THE RELATIONSHIPS OF THE SILKY FLYCATCHERS

CI-IARLES G. SIBLE¾

THE silky flycatchersinclude the generaPtilogonys, Phainopepla, and Phainoptila,which are usuallytreated either as a family, Ptilogonatidae (e.g. Wetmore 1960), or as a subfamily of the family Bomby- cillidae (e.g. Greenway1960). Someauthors have includedthe Palm Chat (Dulus dominicus)of Hispaniola and the (Hypocolius ampelinus) of and in the Bombycillidae (Arvey 1951, Mayr and Amadon1951). Othershave consideredsome of theseallocations to be tentative or unproved and have recognizedseparate families for some or all of these groups (e.g. Wetmore 1960, Greenway 1960). The silky flycatchersare confinedto North and Central America. The Phainopepla(Phainopepla nitens) occursin the arid and semiaridregions of the southwestern United States and in Mexico south to Puebla and Vera Cruz. The Gray Silky Flycatcher (Ptilogonyscinereus) is a montane speciesranging from northwesternand easternMexico to Guatemala. The Long-tailedSilky Flycatcher(P. caudatus)and the Black-and-yellowSilky Flycatcher (Phainoptilamelanoxantha) are eachendemic to the highlands of Costa Rica and western Panama. This paper reviews some of the taxonomichistory and charactersof the silky flycatchersand presentsnew evidencefrom studies of the - white proteins indicating that they are closely related to the ,the solitaires,of the thrushfamily Turdidae. The two of the genusEntomodestes of northwesternSouth America may be part of this natural cluster,but their egg whites have not been available for comparison.

TAXONOMIC H•STOR¾ or THE S•LK¾ FLYCATCHERS AND A•;L•ES

Ptilogonyscinereus, the first of the group to be discovered,was described by Swainsonin 1824. Swainsonalso describedthe Phainopepla (1837), which he placed in Ptilogonys. From the time of their discovery these were consideredto be closelyrelated to the . Gray (1846) placedthem near the waxwingsbecause of similaritiesin bill shape,tarsal scutellation, texture, and the possessionof a crest. But there also are differences. Phainopepladiffers from Bombycilla in having a long tenth primary (vs. a rudimentaryone), a roundedwing (vs. a long, pointedwing), long rictal bristles(vs. the obsoletecondition), unfeathered nasal fossae (vs. denselyleathered), downy nestlings (vs. naked), and unstreakedjuvenal plumage (vs. streaked). Thus those who advocated placementin separatetaxa alsohad a basisfor their opinion.

394 The Auk 9o: 394410. April 1973 April 1973] Silky FlycatcherRelationships 395

Baird (1858) proposedthe genusPhainopepla for Swainson'sPtilogonys nitens, an arrangementthat has since been acceptedwithout debate. The associationof Myadesteswith the waxwingsand silky flycatchers was apparently first challengedby Baird (1864-72: 417-433) who advocated the tranfer of Myadestes from its "usual associationwith Ptilogonys,among Ampelidae [= Bombycillidae],to, or at least very near, the Turdidae." Although Baird noted the "close resemblancein generalappearance" between Myadestes and Ptilogonyshe traceda series of links that showedthe alliance of Myadestes to the typical thrushes. Baird proposedthe subfamily"Myiadestinae" to include "Myiadestes... Cichlopsis...Platycichla...the latter genus is so closelyrelated to Cichlopsisas almost to be the same; Platycichla forming the link with with Turdinae through Planesticus,while such speciesas Myiadestes unicolorshow the affinities of Cichlopsisto Myiadestes." Coues'treatment of the problemprovides an indicationof the transition of taxonomic opinion from the older view of Myadestes as a relative of Phainopeplato the new alliance with the thrushes. In the first edition of his "Key" Coues(1872) includedPhainopepla and Myadestesin the wax- wing family (Ampelidae), but noted that Baird had separatedthem and that this coursewas possibly correct. In the "Birds of the Colorado Valley" (1878) Couesadopted Baird's arrangementand placedMyadestes with the thrushes.Then, in later editionsof the "Key" (e.g. 1887), he recognizedthe subfamily"Myiadestinae" but placedit in the "Ampelidae," followingthe Ptilogonatinae,and stated that althoughMyadestes townsendi "was formerlycalled 'Ptilogonys', it has nothingto do with the foregoing subfamily. The Myiadestinae are in fact nearly related to the Turdidae." Seebohm(1881) left Myadestescompletely out of the Turdidae and Sharpe(1883) includedthe genusin the Timaliidae,noting (p. 368) that this was an unsatisfactoryarrangement but that he "placed them near the Mocking-Thrushes[= Mimidae], which they resemblein the power of song."Seebohm defined the Turdinae primarily on the basisof the booted tarsus and spotted juvenal plumage, a definition relied upon to the present day. Stejneger (1883) praised Seebohm'sdefinition of the Turdinae, but was "provoked by the arrangementproposed." As a reply Stejneger presenteda reviewof the systematicsof the American thrushesin which he adoptedBaird's proposalconcerning Myadestes. But one speciesdid not fit the definitionand Stejnegerdecided that "the groupwill not, however, be naturally limited or clearly defined without removing the species Myadestesleucotis (Tschudi), which is widely different, from the Mya- destinae, being a true member of the Ptilogonatidae." Stejneger then 396 C•^•z•s G. Smz• [Auk, Vol. 90 proposedthe new genus Entomodestesfor M. leucotis and assignedit to the silky flycatchers. An important factor in this decisionseems to have beenthat Stejnegerhad found the tarsusof leucotisto be faintly scutellate anteriorlywhereas Baird had thoughtthat it was "without distinctscutellar divisionsanteriorly, except below" (Stejneger 1883: 457). Thus by applying Seebohm'sdefinition, Stejnegerfelt obliged to separateleucotis from the thrushes. Although Entomodesteslater was replacedin the thrushes (Ridgway 1904, Ripley 1964), Myadesteswas not again consideredto be a relative of the silky flycatchers. The Seebohmdefinition, supportedby Baird, Coues,Stejneger and others,precluded the associationwith the thrushesof a specieswith a scutellatetarsus. But that a relationshipexisted between the waxwingsand silky flycatcherswas not challenged,and they continued to be placedtogether. Lucas (1894) noted that Phainopeplaand Ampelis (= Bombycilla) are alike "in the palatal regionand both possessa large, free, swollen lachrymal,this last beinga point of muchimportance, since such a lachry- mal is of rare occurrenceamong birds." According to Beecher (1953) freelachrymals are alsopresent in (at least) somecorvids (Cissa), Oriolus, Paradisaea, Ailuroedus, Callaeus, Pericrocotus, .Cracticus, Artamus, Laniarius, Vanga, Prionops,Aegithina, and Sitta. Lucas (1894) alsofound that "the quadratesof Ampelisand Phainopepla agreewith eachother in minute as well as generalcharacters, as do also the pneumatic maxillo palatines." This last point is of special interest becausethe thrushes,including Myadestes, also have maxillo-palafines with inflated ("pneumatic") tips. Lucas noted further that Phainopepla and the waxwingsagree in the general contour of the dorsal tract, but this has little or no taxonomicsignificance for Mary H. Clench tells me that nothing in their pterylosesdistinguishes between thrushes, wax- wings, silky flycatchers,and several other passerinegroups. Lucas (1894: 310) also stated that "the skull of Myadestesis rather short,and on its superioraspect bears a considerableresemblance to that of Ampelis." It follows that the skulls of Myadestes and Phainopepla shouldbe similar,and examinationshows that they are. It is curiousthat Lucas stated (p. 311) that "Myadestes . . . has a flat non-pneumatic maxillo-palatine." This is certainly incorrect for I have examined these bonesin Myadestes,Phainopepla, and Bombycillaand all three have an inflated,hollow, i.e. "pneumatic"tip. Thoseof Myadestesand Phainopepla are more alike than either is like that of Bombycilla. It is possiblethat Lucas' specimenof Myadestes was damagedand that this similarity there- fore escapedhis notice. His Figure 10 (p. 311) showsthe palatal region April 1973] Silky FlycatcherRelationships 397

of Myadesteswith the tip of one of the maxillo~palatinesmissing, but the otherseems to be intact. Lucasconcluded (p. 311) that although Myadestes"has someleanings toward the Ampelidaeit seemsto have more decidedaffinities with the thrushes,although it is by no meansa typical ." Ridgway (1904) recognizedthe families Ampelidae,Ptilogonatidae, and Dulidae and consideredthem allied to one another. Concerningthe silky flycatchershe stated(pp. 113-114) that "they are relatedto the Waxwings(family Ampelidae),and have usuallybeen placed with them; but they differ in their roundedwings, with well-developedtenth and short- enedninth primaries,their well-developedrictal bristlesand differentchar- acter of the frontal .Their habits,however, are said to be very similar." The statementabout similar "habits" presumably referred to the fact that both groupseat fruit and insectsand have somewhatsimilar nests. In his diagnosisof the PtilogonatidaeRidgway (1904:113) noted that in adultsof Phainoptila(Salvin 1877) the acrotarsiumis not distinctly scutellateand that "the genusPhainoptila is doubtfullya memberof this group,and as far as the adult is concernedmight easily be referredto the Turdidae without materially affectingthe diagnosisof the latter family; but the younghave the plumageabsolutely plain-colored and the acro- tarsium distinctly scutellate." Ridgwaywas not consistentin his applicationof the Seebohmdefinition of the Turdidae, for he broughtEntomodestes back into that family, in spiteof its havingthe "acrotarsiumdivided ("scutellate") on inner side" (1907: 7) and contraryto Stejneger'sargument that it is a ptilogonatid. Ridgwayplaced next to Myadestesin his key to the genera of AmericanTurdidae and distinguishedbetween them on the basis of bill length and tarsal scutellation. Ridgway (1907: 1) also expresseddoubt about the taxonomicvalue of the spottingin the juvenal plumageof thrushes,citing severalexamples of speciesin which the spottingis indistinct or nearly absent. One of his exampleswas Zeledonia,which has no trace of spottingin the juvenal plumage,but in this he was not citing an exceptionwithin the Turdidae for as I have shown(Sibley 1968) Zeledoniais actually a woodwarbler (Parulini) that had beenmistakenly assigned to the Turdidae. Ridgway's point was that the diagnosticvalue of the spottingin the juvenalplumage of thrushesis lessthan had been supposed.He also referred to his earlier (1904) remarksabout Phainoptila(quoted above) as an exampleof a that "might easily be referred to the Turdidae" but which has un- spotted young. Since1907 the taxonomicdiscussions of the silky flycatchershave con- 398 Ca^•L•s G. SmL•¾ [Auk, Vol. 90 cernedthemselves almost exclusivelywith the questionof their relation- shipsto Bombycillaand Dulus. The possibilitythat they might be related to the thrushes via Myadestes and Entomodestesappears to have been mentionedonly by Hellmayr (1934: 444) who noted that Entomodestes "seemsto be well differentiatedfrom Myadestesby longer bill and scu- tellate inner side of the acrotarsium,and may even prove to belongto the Ptilogonatidae." He thus followed Ridgway but appeasedStejneger with this neutral footnote. Stresemann(1927-34) placedthe silky flycatchersin the Bombycillidae but recognizedthe Dulidae as a separate,but adjacent, family. Wetmore (1930) followedRidgway by recognizingthree separatefamilies and has doneso consistentlythrough the years (1960). He noted (1951: 1, 1960: 20) that "suggestionsfor the union of the Bombycillidae,Ptilogonatidae, and the Dulidae in one family are not substantiatedby examination of the skeleton. Dulus, the , is widely different from the other two, a structural distinction that is further emphasizedby its curious communal nesting habits. The first two seem more closely related but are separatedclearly by charactersfound in the ectethmoid region of the skull, and in the manubrium, to mention only two points that are easily apparent." Arvey (1951) made a comparativestudy of Bombycilla, the silky fly- catchers,and Dulus, basedupon coloration,nesting, food habits, skeleton, and certain soft parts. Arvey tended to explain the differencesbetween the threegroups as due to specializationand concludedthat they are related and shouldbe placedin a singlefamily. He made no comparisonswith Myadestesexcept in tablesof ratiosof the relative lengthsof certain bones. Delacourand Amadon(1949) consideredthe relationshipsof Hypocolius and concludedthat it shouldbe assignedto the Bombycillidaeas a sub- family, with the Ptilogonatinae,and possiblythe Dulinae as additional subfamilies. Mayr and Amadon (1951) adopted this arrangement but noted (p. 23) that "Hypocoliusshares with Eurocephalus,and to a lesser extent with Prionops, the peculiar feature of having the normally un- brokenplates comprising the rear half of the oscininetarsus divided weakly into a number of shieldsor scutes. This might mean that the resemblance of Hypocolius to other bombycillidsis superficial. Another possibility is that the Bombycillidaebelong in the general vicinity of the Prionopidae and related families. We here tentatively follow the latter alternative." In a later paper Amadon (1956) expresseduncertainty about the relation- ship of Hypocolius to the waxwings. Ripley (1952) includedthe two speciesof Entomodestesin Myiadestes April 1973] Silky Flycatcher Relationships 399

[sic] but in a later (1964) treatmentrecognized both generaand placed them next to one another. Beecher (1953) interpreted the patterns of the jaw musclesas sup- porting a relationship among the waxwings, silky flycatchers, palm chat, and Hypocolius, and between them and the Campephagidaeand Pycnonotidae. Greenway(1960) expresseddoubt about an alliancebetween Bombycilla and the silky flycatchersby noting (p. 371, footnote)that "actual relation- ship of this group to Bombycilla has not been proved." Similar skeptical footnotesabout Hypocoliusand Dulus alsowere expressed(p. 373). Skutch (1965) studiedthe nestingof Ptilogonyscaudatus in Costa Rica and presented a useful comparative summary of the life histories of Ptilogonys,Phainopepla, Bombycilla, and Hypocolius, with some notes on Phainoptila,the nest of which was then unknown. All of thesespecies feedon small fruits and flying insects.Only Phainopeplahas a conspicuous, melodioussong, but voice seemsto play a minor role in the biology of the group. The nest in all speciesis an open cup placed in a tree or bush. The male helps to build, but his participation varies. The eggsof all are generally similar, being mostly grayish with darker markings of lilac, brown,and black. The young hatch naked in Bombycilla and have pinkish skin. In Phainopeplaand Ptilogonysthe young have white down (especiallylong in Phainopepla)and duskyskin. Skutchnotes (p. 424) that "a peculiarity of both Ptilogonys and Phainopepla is that feathered nestlings make excursionsthrough nearby branchesand return to the nest, before they finally sever contact with it." Skutch concluded that the resemblances between these birds are more numerousthan the differences,and that (p. 425) "so far as their life historiesare now known, they provide no reasonfor classifyingthese birds in separate families." Skutch notes (p. 425) that Phainoptila contrastswith the others in "its lack of a crest, thrush-like aspect,slight sociability,and forest hab- itat...one who has watched this bird in the field feels the strength of Ridgway's (1904: 113) remark that 'the genusPhainoptila is doubtfully a memberof this group...and might easilybe referredto the Turdidae'" (see complete quotation above). In 1972 Lloyd F. Kiff found the first known of Phainoptila in Costa Rica. He reports (in litt.) that the nest and the egg colors and markings are "like those of Phainopepla and Ptilogonys." The adult plumagecoloration of Phainopeplaseems to differ from that of Myadestes,but the female plumageof Phainopeplais similar to that 400 Ca^RL•S G. S•BL•Y [Auk, Vol. 90 of M. townsendi. Furthermore the plumage of the black , Ento- modestescoracinus, is similar to that of the male of Phainopeplabeing lustrousjet black with areasof white in the wings, tail, cheeksand breast. The shape and proportionsof the bills in Myadestes and Phainopepla are essentially identical. The juvenal plumage of Myadestes is like that of the typical thrushes in havingbuff spotson the body feathersand wing coverts,but Phainopepla has an unspottedjuvenal plumage. In both genera the postjuvenal body molt tends to be complete,but in Phainopepla about 50 percent of the individuals also replace all or most of the remigesand rectrices (Miller 1933). Ames (MS) has found that most of the genera currently placed in the Turdinae and Muscicapinaeshare a pattern of the syringeal musculature that he calls the "turdine," as opposedto the "generalizedoscine" pattern found in most other memberso.f the Passeres.Among the exceptionsAmes found is Myadesteswhich, like Phainopepla,has the generalizedoscine pattern, not the turdine arrangement.Entomodestes, however, has the turdine syringealpattern. Ames further informs me (in litt.) that the syringesof Phainopeplaand Bombycillaare muchalike and that the syrinx of Myadestesis morelike that of Phainopeplathan that of Turdus. The syrinx of Ptilogonysis like that of Phainopepla,but Phainoptila is "more robust in the syrinx than are the other ptilogonatids." Ames' findingsthus suggestan allianceamong the ptilogonatids,Myadestes and Bombycilla,but not betweenthese and Entomodestes. The syringeal charactersprovide an interesting comparisonwith the tarsal scutellationpatterns. Myadestes,which has a bootedtarsus, agrees in its generalsyringeal characters with Phainopepla,which has a scutellate tarsus,and Entomodestes,which resemblesboth Myadestesand Phaino- pepla in severalways, has a typical thrush syrinx but also.has. faintly scutellatetarsi. And Phainoptila, which is closelyrelated to. Phainopepla, has a bootedtarsus and a thrushlikeappearance but a "generalizedoscine" syrinx. The significanceof the syringealpatterns is not yet clear but the taxo- nomic value of tarsal scutellationpatterns is certainly low and has been questionedmany times, notably by Pycraft (1906), Blaszyk (1935), Plotnick and Pergolani de Costa (1955), Rand (1959), and Ames et al. (1968). In summary: From 1824 until 1866 the silky flycatchersand solitaires (Myadestes) were often consideredto be related becauseof their external similarities.Following Baird's transfer of Myadestesto theTurdidae, the silky flycatchers,waxwings and, sometimes,the Palm Chat (Dulus) and April 1973] Silky Flycatcher Relationships 401

the genusHypocolius have been placed together or in separatebut adjacent families.A possiblerelationship between the silky flycatchersand Mya- desteshas been forgotten or ignoredbecause of relianceupon the combina- tion of bootedtarsi and spottedjuvenal plumageto define the Turdidae. The two speciesof the SouthAmerican genus Entomodestes, placed in the Ptilogonatidaeby Stejnegerbecause of having faintly scutellatetarsi, were returnedto the Turdidae, next to Myadestes,by Ridgway and later authors. Ripley (1952) mergedthe two genera,later (1964) separated them again. In external appearanceand skeletal charactersPhainopepla and Myadestesare extremely similar and also similar to Bombycilla. Phainopeplaand Myadesteshave similar syringes and similarfood habits. Phainopeplaand Myadestesdiffer in tarsal scutellationand juvenalplum- agespotting, but Phainoptila,which seems closely related to Phainopepla and Ptilogonys,has a bootedtarsus and hasbeen considered to be thrush- like in appearanceand behavior. Entomodestesshares characterswith both the thrushesand the silky flycatchers.

THE EGG-WHITE PROTEIN EVIDENCE

In a comparativestudy of passerineegg-white proteins using starch gelelectrophoresis (Sibley 1970) I foundthat Phainopeplaand Bombycilla had similar patterns,but that Dulus differed enoughto cast doubt on a closerelationship between it and the other two genera.Although Phaino- peplawas not madethe subjectof a specialcomparison with the thrushes, it clearly sharesa commonpattern with them. Myadesteswas not illus- trated in this paper but its starch gel pattern agreeswith that of the thrushes and with Phainopepla. The limitationsof the starchgel techniquedid not provide sufficient resolutionof the egg-whiteproteins to make detailed comparisons,but I am now able to present data based upon the electrophoreticmethod of isoelectricfocusing in acrylamidegel (abbreviatedIFAG). The technique wasdescribed by Sibleyand Frelin (1972) and usedin a studyof Opistho- comusby Sibleyand Ahlquist (1973). With IFAG it is possibleto resolve from 20 to 30 protein bandsin most passerineegg-white spedmens, com- pared with fewer than 10 in starch gel. The IFAG techniqueseparates proteinson the basis of their isoelectricproperties and it is possibleto examinelimited portionsof the pH spectrum,thus achievinga "magnified" view of those proteins that are isoelectricwithin the chosenpH range. This hasbeen donefor the pH rangesof 6-4 and 8-3 in the presentstudy, as well as for the completerange of pH 3-10. The numerical direction in each case indicatesthat the samplewas applied at the pH indicated by the first value, and thus the movementof proteins took place from 402 CHARLESG. SX•LEY [Auk, Vol. 90

MYADESTESOBSCURUS

PHAINOPTILAHELANOXANTHA

PHAINOPEPLANITENS

MYADESTESOBSCURUS

BO•YCILLA CEDRORUM

PHAINOPEPLANITENS

•- DI]LUSDOmINICIlS

BOI•BYCILLACEDRORUI•

PH 6,0 PH q,O

Figure 1. The egg-white proteins of the silky flycatchersMyadestes, Bombycilla, and Dulus comparedusing the techniqueof isoelectricfocusing in acrylamidegel with an Ampholinerange of pH 6-4, gel A-1202. See text for description. that point towardthe secondpH value during the periodof migration to the isoelectricpoint of eachprotein. The reasonfor usinga limitedpH rangeis to examinea particularset of proteinsin greater detail. For examplepH 6-4 (or 4-6) coversthe region in which the ovalbumin fractionor fractionsoccur. This regiontends to containa largepercentage of theproteins in arian eggwhite and experience with some10,000 samples has shownthem to be especiallyinformative for taxonomiccomparisons. The directionof migration(i.e. 6-4 or 4-6) is dictatedby technicalcon- siderationsthat makethe 6-4 directionpreferable. All criticalcomparisons are madeonly amongthe eightsamples in a givengel, thus insuringthat all have beenexposed to identicalconditions during analysis. In all comparisonsusing pH rangesof 3-10, 6-4, and 8-3, Phainopepla agreesin detailwith Myadestes.The 6-4 patternsare the mostinformative and are the onlyones illustrated in thispaper (Figures 1-3). At least 15 proteinbands can be seenin the originalgels (somemay be lost in the April 1973] Silky Flycatcher Relationships 403

DULUS DOMINICUS

BOMBYCILLACEDRORUH

]1 PHAINOPEPLANITENS

HYADESTESOBSCURUS

" RALLOIDE$

PHOENICURUSPHOENICURUS

• • TURDUSHIGRATORIU$

PH 6.0 PH 4.0

Figure 2. The egg-white proteins of Dulus, Bornbycilla, Phainopepla, Myadestes, and two thrushes. Conditions as in Figure 1. Gel A-1203. See text for description. photographicreproduction) and the two generaagree in the positionsof all of them. Phainoptila also agrees with Phainopepla and Myadestes althoughone protein near pH 6.5 has a slightly different position. When the comparisonsare extended to various thrushesboth similarities and differencesare apparent. It shouldbe noted that all passerinesseem to possesscertain of the proteinsin this region, that is, all have homologs of the samebasic set. Thus it is usually possibleto identify the homol- ogousproteins when comparing different species. The differences among a closelyrelated group of speciesthus take the form of differing degrees of agreementin isoelectricpoints, which presumably reflect structural differencesin amino acid sequenceand, hence, genetic differences. At the magnificationachieved with the pH 6-4 rangeit is reasonableto assume that even the smallestdifference in the isoelectricpoints of two homol- ogousproteins is due to a differenceof at least one amino acid and, therefore,of at least one nucleotideat the DNA level. Thus when two speciesare identical in the isoelectricpoints of 15 proteinsit is reasonable to assumethat they are closelyrelated, providing there is no important conflicting evidence. More detailed comparisonsof the pH 6-4 IFAG patterns follow: In 404 CHARLESG. SIBLEY [Auk, VoL 90

CORVUS•RACHYRI'IYNCHOS

RIMUS POLYGLOI-rOS

PHAINOPEPLANITENS

MYADESTESO•SCURUS

PARUSATRICAPILLUS

ICTERUSGALBULA

TROGLODYTESAEDON

LANIUSLUDOVICIANUS

pl.I 6,0 pl.Iq,O Figure 3. The egg-white proteins of several passerinegenera compared with those of Phainopepla and Myadestes. Conditions as in Figure 1. Gel A-1204. See text for description.

Figure 1 at least nine protein bands can be seen in Phainopepla and Myadestesobscurus and they are essentiallyidentical. In gel A-803 (not illustrated) 13 protein bands can be seen in the pI-I 6-4 region and the two generaare identicalin all of them, that is, the isoelectricpoints and quantities of each pair of homologousproteins correspondin the two genera. Phainoptila,in Figure 1, showsclose agreement with Phainopepla, althoughat least two proteins seem to have slightly different isoelectric points from their homologsin Phainopepla. The pattern of Bombycilladiffers strikingly from thoseof Phainopepla, Phainoptila, and Myadestes. The well-resolvedproteins near pI-I 6 do not have obviouscounterparts in Phainopeplaand the remainderof the pattern in the pI-I 5-4 regionlacks well-defined bands in all of the many gels that have been studied. In gel A-757, pI-I 3-10 (not illustrated), Bombycilla and Phainopepladiffer in the way reflected in the pI-I 6-4 April 1973] Sill•y Flycatcher Relationships 405

patterns, that is, the major bands in Phainopeplaoccur near pH 5 but in Bombycilla they are near pH 6. These differencesare real, but in somegels (A-755, A-803, A-814) it is possibleto see that the two genera do have severalproteins with identical or similar isoelectricpoints in the pH 5-4 region. I concludethat Phainopeplaand Myadestesare more alike than Phainopeplaand Bombycilla. The pattern of Dulus is similar to that of Phainopeplain the pH 5-4 region,but less so near pH 6, although some faint bands in the Dulus pattern do have strong counterpartsin Phainopepla. In pH 3-10 gels (e.g. A-757) Dulus has a clusterof strongbands near pH 4, Phainopepla near pH 5, and Bombycillanear pH 6, thus reflectingthe situationsin their pH 6-4 patterns. All three generahave the same set of homologous proteinsin the pH 5-6 regionbut so also do certain other passetines,as illustratedin Figure 3. In this gel, .Corvus,Mimus, Troglodytes,Lanius, Phainopepla,and Myadestesare similaror identicalin the pI-I 5-4 region, but only the latter two alsoagree in the pH 6-5 sectionof the gel. Parus and Icterus differ from the others and from one another. In Figure 2 the patternsof the two speciesof Myadestesare more like that of Phainopepla than like those of Phoenicurus and Turdus. It is also clear that the two speciesof Myadestes differ slightly from one another in the isoelectric points of some proteins. The faint bands next to strong ones in two positionsin the M. ralloides and Phoenicuruspatterns may be due to geneticpolymorphisms and may be ignoredfor our presentconsiderations. I concludethat Myadestesis more like Phainopeplathan it is like Turdus or Phoenicurus. The patterns of other thrushes, including Erithacus rubecula,Cossypha caffra, C. heuglini,P. phoenicurus,Saxicola caprata, S. torquata,Myiophoneus horsfieldii, Zoothera citrina, Catharus fuscescens, Hylocichla mustelina,Platycichla •lavipes, Turdus pilaris, and Turdus leucomelasalso have been used in the comparisons.All show somesimi- larities to Myadestes,but none is as similar to it as is Phainopepla.

CONCLUSIONS

From thesecomparisons, and in the light of the other evidencecited above, I concludeas follows: 1. The silky flycatchers, Phainopepla, Ptilogonys, and Phainoptila, are moreclosely related to Myadestesthan to any other genus(or genera). 2. The relationshipbetween the silky flycatchersand the waxwingsis unclearand more evidenceis neededbefore any conclusionis made. 3. Similarly the relationshipsof Dulus and I-Iypocoliusremain to be determined beyond question. 406 C•IARLESG. SIBLE¾ [Auk, Vol. 90

I)ISCUSSION

The resultsof this study are several. That the silky flycatchersare closelyrelated to the solitairesseems clear. But the implicationsof the discoverythat the silky flycatchersare more closelyallied to Myadestes than Myadestesis to most (or all?) typical thrushes,is the mostsignif- icant result for it forces a reevaluation of certain taxonomic characters and of the definitionof the boundariesof one or more passerinegroups. For nearly a century the Turdidae have been defined on the basis of the combinationof charactersfirst clearly enunciatedby Seebohm(1881). Stejneger (1883: 450) expressedhis approval in words that have been consideredacceptable to the presentday. "The definition of the group Turdidae... givenby Mr. Seebohm,seems to be a very properone, and I think he has therein expressedthe only chief characterwhich really in- dicatesthe relationshipof the birds to be includedin this family. The peculiar spottedfirst plumageof the Turdidae is a very striking feature, and its coincidencewith booted tarsi very remarkable. A careful compar- isonwith forms,which, without showingthese characters, have at different timesbeen referred to the Turdidae,will convinceus that the limits traced by Mr. Seebohmare the only reliableones, and that the family thus defined is a very natural group,and, indeed,one of the bestamong the Passeres.... It is not difficult to foreseethat his definitionof the family will be heartily accepted by ornithologists." Stejneger proved to be a good prophet for Seebohm'sdefinition was "heartily accepted." Ridgway (1907: 3) gave it his approval and the definitionhas been used to includeor excludespecies from the thrushes ever since. But was it ever precisely definitive? It was not. Seebohm himselfnoted that the "very young birds of one or two species"might showevidence of tarsalscutellation and Ridgway (1907: 1) calledattention to somespecies in which the spottingof the juvenal plumageis indistinct or almost absent. The definition held up becauseit includes over 300 speciesand the correlation between plumage and tarsal charactersis remarkably constant for such a large group. But eachof thesecharacters is foundin other groups.A bootedtarsus, or a tendencytoward that condition,occurs in Pitta, in somegenera of the Formicariidae(Gymnopithys, Phaenostictus, Hylophylax), and in the Acanthisittidae,Cracticidae, Grallinidae, Ptilonorhynchidae, Paradiseidae, some Corvidae, Callaeidae, and some Dicaeidae. The booted tarsus in Cinclus,some sylviids, and a tendencytoward the conditionin Prunella have been usedas evidenceof a relationshipbetween these birds and the thrushes. A spottedjuvenal plumageis usuallypresent in the Muscicapidaeand April 1973] Silky Flycatcher Relationships 407 has beenused as evidenceof a relationshipto the thrushes,although the muscicapidshave scutellatetarsi. The membersof the thrush genus Myiophoneusare only slightly spottedor unspottedas juvenilesand the Sylviidae,often consideredto be closerelatives of the Turdidae, are un- spottedin the juvenal plumage. The adaptivesignificance of bootedtarsi is not easy to discernbecause specieswith a great variety of habits possessthe condition. It is this apparentlack of a clear functionalrelationship that has made this char- acter so resistant to criticism. If there is a single adaptive correlation between a smooth tarsus and some environmental or behavioral selective pressureit has not yet been identified. A detailed study of the question may revealthat there are actually severaltypes of bootedtarsi, eachcor- related with a different sourceof selection,or that all are the result of a singleselective force and thus similar by convergence. Somewhatthe sameargument can be presentedin relation to the spotted juvenal plumagesof thrushesand Old World flycatchers. Presumably juvenal plumage colorationperforms at least two functions. It is pro- tective by beingconcealing, and it relievesthe youngbirds from the attacks of territory-holdingadults by lacking the "signal" charactersthat elicit hostile responses.In some sexuallydimorphic groups (but not in the thrushes)it is convergentlysimilar to the adult female plumagefor these samereasons. Any pattern that will satisfythese or additionalrequirements of selectionwill serve,and a variety of plumagepatterns have evolvedin differentgroups. The particulartype of spottingshared by most thrushes and many muscicapidsis a significant character becausesuch spotting is rare outsidethese groups. They are thereforeoften believedto be related, whereasspecies with streaked juvenal ,which occur in many groupsof ,are not. The resolution of these questionslies outside the present paper, but Seebohm's definition of the thrushes must either be modified to include the silky flycatchersor to excludethe genusMyadestes. Either solution breachesthe definitionand castsdoubt on the validity of the Turdidae as presently understood.

RECOMMENDATIONS

At least two taxonomic treatments might be employed to reflect the relationshipsrevealed by this study. Myadestescould be includedin the Ptilogonatidae,or the silky flycatchersplus Myadestes (and perhaps Ento•nodestes)could be placed in a subfamily (Ptilogoatinae) in the Turdidae. Becauseso many facets of the larger problemsinvolving the thrushes, 408 CItARLES G. S•rE¾ [Auk, Vol. 90 muscicapids,and their real or assumedallies remain unclearI prefer a treatment that, for the moment, disturbs the present classificationleast and yet expressesthe close relationshipbetween Myadestes and the silky flycatchers.I thereforerecommend that Wetmore's(1960) arrangement be modifiedby includingMyadestes, and perhapsEntomodestes, in the Ptilogonatidaeand that this family be placed next to the Turdidae. At this time I prefer to reservejudgment on the relationshipsof Bombycilla, Hypocolius and Dulus.

ACKNOWLEDGMENTS

The specimensused in this study were made available by the generoushelp of the followingpersons: J. E. Ahlquist, C. Best, R. Clapp, C. T. Collins, K. W. Corbin, W. R. J. Dean, A. and D. Dod, A. Garza de Leon, D. Gates, J. D. Goodall, D. A. Humphrey, H. L. Jones,L. F. Kill, E. Klaas, W. Makatsch,E. S. Morton, A. Navarro, D.C. Neville, T. B. Oatley, C. M. Perrins,R. Pickering,N. Proctor,A.M. Rea, J. C. Robjent, S. Rothstein, A. Ruschi, A. and L. Schifferli, F. Skou, J. A. Spendelow, G. Spitzer, W. R. Spofford. P. L. Ames, Mary H. Clench, L. F. Kill, and K. C. Parkes provided helpful sug- gestionsand information and J. E. Ahlquist gave his advice and assistance.Dorothy J. Moore carried out the laboratory proceduresand F. C. Sibley provided technical assistance. The National ScienceFoundation supported the study under grant GB-6192X.

SUmmArY

From the time of the discoveryof Ptilogonysc{n½rcus in 1824until 1866, the silky flycatchers(Pti]ogonatidae) and the genusMyad½st½s were con- sideredto be related to one anotherand to the waxwings(Bombycilla). Since 1866 Myad½st½shas been placed in the Turdidae and the other genera have been placed together or in adjacent taxa. Data from com- parative studiesof the egg-whiteproteins, using the e]ectrophoretictech- nique of isoe]ectricfocusing in acry]amidegel, indicatethat Myad½stes and the silky flycatchersactually are closelyrelated. Evidence from other sourcesalso supportsthis conclusion.It is recommendedthat Myad½$t½$ and, perhaps, Entomodestes,be transferred to the Ptilogonatidae and that this family be placed next to the Turdidae.

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Peabody Museum of Natural History, Yale University, New Haven, Connecticut 06520. Accepted 19 December 1972.