(SUIRIRI SUIRIRI) COMPLEX

Transcription

1 The Auk 118(2): , 2001 GEOGRAPHIC VARIATION, HYBRIDIZATION, AND THE LEAPFROG PATTERN OF EVOLUTION IN THE SUIRIRI FLYCATCHER (SUIRIRI SUIRIRI) COMPLEX FLOYD E. HAYES 1 Department f Bilgy, Caribbean Unin Cllege, P.O. Bx 175, Prt f Spain, Trinidad and Tbag ABSTRACT.--The Suiriri Flycatcher (Suiriri suiriri) f Suth America is represented by three distinct frms ccurring in parapatry: (1) S.s. suiriri t the suthwest in the Chac/Pampas; (2) S. lcated centrally in the Cerrad/suthern Amaznia; and (3) S.s. bahiae t the nrtheast in the Caatinga. On the basis f an analysis f 366 specimens f S. suiriri, I fund meager evidence fr lng-distance migratin, little supprt fr Bergmann's rule, and mre supprt fr Glger's rule. I pstulate that an ancestral ppulatin f S. suiriri split int three islated ppulatins, with the central ppulatin differentiating mst rapidly int affinis, thus explaining the leapfrg pattern f greater similarity between peripheral suiriri and bahiae. After secndary cntact, affinis freely hybridized with nminate suiriri in a hybrid zne t the suthwest, where specimens demnstrate mrphmetric intermediacy and increased plumage variability; the rarity f parental phentypes within the hybrid zne suggests that the tw frms are cnspecific accrding t the bilgical species cncept. In the nrtheast, affinis may have hybridized with a remnant ppulatin f suiriri, pssibly represented by prly knwn bahiae. The intermediate size and increased plumage variability f bahiae resemble that f suiriri x affinis hybrids, supprting a hypthesis f hybrid rigin fr bahiae, but alternative hyptheses cannt be ruled ut. This hypthesis f differentiatin is supprted by the cncrdant patterns f disjunctin amng several pairs f sister taxa f bird species that ccur in the Chac and Caatinga, with n intervening ppulatins in the Cerrad, implying a shared histrical prcess f vicarlance. Genetic and behaviral studies are needed t elucidate further the status and histry f differentiatin within S. suiriri. Received 31 January 2000, accepted 13 December EVOLUTIONARY BIOLOGISTS have lng recg- terbreeding between ppulatins in secndary nized the imprtance f dcumenting ge- cntact" (Sibley and Shrt 1964:148), is a gegraphic variatin in rganisms (e.g. Mayr 1963, netic phenmenn that is widespread in birds 1970; Guld and Jhnstn 1972, Zink and Rem- (Grant and Grant 1992) and can be inferred sen 1986). The data derived frm such studies phentypically. Schueler and Rising (1976:284) are crucial fr testing hyptheses f phylge- prvided evidence that "an increase in varinetic relatinships and speciatin prcesses ability and intermediacy in cncert cnstitute amng sister taxa. Analyses f the nature f the nly phenetic evidence fr hybridizatin." cntact znes between sister taxa are imprtant The Suiriri Flycatcher (Suiriri suiriri), a mefr evaluating the extent f gene flw and the dium-sized species f the avian family Tyrandevelpment f islating mechanisms between nidae, is widely distributed t the east f the differentiating ppulatins (e.g. Remingtn Andes in Suth America (Shrt 1975, Ridgely 1968, Shrt 1969, Wdruff 1973, Schueler and and Tudr 1994). As with many Netrpical Rising 1976, Mre 1977, Bartn and Hewitt birds, little is knwn abut its natural histry 1985a, b, Harrisn 1990). Where znes f phe- (e.g. Sick 1984, Beltn 1985, Ridgely and Tudr ntypic intermediacy ccur between sister 1994, Zimmer et al. 2001). It is an uncmmn taxa, it is particularly imprtant t distinguish resident f relatively dry, pen frest and frest between primary intergradatin and secndary edges, usually ccurring alne r in pairs. As intergradatin. Hybridizatin, defined as "in- with mst flycatchers, the sexes are mnchrmatic. Althugh altitudinal migratin appar- Present address: Unit f Zlgy, Department f ently ccurs (Chesser 1997), it is nt knwn t Life Sciences, University f the West Indies, St. Au- be a lng-distance latitudinal migrant (Zimmer gustine, Trinidad and Tbag flyd_hayes@ 1955). The species is represented by three dishtmail.cm tinct races: (1) nminate S.s. suiriri f suthern 457

2 458 FLOYD E. HAYES [Auk, Vl. 118 Blivia, suthern Brazil, Paraguay, Uruguay, and nrthern Argentina; (2) S. f suthern Surinam, central and eastern Brazil, and eastern Blivia; and (3) S.s. bahiae in a small area f the Caatinga in eastern Brazil. Thus far there have been n attempts at rigrusly examining mrphlgical and gegraphic variatin within the species. The purpses f this study are t evaluate the relatinships between the three frms f S. suiriri by examining (1) gegraphic variatn f mrphmetric and plumage characters, (2) sexual dimrphism, (3) the character and extent f hybridizatin between frms, and (4) the strength f crrelatin between mensural and envirnmental variables. The results f these analyses are further used t (1) evaluate quantitatively Bergmann's rule, which predicts that bdy size is inversely crrelated with temperature and humidity (James 1970, Zink and Remsen 1986); (2) evaluate qualitatively Glger's rule, which predicts that ppulatins in mre humid areas are mre heavily pigmented than thse in drier areas (Zink and Remsen 1986); and (3) infer the evlutinary histry f the three frms. TAXONOMIC BACKGROUND In an early study f the taxnmic status f S. suiriri, Cry and Hellmayr (1927) recgnized nminate suiriri and affinis as distinct species, and cnsidered bahiae as a subspecies f affinis. The tw "species" appeared t be allpatric until Laubmann (1940) reprted five presumed hybrid specimens f suiriri x affinis frm Departament (hereafter Dpt.) Cncepci6n, nrtheastern Paraguay, and tw specimens frm the same area that appeared t represent typical affinis. Zimmer (1955) subsequently examined a large series f suiriri at the American Museum f Natural Histry (AMNH) and reprted 17 suiriri x affinis hybrids frm the same area in nrtheastern Paraguay, plus an- ther frm Campanari, Mat Grss d Sul, Brazil. Because f the variable and intermediate Campanari, and reprted them t be as described by Zimmer (1955). Traylr (1979, 1982) cncurred with Zimmer (1955) in recgnizing suiriri, affinis, and bahiae as three races f a plytypic species. Nevertheless, subsequent authrities differed in their taxnmic treat- ment f S. suiriri. Remsen and Ridgely (1980) and Sibley and Mnre (1990) recgnized tw species. Remsen and Traylr (1983, 1989), Ridgely and Tudr (1994), and Hayes (1995) recgnized a single species. The suthern race, nminate suiriri, has upperparts ranging frm dark gray t dark live, a whitish belly, a dark, pale-tipped tail, and a relatively shrt and black bill. The nrthern race, affinis, differs markedly frm suiriri by having a bright yellw belly, paler gray r live upperparts cntrasting with a whitish r yellwish rump extending nt the rectrices, a lnger bill that is ften partially pale, and verall larger size. The tw races are illustrated in Ridgely and Tudr (1994:plate 31). Unfrtunately the behavir and eclgy f thse tw races have nt been studied in detail. Althugh Sick (1984) described their sngs as being markedly different and suggested that they were separate species, Zimmer et al. (2001) pr- vided evidence that their vcalizatins were similar and that Sick's descriptin f affinis was based n bservatins f an unrecgnized cryptic species (see belw). The prly knwn nrtheastern race, bahiae, restricted t the Caatinga f Brazil, has grayish upperparts including a dark rump (as in suiriri), a belly clr allegedly varying frm whitish t bright yellw, and is intermediate in size between suiriri and affinis. Its vcalizatins are apparently similar t thse f suiriri and affinis (Zimmer et al. 2001). Amng a series f affinis specimens at the AMNH, Zimmer (1955) nted that five were remarkably shrt-billed with an unusually wide, pale terminal band n the rectrices. Enigmatically, thse specimens were btained frm lcalities in central Brazil where typical affinis were als taken. Traylr (1982) further nted that thse five specimens and a sixth in the Field Museum f Natural Histry (FMNH), initially described by Hellmayr (1929), pssessed characteristics f the ppulatin in the zne f intergradatin, Zimmer (1955) cnsidered the tw frms cnspecific. Shrt (1975), hwever, failed t find the AMNH hybrid specimens and distinctly brader central rectrices lacking the recgnized suiriri and affinis as distinct species. pale edges f bth suiriri and affinis, and a mre Traylr (1982) relcated the AMNH hybrid gradual transitin frm the pale-gray crwn t specimens frm nrtheastern Paraguay and the pale-live back than in typical affinis. Tray-

3 April 2001] Evlutin in the Suiriri Flycatcher 459 lr (1982) and Ridgely and Tudr (1994) speculated that the shrt-billed specimens represented an undescribed sibling species. Zimmer et al. (2001) prvided mrphlgical, vcal, and behaviral evidence that the shrt-billed frm was sufficiently distinct frm ther Suiriri taxa under any f the widely accepted species cncepts (McKitrick and Zink 1988) t warrant recgnitin f a new species, the Chapada Flycatcher (S. islerrum). Althugh Zimmer (1955) gave the range f measurements fr bill length (frm base) in suiriri and affinis (including shrt-billed affinis), n measurements f bahiae and the intermediate ppulatins have been published. Because f the larger size f nrthern affinis, Shrt (1975) cnsidered the genus t be an exceptin t Bergmann's rule. Hwever, there have been n quantitative analyses f mrphmetric and plumage variatin within the species. METHODS I examined and measured 366 study skins f adult S. suiriri (specimens with evidence f juvenal plumage-speckled feathers n upperparts--were excluded; Wetmre 1926) in American museums (see Acknwledgments). These included 298 specimens f suiriri, 44 specimens f affinis, 4 specimens f bahiae, and 20 specimens f presumed hybrid suiriri x affinis (all with intermediate and variable characters, and taken frm the zne f cntact between the tw Mrphmetric variatin.--using digital calipers, I measured (nearest 0.1 mm) the fllwing mensural variables: bill length (BL), frm nstril t tip f maxilia; wing chrd length (WL), frm bend f flded wing t tip f lngest primary feather; and tarsus length (TS), frm junctin f tibitarsus and tarsmetatarsus t distal junctin f hind te and tarsmetatarsus. Tail length (TL), frm base f tail t tip f lngest rectrix, was measured (nearest millimeter) with a blunt wden ruler t minimize damage t the specimens. I did nt attempt t evaluate mea- surement errr, which undubtedly cntributed t the variatin in the data. I used thse data t cmpute fur dimensinless (scale-free) shape variables. These included bill length/wing length (BL/WL), tarsus length/wing length (TS/WL), and tail length/wing length (TL/ WL). Thse variables were used t evaluate relative shapes, but because ratis are nnlinear, ften intrducing spurius crrelatins (Atchley et al. 1976), they were nt subjected t statistical analyses. Fr each specimen, I als recrded the sex, lcality, date, and mass (MS; nearest 0.1 g) when available, frm the attached specimen labels. The sex was nt identified in 15 specimens f suiriri, 6 f affinis, and 2 f suiriri x affinis; nne f thse specimens was used in the mrphmetric analyses described belw. Tw-sample (Student's) t-tests (Skal and Rhlf 1981) were used t cmpare the means f each men- sural variable between the sexes fr suirir, affinis, and Paraguayan suiriri x affinis hybrids. One-way analyses f variance (Skal and Rhlf 1981) were used t cmpare the means f each mensural variable between suiriri, affinis, and the Paraguayan sui- riri x affinis hybrids; a psteriri multiple cmparisn parental phentypes). Additinal data n bahiae were tests were cmputed using t-tests (t statistic) with btained frm Brazilian museums (E. Willis pers. Bnferrni adjustments f the alpha level (Lentner cmm. and published literature) and frm field band Bishp 1986). I did nt attempt t cmpare staservatins by Zimmer et al. (2001) and B. Whitney tistically variables fr specimens f bahiae and hy- (pers. cmm.). brids frm Blivia and Brazil due t the small sample Gegraphic distributin.--the lcality fr each spec- sizes. imen was taken frm the specimen label and its ln- A principal cmpnents (PC) analysis (Skal and gitude, latitude, and elevatin were btained (when Rhlf 1981) was cmputed using the crrelatin maavailable) frm rnithlgical gazetteers fr each trix f raw data fr the fur mensural variables (BL, cuntry (Paynter 1985, 1989, 1992; Paynter and Tray- WL, TS, TL) t prduce individual PC ladings fr lr 1991) r frm the specimen label. Each lcality all 366 specimens f S. suiriri and 11 specimens f S. was pltted n a map. islefrum. PC1 and PC2, lsely interpreted as size Seasnal distributin.--evidence fr lng-distance and shape axes (e.g. Rising and Smers 1989), were migratin, particularly fr the suthern race suiriri, pltted t cmpare multivariate trends amng Suiriri wuld cmplicate an analysis f gegraphic varia- taxa. All statistical analyses were cmputed with tin. T examine the seasnal distributin f S. sui- Statistix 3.1 sftware (Annymus 1990). riri, the lcality fr each specimen (as described abve) and date f cllectin were taken frm the specimen label. A bivariate plt f latitude and mnth f cllectin was made fr all specimens. An absence f specimens frm the suthern latitudes and a cncentratin f specimens at nrthern latitudes during the austral winter (June-August) wuld prvide evidence fr lng-distance migratin. Plumage variatin.--i used a simple "hybrid index" t scre three plumage characters fr all specimens f S. suiriri. I initially used a five- scre hybrid index, but fund it easier t place specimens int a threescre hybrid index. Althugh sme criticize the use f a hybrid index as crude and subjective, independent studies btain essentially identical results (Crbin and Barrwclugh 1977). Back clratin was

4 460 FLOYD E. HAYES [Auk, Vl. 118 affinis \ white-bellied bahiae yellw-bellied bahiae Paraguay hybrids Brazil hybrid Blivia hybrids _ m suiriri ß affinis bahiae. hybrds ' ß. ß ß white-bellied ß I : e * *. O ß O 0 ' _ J FMAM J J ASOND MONTH FIG. 1. Distributin f Suiriri suiriri specimens, including S.s. suiriri x S. hybrids, examined in this study. Fr S. suiriri bahiae, lcalities indicated with numeral 1 refer t specimens examined during this study; lcalities indicated with numeral 2 refer t published specimen recrds and recent sight recrds. scred as 0 if dark grayish r live as in typical suiriri, 2 if light grayish r live as in typical affinis, and 1 if intermediate between the tw taxa. Rump clr was scred as 0 if dark as in typical suiriri, 2 if light as in typical affinis, and 1 if intermediate between the tw taxa. Belly clr was scred as 0 if whitish as in typical suiriri, 1 if light yellw, and 2 if bright yellw as in typical affinis. The three scres were summed as a measure f plumage variability (0 fr typical suiriri, 6 fr typical affinis, and 1-5 fr intermediate between the tw taxa). Gegraphic variatin.--t assess gegraphic variatin in mrphlgical traits, I cmputed Pearsn crrelatin cefficients (r; Skal and Rhlf 1981) fr each mensural variable with latitude, lngitude, and elevatin. This was dne fr each sex f all S. suiriri cmbined and separately fr suiriri and affinis. Because temperature is crrelated with bth latitude and elevatin, a significantly psitive crrelatin between mensural variables and either latitude r ele- vatin wuld prvide supprt fr Bergmann's rule. RESULTS Gegraphic distributin.--the three races f S. suiriri are parapatrically distributed, with the range f each species rughly cinciding with well knwn bigegraphic regins (e.g. Shrt 1975): nminate suiriri ccurs primarily in the FIG. 2. Seasnal variatin in latitudinal distribu- tin f Suiriri suiriri specimens, including S.s. suiriri x S. hybrids, examined in this study. Chac and Pampas, affinis in the Cerrad and suthern Amaznia, and bahiae in the Caatinga (Fig. 1). The ranges f suiriri and affinis meet between the Chac and Cerrad in eastern B- livia, nrtheastern Paraguay and suthwestern Brazil, where hybridizatin apparently ccurs but is well dcumented nly in Paraguay. The ranges f affinis and bahiae meet between the Cerrad and Caatinga, where hybridizatin may ccur, but is prly dcumented. Seasnal distributin.--althugh shrt-distance altitudinal migratin apparently ccurs in nminate suiriri, at least in Blivia (Chesser 1997), there is little evidence fr lng-distance latitudinal migratin (Fig. 2). Specimen recrds frm Argentinian and Uruguayan museums indicate that the suthernmst ppulatins f nminate suiriri remain present thrughut the year (L. Jseph unpubl. data). Mrphmetric variatin.--in nminate suiriri, sexual dimrphism ccurred in all size variables, with males averaging significantly larger (Table 1). In the cnsiderably smaller sample f affinis specimens, females als averaged smaller fr each measure f bdy size (except mass, fr which there was insufficient data), but did nt differ significantly fr BL (Table 1). In bth suiriri and affinis, TL/WL averaged slightly higher fr females than fr males, suggesting that females had relatively shrter WL cm- pared t TL, but n ther shape variables differed appreciably between sexes fr either tax-

5 April 2001] Evlutin in the Suiriri Flycatcher 461

6 462 FLOYD E. HAYES [Auk, Vl. 118 TABLE 2. Descriptive measures f size and shape variables (see Table i fr definitins) fr three hybrid ppulatins f Suiriri s. suiriri x affinis. Statistical cmparisns are given between the Paraguay hybrids and suiriri and affinis fr each sex. N significant sexual dimrphism was fund in the Paraguay hybrid pp- ulatin. Paraguay Blivia Brazil Variable SD Min,Max n SD Min, Max n R n BL c 8.77,c , , , WL c 80.01,c , , , TS c b , , b , TL c d , , b , BL/WL c , , , TS/WL c , , , TL/WL c , , , PC1 C , , , PC2 c , , , Differs frm suiriri f same sex, P < Differs frm suiriri f same sex, P < Differs frm affinis f same sex, P < Differs frm affinis f same sex, P < n (Table 1). Females f bahiae likewise averaged smaller than males fr each size variable (except mass, fr which there was n data) and had a larger value fr TL/WL (Table 1), but the data were insufficient fr statistical cmparisns. Bth sexes f affinis averaged significantly higher fr each size variable (except mass, fr which there was insufficient data) than suiriri (Table 1). The higher TS/WL and TL/WL fr suiriri suggested that WL is relatively shrter fr suiriri than fr affinis. In the small sample f bahiae, the means f BL were similar t thse f affinis, WL and TS averaged slightly lnger than suiriri, TL averaged slightly shrter than suiriri, and TL/WL was intermediate (Table 1). Hybrid suiriri x affinis frm Paraguay shwed n significant sexual dimrphism fr any size r shape variable, but the sample sizes were small (Table 2). Althugh the hybrids tended t be intermediate in size between sui- riri and affinis, the nature f variatin was cmplex. Fr males, BL and WL were intermediate between suiriri and affinis, differing significantly frm bth; TS averaged significantly lnger than suiriri and apprached the mean fr affinis; and TL was significantly shrter than affinis and apprached the mean fr suiriri (Table 2). The female hybrids differed significantly frm suiriri, but nt frm affinis fr all fur size variables; WL was intermediate, thugh clser t affinis, whereas BL, TS, and TL averaged the same r even higher than affinis (Table 2). Fr the three shape variables, males appeared t differ frm suiriri fr TS/WL and TL/WL, whereas females appeared t differ frm suiriri nly fr BL/WL; neither sex differed frm affinis fr any shape variable. The tw Blivian hybrids resembled suiriri in size variables and affinis in shape variables; the Brazilian hybrid resembled affinis in BL, WL, and TL, and suiriri in TS and TS/WL. A bivariate plt (Fig. 3) f PC1 (59.7% f variance) and PC2 (19.4% f variance) separated mst suiriri and affinis specimens. The bahiae specimens appeared t be intermediate between suiriri and affinis. The male suiriri x affinis hybrids frm Paraguay and Brazil appeared intermediate between suiriri and affinis, but the tw males frm Blivia clustered with- in the range f variatin f suiriri. In cntrast, the female suiriri x affinis, all f which were frm Paraguay, clustered within the range f variatin f affinis. The widely scattered speci-

7 April 2001] Evlutin in the Suiriri Flycatcher ca 0 O _ MALES [] Blivia - Brazil --_ 0 / ø- ø% O/1ø _ wh te-belhed ß. ' _ _. -. ß " S.s. suiriri S.s. suiriri x affinis u S.s. bahiae _ es. S. islerrum PC1 - [] [] FEMALES 2 [] - # O O O 1 ' %. øø _..-[] ß? CJ c O - white_bellied// -2 ß yellw:bellied -3 ' I ' I ' i ' i ' I ' i,, i ' PC1 FiG. 3. fi cipa] cmp e [s a a]ysis specimens examined i [his study. mens f S. islerrum tended t cluster mre with affinis than suiriri. Plumage variatin.--the ttal plumage scre was 0 fr all but tw specimens f nminate suiriri, and 6 fr all but tw specimens f affinis (Table 3). Intermediate belly clr ccurred in a specimen f suiriri frm Tucuman, Argentina (AMNH ), anther specimen f suiriri frm Yut, Jujuy, Argentina (Museum f Cmparative Zlgy; MCZ ), and in a female specimen f affinis frm Chapada, Mat O Grss, Brazil (AMNH 33101); a specimen f unknwn sex frm Miranda, Mat Grss d Sul, Brazil (MCZ ), had a nearly-white belly within the nrmal range f variatin f suiriri. Nne f thse specimen shwed ther yellw-bellied indicatins f hybridizatin. Wetmre (1924, 1926) described an additinal yellw-bellied specimen (nt examined by me) frm Tucu- man, Argentina, as a new species (S. imprvisa), but Cry and Hellmayr (1927:443) pinted ut that ther specimens frm the type lcality (e.g. AMNH described abve) varied frm the typical "white" f nminate suiriri t the yellw f imprvisand regarded the type spec- imen f the latter as a mutant suiriri. The fur bahiae specimens examined resembled suiriri in back and rump clr, but had highly variable belly clratin: white in tw specimens and yellw in the ther tw (Table 3, Appendix). The tw white-bellied bahiae specimens, bth taken frm the same lcality, were indistinguishable frm suiriri n the basis f plumage, but BL f bth specimens (male, 9.5 mm; female, 9.0 mm) exceeded the maximum BL f suiriri (9.0 and 8.7 mm fr males and females, respectively; see Table 1). Furthermre, Zimmer (1955) pinted ut that the uter web f the uter rectrix was dull as in affinis and bahiae, in cntrast with the lighter uter web f suiriri. Anther white-bellied specimen at the Museu de Zlgia da Universidade de Sa Paul (MZUSP 8418), a male cllected at Pirapra, Minas Gerais, in August 1912, was regarded as indistinguishable frm suiriri and cnsidered a migrant (Cry and Hellmayr 1927, Pint and TABLE 3. Number f individuals fr each plumage scre fr each taxn r ppulatin f Suiriri suiriri. Back Rump Belly Ttal scre Taxa/ppulatin 0 I 2 0 I 2 0 I 2 0 I S.s. suiriri S I I I I 46 S.s. suiriri x affinis Blivia Paraguay cmbined San Luis de la Sierra ZanjaMrtf Brazil S. s. bahiae

8 464 FLOYD E. HAYES [Auk, Vl. 118 Camarg 1961, Ridgely and Tudr 1994). Zimmer (1955) and Pint and Camarg (1961) pinted ut that a female (MZUSP 8415) cllected frm the same lcality was assigned t affinis (identity recnfirmed by E. Willis, pers. cmm.). Zimmer (1955), wh did nt examine the specimens, queried the identity f the white-bellied bird as suiriri and suggested it was a white-bellied affinis; hwever, the dark rump and back described by Pint and Camar- g (1961) are mre characteristic f bahiae than affinis. The specimen was recently reexamined by E. Willis (pers. cmm.), wh reprted BL t be 9.0 mm, the maximum fr male suiriri (Table 1); ther measurements (WL, 81.5 mm; TS, 20.4 mm; TL, 74.3 mm) were within the range f variatin f male suiriri and affinis. Further- mre, Willis described the specimen's back as "rather live but with rusty r brwnish wash,... mre like affinis than like the few suiriri in the cllectin." This specimen appears t represent a white-bellied bahiae. Other bahiae specimens reprted in the literature (but nt examined by me) were apparently yellw-bellied, including the type specimen frm "Bahia" in the Berlepsch cllectin (Cry and Hellmayr 1927). Hwever, E. Willis (pers. cmm.) reexamined the MZUSP specimens assigned t bahiae by Cry and Hellmayr (1927) and Pint and Camarg (1961), and fund that all specimens, including tw frm Juazeir, Bahia (09ø25'S, 40ø30'W), tw frm Cremas, Paraiba (07ø01'S, 37ø58'W), and tw frm Ibipetuba (= Santa-Rita-de-Cassia), Bahia (11ø00'S, 44ø32'W), were all pale-rumped, apparently representing affinis. I fund that tw Ls Angeles Cunty Museum (LACM) specimens assigned t bahiae, als frm Ibipetuba, Bahia, were als pale-rumped, representing affinis. All recent recrds f bahiae were apparently f yellw-bellied birds. Thse include bservatins by R. Ridgely (pers. cmm.), Zimmer et al. (2001) near Laga Grande, Pernambuc, frm , and B. Whitney (pers. cmm.) at the fllwing lcalities: 20 km suth f Carinhanha, Minas Gerais (14ø17'S, 43ø47'W), n 11 Nvember 1994 (ne bird); 25 km east f Guanamb, Bahia (14ø07'S, 42ø37'W), n 12 Nvember 1994 (fur); and 15 km nrth f Petrlina, Pernambuc (09ø24'S, 40ø30'W), n 21 Nvember 1994 (tw). Twenty specimens taken frm the zne f cntact between suiriri and affinis shwed intermediate and variable plumage characters. The tw hybrids frm eastern Blivia were intermediate in back and rump clr, but belly clr resembled suiriri (Table 3, Appendix). The 17 hybrids frm tw lcalities (in nrth- eastern Paraguay) were cnsistently dark-rumped (as in suiriri), intermediate in back clratin, and highly variable in belly clratin (Table 3, Appendix). The distributin f ttal plumage scres fr the tw Paraguayan lcalities did nt differ significantly (X 2 = 8.02, df = 4, P = 0.09). Traylr (1982) reprted tw additinal hybrid specimens (AMNH) frm Be- 16n, Dpt. Cncepci6n, which is suth f the ther Paraguayan hybrid lcalities; hwever, I fund the Be16n specimens t be indistinguishable frm suiriri (Hayes 1995). The single Brazilian hybrid specimen was intermediate in back clratin, but therwise resembled affinis (Table 3, Appendix). Gegraphic variatin.--significant gegraphic variatin ccurred fr several mensural vari- ables. Amng all S. suiriri specimens cmbined, latitude was negatively crrelated with BL, WL, TS, and TL, and psitively crrelated with MS (but nly ne affinis male in sample) and PC1 (Table 4). Lngitude was negatively crrelated with BL, WL, TS (males nly), PC1, and PC2 (males nly), and psitively crrelated with PC2 (females nly; Table 4). Elevatin was psitively crrelated with WL (females nly), TL, and PC2, and negatively crrelated with WL (males nly, in cntrast with females), MS (males nly), and PC1 (Table 4). Within nminate suiriri, latitude was psitively crrelated with MS and negatively crrelated with TL (females nly) and PC2 (females nly; Table 4). Lngitude was psitively crrelated with TL and PC2 (males nly), and negatively crrelated with MS (males nly) and PC1 (females nly; Table 4). Elevatin was psitively crrelated with WL, TL and PC2, and negatively crrelated with MS (males nly) and PC1 (Table 4). Sample sizes were limited within affinis, especially fr males (Table 4). Fr males, elevatin was psitively crrelated with TS; fr females, latitude was psitively crrelated with WL and TL and negatively crrelated with PC1, and lngitude was psitively crrelated with TL (Table 4).

9 April 2001] Evlutin in the Suiriri Flycatcher 465 TABLE 4. Gegraphic crrelates (r) f bdy size and shape variables (see Table 1 fr definitins) in S. suiriri. Nte that S. suiriri refers t all frms cmbined, including hybrids; sample sizes fr S.s. bahiae and S.s. suiriri x affinis were inadequate fr analysis. See Methds fr definitins f variables. Latitude and lngitude Elevatin Variable Sex/taxn n Lat. Lng. n Elev. BL c S. WL c S. TS c S. TL c S. 2S. MS c S. PC1 c S. PC2 c S. 2S. 2S suiriri s. suiriri su rirl s. suirlri s. suirlrl s. suirm sulrin s. suinrl sulrirl s. suir r su rir s. suinrl su rir s. suir n sulrin s. suirlrl sulrirl s. suinn sulrin s. suinn s. affims sulrir s. suirlrl s. affinls su rirl s. suirlrl s. affinls sulrirl s. suirlrl a a b a a c b b c c c c b a b 0.23 c c 0.48 c c b c b b a c c b c * a c DISCUSSION This study dcuments cnsiderable gegraphic variatin in mrphmetric and plumage traits within S. suiriri. Althugh it remains unclear whether such variatin is attributable t natural selectin, envirnmental inductin, r stchasticity, the gegraphic patterns f variability fr temperature and humidity are well knwn within the range f S. suiriri (e.g. Hffman 1975, Prhaska 1976), and an assessment f their ptential effects n mrphlgical variability is warranted (Zink and Remsen 1986). Temperature is inversely crrelated with bth latitude and elevatin. Because trpical affinis is clearly larger than mre temperate suiriri, Shrt (1975) pinted ut that S. suiriri vilated Bergmann's rule. When all specimens

10 466 FLOYD E. HAYES [Auk, Vl. 118 are cmbined, the negative crrelatin f mst mrphmetric traits with latitude supprts Shrt's (1975) cnclusin. Hwever, within the mre sutherly race suiriri, the psitive crrelatin f MS with latitude supprts Bergmann's rule, thugh the negative crrelatin f MS with elevatin is puzzling. Althugh WL is f- ten used as an index f bdy size fr studies f gegraphic variatin (Zink and Remsen 1986), MS is ften cnsidered t be a better measure f bdy size (e.g. Rising and Smers 1989, Freeman and Jacksn 1990). In the case f suiriri, WL was significantly crrelated with elevatin as nted in Blivian specimens by Traylr (1950), but nt latitude. PC1, ften used as a multivariate measure f bdy size (Rising and Smers 1989), was nt crrelated with latitude and was negatively crrelated with elevatin. The effects f temperature n mrphlgical variatin remain unclear, as it is with many Nrth American species f birds (Zink and Remsen 1986). The peripheral races suiriri and bahiae ccur in drier regins than des affinis. Althugh affinis is generally paler drsally (primarily n rump) than either suiriri r bahiae, the latter races are paler n the underparts and grayer n the upperparts, in general cnfrmity with Glger's rule, which is well supprted amng Nrth American birds (Zink and Remsen 1986). Several histrical prcesses may be pstulated t explain the gegraphic pattern f differentiatin demnstrated by S. suiriri. First, differentiatin may have evlved in parapatry, with current cntact znes between adjacent races cinciding with relatively abrupt envirnmental changes (Endler 1982a, b). In that scenari, each cntact zne wuld represent a "step cline" f primary intergradatin where gene flw is reduced between cntiguus ppulatins that have never been islated. Secnd, differentiatin may have evlved in allpatry fllwing lng-distance dispersal f "funder" individuals acrss pre-existing barriers. And third, differentiatin may have evlved in allpatry as a cntiguus ancestral ppulatin became fragmented by vicariance. Fr bth the secnd and third hyptheses, the current cntact znes wuld represent secndary intergradatin (i.e. hybridizatin) ccurring where the expanding ranges f adjacent ppulatins met. The parapatric differentiatin mdel is supprted by the psitin f current cntact znes cinciding with the bundaries between majr bigegraphic regins (Chac-Cerrad-Caatinga), which may be characterized by relatively abrupt envirnmental changes. Hwever, the bundaries between thse regins may als represent lcatins f previus vicariance events (e.g. Cracraft and Prum 1988). Furthermre, the mrphlgical intermediacy and increased plumage variability f specimens frm the cntact zne between suiriri and affinis in nrtheastern Paraguay prvide cnvincing evidence fr secndary intergradatin (hybridizatin) rather than primary intergradatin (Schueler and Rising 1976). The ccurrence f hybridizatin effectively falsities the parapatric mdel and is cnsistent with bth the dispersal and vicariance mdels f allpatric differentiatin. Lng-distance dispersal is unlikely t ccur amng relatively sedentary taxa, including S. suiriri, fr which there is little evidence f lngdistance migratin. This suggests that the nrtheastern race bahiae, which resembles suthern suiriri mre than affinis n the basis f plumage scres, was unlikely t have been established by lng-distance migrants f suiriri frm suthern Suth America. Because the gegraphic range f affinis ccurs between mrphlgically similar suiriri and bahiae, S. suiriri prvides yet anther example f the leapfrg pattern f gegraphic variatin (Remsen 1984) in which tw similar ppulatins are separated frm each ther by ne r mre different, intervening ppulatins f the same species. Remsen (1984) suggested several hyptheses that may explain such a pattern, including (1) cnvergent evlutin in the phentypically similar but gegraphically separated taxa; (2) mre rapid, divergent evlutin in the central, intervening taxa; (3) centrifugal speciatin; (4) ancient crridrs cnnecting the currently separated but phentypically similar taxa; and (5) randm phentypic changes in gegraphically islated ppulatins f a taxn, with the changes ccurring first--by chance-- in the intervening ppulatin(s). The lack f cncrdance in the ranges f Andean bird taxa exhibiting the leapfrg pattern supprts the fifth hypthesis, suggesting that phentypic changes appeared randmly with respect t gegraphy, and were nt necessarily induced by the envirnment (Remsen 1984). Can this hypthesis explain the gegraphic pattern f differentiatin in S. suiriri?

11 April 2001] Evlutin in the Suiriri Flycatcher 467 Many species f Suth American birds are represented by pairs f clsely related taxa with disjunct ppulatins in the Chac and Caatinga, with essentially n intervening ppulatin in the Cerrad (except perhaps in the periphery; e.g. see distributinal maps f Shrt 1975, Ridgely and Tudr 1989, 1994). Essentially mntypic species with little differentiatin between ppulatins include the Whitebreasted Tinamu (Nthura braquira) and tive mating ccurred. Thus, the tw frms Chty Spinetail (Scheniphylax phryganphi- appear t be cnspecific when the criterin f la). Other species with mre distinct racial differences include (races given in Chac/Caatinga sequence): the Blue-crwned Parakeet (Aratinga acuticaudata acuticaudata /hemmrhus), Rufus Cachalte (Pseudseisura cristata unirufa / reprductive islatin--a hallmark f the bilgical species cncept (e.g. Mayr 1970)--is applied. In the Caatinga, hybridizatin may als have ccurred when affinis and the islated remnant cristata), Stripe-backed Antbird (Myrmrchilus suiriri ppulatin came int cntact, but the sitstrigilatus suspicax/strigilatus), White Mnjita (Xlmis iruper iruper / nivea), Greater Wagtailuatin there remains less clear. Prly knwn bahiae may have shared a mst recent cmmn Tyrant (Stigmatura budytides inznata/gracilis) ancestr with (1) an islated ppulatin f afand Bay-winged Cwbird (Mlthrus badius finis (Fig. 4A), (2) a remnant ppulatin f suibadius/fringillarius). Other examples ccur at the species level. The cncrdance f this disjunct distributin pattern amng species belnging t unrelated taxnmic grups implies a shared histrical prcess f vicariance (e.g. riri (Fig. 4B), r (3) may instead cnstitute a hybrid ppulatin f remnant suiriri x affinis (Fig. 4C). The prximity f affinis and the distant current distributin f suiriri supprt the first hypthesis; if that is true, the shared traits f Cracraft and Prum 1988). Althugh plausible, bahiae and suiriri wuld have evlved indepenlng-distance dispersal f a taxn acrss un- dently. The secnd hypthesis is supprted by favrable Cerrad habitat frm ne side t the the greater resemblance between bahiae and suither is unlikely t ccur in relatively sedentary taxa nt knwn fr lng-distance migratin (e.g. White-breasted Tinamu). Rather, a relatively recent vicariance event r series f events prbably ccurred, which subdivided a cntiguus ancestral ppulatin int islated Chac and Caatinga units. Such an event likely ccurred with S. suiriri. The mst plausible agent fr vicariance was the frmatin f huriri and the cncrdant pattern f disjunct Chac-Caatinga taxa. The intermediate size and increased variability in plumage f the few bahiae specimens available, cmbined with descriptins f seemingly intermediate affinis x bahiae specimens in the regin, supprt the third hypthesis f hybrid rigin fr bahiae. Indeed, the bahiae specimens exhibit a remarkable similarity with the suiriri x affinis hybrids frm Par- mid frest barriers during previus interglacial perids (e.g. Haffer 1985, 1987; Willis 1992). In cnfrmity with the vicariance mdel f differentiatin, I pstulate the fllwing scenari fr differentiatin in S. suiriri. Initially, an ancestral ppulatin f S. suiriri resembling nminate suiriri ccupied the entire arid diagnal frm the Chac t the Caatinga. After ne r mre vicariance events islated Chac, Cerrad, and Caatinga ppulatins, the Cerrad ppulatin differentiated mre rapidly than the tw peripheral islates, eventually acquiring the traits f affinis. As the suthern ppulatin f nminate suiriri spread nrthward and affinis spread suthward, they met and freely hybridized in the absence f islating mechanisms, thus frming a hybrid swarm f individuals exhibiting intermediacy and increased variability. Because the ppulatin frm the hybrid zne in nrtheastern Paraguay appears t be cmpsed entirely f hybrids with few parental phentypes (22 hybrid specimens vs. 2 purprtedly typical affinis; Laubmann 1940, Zimmer 1955), little r n assrta- aguay; that was first nted by Zimmer (1955: 19), wh stated that "sme f the specimens f the intermediate ppulatin f nrtheastern Paraguay almst meet the requirements [f bahiae], but shw, in mst cases, t much f the yellwish basal area f the rectrices t agree fully." If bahiae is indeed a hybrid taxn, as the mrphlgical data suggest, it shuld nt be cnsidered a valid subspecies. Furthermre, it wuld appear t represent a disjunct hybrid ppulatin situated >1,000 km frm the nearest current ppulatin f suiriri, a presumed parental phentype, unless the white-bellied specimens tentatively assigned t bahiae actually represent a remnant suiriri ppulatin be-

12 468 F] Y]::) E. HAYES [Auk, Vl. 118 S. islerrum S. s. suiriri S. S. g. bahiae S. islerrum S. S. s. suiriri S. s. bahiae S. islerrum S. S. s. bahiae S. s. suirid S. islerrum S. S. s. suiriri S. s. bahiae FIG. 4. Hypthetical relatinships amng Suiriri taxa illustrated by fur cladgrams. Nte that S. suiriri is mnphyletic in cladgrams A-C and paraphyletic in cladgram D. cming extinct thrugh cmpetitin r intrgressin frm affinis (e.g. Rhymer and Simberlff 1996). If indeed the white-bellied bahiae specimens represent a remnant suiriri ppulatin, then bahiae may be a distinct taxn less variable than it appears, represented exclusively by yellw-bellied birds. If that hypthesis is crrect, bahiae may have shared a mst recent cmmn ancestr with either an islated ppulatin f affinis r remnant suiriri. Finally, these hyptheses are based n the assumptin f mnphyly within S. suiriri (Fig. 4A-C). The superficial similarity between S. islerrum and S. suggests that they may be sister taxa, in which case S. suiriri wuld be paraphyletic (Fig. 4D; McKitrick and Zink 1988 discuss the implicatins fr species cncepts), but S. islerrum appears behavirally and vcally distinct (Zimmer et al. 2001). Genetic studies are clearly needed t elucidate further the relatinships and histry f differentiatin amng the frms f S. suiriri and S. islerrum. ACKNOWLEDGMENTS I thank J. Bates and D. Sttz fr reviewing the manuscript. This study was funded by grants frm the American Museum f Natural Histry, Field Museum f Natural Histry, Lma Linda University, and the University f the West Indies. Assistance was prvided by C. Griffiths, M. LeCry, L. Shrt, and E Vuilleumier at AMNH; D. Maurer, T. Schulenberg, and D. Willard at FMNH; K. Garrett at the Ls An- geles Cunty Museum; R. Brwning and G. Graves at the Natinal Museum f Natural Histry; and P. Chu, J. Hinshaw, R. Payne, and R. Strer at the University f Michigan Museum f'zlgy. I thank the fllwing individuals fr the lan f specimens frm their institutins: E Gill f the Academy f Natural Sciences f Philadelphia; K. Parkes f the Carnegie Museum; and R. Paynter f the Museum f Cmparative Zlgy. Fr supplying literature, specimen data, recent sight recrds r insightful cnversatin, I thank M. Berres, N. Cllie, T. Gdwin, L. Jseph, R. Prum, J. da Silva, B. Whitney, and E. Willis. LITERATURE CITED ANONYMOUS Statistix Manual. Analytical Sftware, St. Paul, Minnesta. ATCHLEY, W. R., C. t. GASKINS, AND D. ANDERSON Statistical prperties f ratis. I. Empirical results. Systematic Zlgy 25: BARTON, N.H., AND G. H. HEWITT. 1985a. Hybrid znes and speciatin. Pages in Essays n Evlutin and Speciatin in Hnr f M. J. D. White (W. R. Atchley and D. S. Wdruff, Eds.). Cambridge University Press, Cambridge, United Kingdm. BARTON, N.H., AND G. H. HEWITT. 1985b. Analysis f hybrid znes. Annual Review f Eclgy and Systematics 16: BELTON, W Birds f Ri Grande d Sul, Brazil, part 2. Frmicariidae thrugh Crvidae. Bulletin f the American Museum f Natural Histry 180: CHESSER, R. t Patterns f seasnal and gegraphical distributin f austral migrant fly-

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15 April 2001] Evlutin in the Suiriri Flycatcher 471 APPENDIX. Specimen number, sex, lcality, date (1990s), and hybrid index scre fr three plumage clr variables fr all Suiriri suiriri bahiae and hybrid S.s. suiriri x affinis specimens examined in this study. Date Plumage scre Specimen Sex Lcality (D-M-Y) Back Rump Belly S.s. bahiae AMNH ct Bel Jardim, Pernambuc, Brazil AMNH c Gilbfies/Pindal'ba, PiauL Brazil AMNH Gilbfies/Pindal'ba, PiauL Brazil FMNH Queimadas, Bahia, Brazil S.s. suiriri x affinis AMNH AMNH AMNH AMNH AMNH AMNH AMNH AMNH AMNH AMNH AMNH AMNH AMNH AMNH AMNH AMNH AMNH AMNH CM CM Zanja Mrti, Cncepc16n, Paraguay Zanja Mrif, Cncepc 6n, Paraguay Zanja MrtL Cncepc16n, Paraguay Zanja Mrti, Cncepc16n, Paraguay Zanja Mrti, Cncepc16n, Paraguay Zanja Mrif, Cncepc16n, Paraguay Zanja MrtL Cncepc16n, Paraguay San Luis de la Sierra, Cncepci6n, Paraguay San Luis de la Sierra, Cncepc16n, Paraguay San Luis de la Sierra, Cncepc16n, Paraguay San Luis de la Sierra, Cncepc16n, Paraguay San Luis de la Sierra, Cncepcl6n, Paraguay San Luis de la Sierra, Cncepc16n, Paraguay San Luis de la Sierra, Cncepc16n, Paraguay San Luis de la Sierra, Cncepc16n, Paraguay San Luis de la Sierra, Cncepc16n, Paraguay San Luis de la Sierra, Cncepc16n, Paraguay Campan ri, Mat Grss d Sul, Brazil R Quizer, Santa Cruz, Blivia Ri Quizer, Santa Cruz, Blivia