseof Guira and Crotopkaga. This resemblanceis January 1973] Hoatzin Relationships 9

œRINIFERAFRICANUS

• •.•.,•i' OPISTHOCOMUSHOAZIN

CENTROPUSBENGALENSIS I CUCULU•CANORUS

J • • CHALCITESfiALAYANU$

,i i • CLA•ATORLEVAILLANTI]

t : RALLUSLOfiGZROSTRIS

C0LU•A L]VIA

Pfi $.0 PH 10.0

Figure 3. The egg-white proteins of O•sthocomus compared with those of some cuculiforms, a raft, and a dove using isoelectfic foscusingwith an ^mphoEne range of pH 3-10. The similarities among Opisthocomusand the others are lessstrik- ing than those among Opisthocom•s,G•ira, and Crotophagain Figure 1. most striking.in the ovalbuminregion around pH 4, but there are other componentsas well that have identical isoelectricpoints. The general pattern of Opisthocomusis similar to thoseof the musophagidCriniJer and the coucalCentropus, but few componentshave identicalisoelectric points. The pattern of Opisthocomusdiffers considerablyfrom thoseof the New World cuckoosCoccyzus, Geococcyx, and Morococcyx. The patterns of Coccyzusand Morococcyxin turn resemblethose of the Old World Cuculusand . Finally thereare no similaritiesbetween Opisthocomus,Rallus, and Columba. Figures3 and 4 depictthe egg-whiteproteins of the samespecies in the pH range 6 to 4. In these separationsgreater resolutionof those com- ponentsin the ovalbuminregion is achieved.For example,in the pI-I 3-10 range four major bands and an additional four minor bands can be seen in the ovalbuminregion of Opisthocomus.In the pH 6-4 patternsOpis- thocomushas 18 components.Opisthocomus again bears the greatest resemblanceto Guira and Crotophaga,although small differencesin the 10 StorEy AND AItLQUIST [Auk, Vol. 90

CRINIFER AFRICANUS

OPISTHOCOMUSHOAZIN

CENTROPUSBENGALENSIS

CUCULUSCANORUS

CHALCITESMALAYANUS

CLA•ATORLEVAILLANTII

PALLUSLONGIROSTRI$

COLUIIBALIVIA

PH 6,0 PH/4.0 Figure 4. The same speciesas in Figure 3 but using an Ampholine range of 6-4. isoelectricpoints of a numberof proteinscan be discerned.The Opistho- comuspattern showssome similarity to that of Crinifer, but less to those of Centropusand the other cuckoos,and is unlike those of the cracids, RaNus, or Columba.

ACK•TOWLEDG/WE•TT$

We are especiallygrateful to J. L. Grimmer, Woodbridge Williams, and Ram E. Singh for providing the specimensof I-Ioatzin egg white. Other specimensused in the study were provided by A. Baddokwaya, F. Confino, P. Davis, R. Dean, J. Doebeli, A. Garza, M. Goddard, J. Griffith, I-I. Hendrickson, L. Kiff, P. Miles, C. Perrins, N. Proctor, G. Ragless,J. Robjent, S. Rothstein, A. Ruschi, F. Sibley, W. Spofford, F. Todd, D. Wells, and A. Williams. Dorothy J. Moore and Karen Heckmann provided laboratory assistanceand A. I-I. Coleman prepared the photogTaphs.The National ScienceFoundation provided support under GB-6192X.

SUMMARY The systematicrelationships of the Hoatzin (Opisthocomushoazin), long classifiedwith the Galliformes,have been re-evaluatedin the light of data from electrophoreticcomparisons of the egg-white proteins. All January 1973] Hoatzin Relationships 11 evidencehas been found to be consistentwith an assignmentto the sub- family Crotophaginaeof the Cuculidae. There is no substanceto the long-acceptedalliance to the Galliformesand the relationshipto the Musophagidaeis simply that of the two familieswithin the Cuculiformes.

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HUDSON,G. E., ANDP. J. LANglI.I.OTTI. 1964. Muscles of the pectoral limb in galli- form birds. Amer. Midland Nat., 71: 1-113. HE)soN, G. E., P. J. LANzIL•;OTTI,ANI) G. D. EI)WARI)S.1959. Musclesof the pelvic limb in galliform birds. Amer. Midland Nat., 61: 1-67. HXJX•;E¾,THOMAS H. 1867. On the classificationof birds; and on the taxonomic value of the modifications of certain of the cranial bones observable in that . Proc. Zool. Soc. London, 415-472. HXJX•,E¾,THO•fAS H. 1868. On the classificationand distribution of the Alectoro- morphae and Heteromorphae. Proc. Zool. Soc. London, 294-319. I•;•,mER,JOI•ANN KAR•, WI•;Hm;M. 1811. Prodromus systematisMammalium et Avium, additis terminis zoographicisutriusque classis,eorumque versione germanica. Ber- lin, C. Salfield. LE•XVrR•CH,WERNER. 1931. Der Skierairingder Viigel. Jena. Z. Naturwiss., 65: 513- 586. L'HER•r•NIER, M. 1837. Recherchesanatomiques sur quelquesgenres d'oiseaux rares ou encore peu connus sous le rapport de l'organisation profonde. Comptes rendus Acad. Sci., Paris, 12: 433-441. L•NNAEUS,CARO•;XJS. 1788. SystemaNaturae per regna tria naturae. 13th edition, ed. by J. F. Gmelin. Vol. 1. Lipsiae. MAYR,ERNST, AND DEAN A2VIADON. 1951. A classificationof recentbirds. Amer. Mus. Novitates• No. 1496. MiiZ•,ER,PmmIvV LUDWIG STATIXJS. 1776. Des Ritters Carl von Linne . . . Vollstiindi- gen Natursystems Supplements-und Register-band iiber alle sechs Theile oder Classen des Thierreichs. Niirnberg, G. N. Raspe. NEWTON,A. 1884. Ornithology. Pp. 1-50 in EncyclopaediaBritannica, vol. 18. NITZSCH,C.L. 1829. Observationesde avium arteria catotidecommuni. Halae, Mus. Zool. Direct.; Plurium Acad. Soc. Lit. Sodali. NITZSCH,C.L. 1840. System der Pterylographie (C. C. H. Burmeister, Ed.). Halle, Eduard Anton. PETERS,J. L. 1934. Check- of the world, vol. 2. Cambridge, Harvard Univ. Press. P¾CRA•rT,W.P. 1895. On the pterylographyof the Hoatzin, Opisthocomuscristatus. Ibis, 37: 345-373. QXJEZCH,J.J. 1890. On the habits of the Hoatzin (Opisthocomuscristatus). Ibis, 32: 327-335. REICHENOW,ANTON. 1882. Die ViSgelder zoologischenGlirten, vol. 1. Leipzig, L. A. Kittier. SC•;ATER,P.L. 1880. Remarks on the present state of the system avium. Ibis, 22: 340-350, 399-411. SEEBOI•, H. 1888. An attempt to diagnose the suborders of the great Gallino- Gralline group of birds by the aid of osteologicalcharacters alone. Ibis, 30: 415- 435. SEEBOH•, H. 1890. Classificationof birds. London, R. H. Porter. SEEBOH•,H. 1895. Classificationof birds. Supplement.London, R. H. Porter. SHARPE,R. BOWIH;ER. 1891. A review of recentattempts to classifybirds; an address delivered before the Second International Ornithological Congress,Budapest. SIrmrm;i)T, R. W. 1904. An arrangement of the families and the higher groups of birds. Amer. Nat., 38: 833-857. SIB•,E¾,C.G. 1970. A comparative study of the egg white proteins of passefinebirds. Peabody Mus. Nat. Hist. Bull. 32. January 1973] Hoatzin Relationships 13

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PeabodyMuseum of Natural History and Department of Biology, Yale University, New Haven, Connecticut06520. Accepted28 April 1972.