The Condor 89:760-766 0 The Cooper Ornithological Society 1987

MOLTS OF THE AND NEW INTERPRETATIONS OF THE PLUMAGE SEQUENCE ’

PAMELA C. RASMUSSEN Museumof Natural History,University of Kansas,Lawrence, KS 66045

Abstract. The Rock Shag( rnagellanicus) was foundto be similarin molt patternsto othercormorants studied. It waspreviously thought that two predefinitiveplum- agesoccur in this ;this study shows there is only one predefinitive plumage, which is polymorphic and geographicallyvariable. In addition, although the alternate and basic plumageswere thought to be very distinct in aspect,I found that these plumagesare similar, and specimenspreviously thought intermediate between alternate and basic plumage com- bine individual and geographicalvariation. Differences in frequenciesof juvenal plumage phasesand adult plumage variants between Chilean and Argentinian coastal populations suggestevolution in isolation by populations separatedby a vicariance event such as the Llanquihue Glaciation (20,000 to 10,000 ybp). Key words: Rock Shag; Phalacrocorax magellanicus; molts;juvenal plumage;winter plumage;vicariance efects.

INTRODUCTION the Rock Shag.This paper provides data on molts The Rock or Magellanic Shag (Phalacrocorax of this species, documents that what had been magellanicus) is a small inhabiting interpreted as two sequential immature plum- marine coastal regions of southern South Amer- ages is actually geographic variation in pheno- ica and the (Fig. 1). No geo- type of the juvenal plumage, showsthat the win- graphic variation has been described in this ter plumage is not distinct in aspect from the species,and little has been published on its molts breeding plumage, and demonstrates that adults (Murphy 1936). The complex molts of cormo- from the Atlantic coast frequently show a plum- rants are not well-known except for those of the age variant rare on the Pacific coast. , P. aristotelis (Potts 1971) and the Blue-eyed Shag,P. atriceps, including P. “al- METHODS biventer” (Bernstein and Maxson 1981; Ras- Numbers, ageclasses, and localities of Rock Shag mussen, in press).Neither of these speciesis con- specimensexamined for molt and juvenal plum- sidered closely related to the Rock Shag (Voisin age phase are given in Table 1, and localities are 1973; Siegel-Causey 1986). shown in Figure 1. I considered specimensto be The Rock Shagis the only cormorant for which juveniles if no juvenal feathershad been molted; two distinct sequential immature plumages at- subadults when molting from juvenal to first ba- tained by molt have been described (Murphy sic plumage; and adults when completely in de- 1936). Alternate plumages of several cormorant finitive plumage with no retained juvenal feath- speciesdiffer to some degree from basic plum- ers. Because the stepwise molt of ages (e.g., the Long-tailed Cormorant, P. afii- results in of varying agesbeing present cams; the Blue-eyed Shag; the , P. at the same time, the following molt scoring sys- punctatus; and the Red-faced Cormorant, P. tem was used for flight feathers: (0) juvenal; (1) urile; Tuck 1978, Harrison 1983). The Rock Shag missing; (2) emerging; (3) to one-fourth has been considered to show the greatestdiffer- grown; (4) to one-half grown; (5) to nearly full- ence between basic and alternate plumagesof any grown; (6) no wear or fading; (7) little wear or cormorant species. fading; (8) light but obvious wear and fading; (9) Results of field work on austral South Amer- moderate wear and fading; and (10) heavy wear ican cormorants indicate that confusion exists in and fading. Areas of the body plumage examined the literature regarding molts and plumages of for molt were head (nape and upper throat to face); neck; back (from top of mantle to rump); I Received 20 October 1986. Final acceptance 16 venter (from upper breast to vent); scapulars;and March 1987. tertials. Juvenal-plumaged specimens were

[7601 ROCK SHAG MOLTS AND PLUMAGES 761

TABLE 1. Specimensof the RockShag examined for molt and juvenal plumagephase. :h KEY I JUV.3 Sub- non-Fuegian niles adults Adults TM& III Argentina Season Fuegia Winter (Jun-Aug) 2 : 2 5 Spring(Sep-Nov) 3 2 9 Summer(Dee-Feb) 27 20 15 48 Fall (Mar-May) 12 2 11 25 Number molting 0 23 23 37 Locality Non-Fuegian Argentina 21 8 3 32 Tierra de1Fuego 7 0 7 14 Falkland FalklandIslands 3 6 8 17 IS. Non-FuegianChile 13 13 12 24 Total specimens 44 27 30 101

500 km classedinto five ventral feathering types (Table 2). Adventitious white feathering refers to white FIGURE 1. Localitiesgiven in text, and generalareas areas on the head and neck other than on the of the breeding range of P. magellanicusin the Fuego- cheek or chin. Patagonianregion of Argentina and .

RESULTS Primaries of adult and subadult Rock Shagsare of both wings of three specimensthe distal wave molted by continual stepwisemolt (Stresemann of PBl started before the proximal wave, but in and Stresemann 1966); the first prebasic molt of three others, the proximal started first in both primaries (PB 1) commencesat PI (primary near- wings. Molt data for remiges and rectrices are est carpal joint) and proceeds peripherally. Be- quantified in Table 3. Molt of rectricesappeared fore completion of PB 1, another wave begins at to be irregular in both subadults and adults. Pl and proceedsperipherally. The molt of sec- Body molt in three subadult specimens had ondaries of subadult and adult Rock Shagsis by begun by December, when each was probably no bidirectional stepwise molt, as in P. atriceps more than 1 year old. PBl of contour feathers (Rasmussen, in press). PBl molt starts in the began at about the same time in all body areas most proximal (S15) and most distal (Sl) sec- examined. I found no evidence for a molt of ondaries. Before completion of PBl, PB2 com- juvenal feathers of the venter into a second pre- mences at the most distal and most proximal definitive plumage;juvenal feathersof the venter secondaries.Some irregularities occur, but they were replacedby definitive feathers.Down-tipped do not changethe general pattern. In secondaries head feathers occurred on some specimenswith

TABLE 2. Ventral plumagetype (from Fig. 2) of specimensand photographsof juvenal Rock Shags,and numberof specimensnot molting venterfeathers.

No?- Non-Fue- FFhSZ Tierra del gian Falkland No. not FUegO Argentina Islands molting

A. Venter pure or almost pure white 6 0 0 8 6 B. Venter with a few dark tips to one-fourth dark 15 3 1 11 C. Venter with about half the feathers dark-tipped 1 2 5 4 8 D. Venter with about three-fourths feathers dark-tipped 1 3 10 2 4 E. Venter with no white-tinned feathers 1 1 5 2 5 All localities separately:G, = 50.9, df = 12, P < 0.001, G-statistic,S&al and Rohlf 1981. Chile vs. other localitiespooled: G, = 48.9, df = 4, P < 0.001. 762 PAMELA C. RASMUSSEN

TABLE 3. Summary of molt data for flight feathers of the Rock Shag;number in parentheses= n.

Subadults Adults

Primaries No. waves per wing - _z= 1.83 + 0.78 (23) range = l-3 No. specimenswith bilateral symmetry in molt 6 (10) 4 (12) No. molting primaries per molting wing K= 1.87 f 0.74(15) K = 1.68 + 0.65 (50) range = l-3 range = l-3 No. molting primaries per molting wave x = 1.65 f 0.79 (17) x = 1.42 + 0.57 (54) range = l-3 range = l-3 Secondaries No. waves per wing - 5t = 2.0 * 1.04 (12) range = l-4 No. specimenswith bilateral symmetry in molt O(l1) 0 (12) No. molting secondariesper molting wing K= 2.27 t 1.19(11) .x = 2.65 i 1.14 (40) range = l-4 range = l-5 No. molting secondariesper molting wave x = 1.37 t 0.50 (19) x= 1.31 * 0.49(74) range = l-2 range = l-3 Rectrices No. specimenswith bilateral symmetry in molt 1 (7) 1 (7) of rectrices No. molting rectrices per molting tail .%= 4.0 + 1.58 (9) K = 2.4 t 1.65 (10) range = 2-6 range = l-5

dark ventral featheringas well as thosewith white neck. I have seen no fall or winter Rock Shag (Figs. 2A-E). Juvenal specimens from non-Fue- specimens with the throat and ventral neck en- gian Chile usually had white venters, while most tirely or nearly white. non-Fuegian Argentine and Falkland specimens All but two Rock Shag specimens examined were dark ventrally; Fuegian specimens were with adventitious white feathering were collected more variable than those from farther north on in non-Fuegian Argentina and in the Falkland either coast, although no Fuegian specimenshad Islands. Two specimensfrom non-Fuegian Chile pure white venters (Table 2). (AMNH 443181 and CMNH 123547) have a Figure 3 showsseasonality of molt for subadult trace of white on the neck, no Fuegian specimens and adult Rock Shags.The presenceof an alter- examined have these irregular markings, and of nate plumage of head and neck is inferred from the scoresof adults P. S. Humphrey and I saw two molting adult specimens (SDNHM 38273 in January and February of 1986 in the Beagle and LSUMZ 69634) collected on 9 August and Channel at Ushuaia, Tierra de1Fuego, none had 18 July, respectively. adventitious white feathering (Table 5). There is no evidence for a molt into a phe- notypically distinct “winter” plumage in the Rock DISCUSSION Shag.Nine adult specimens(from the Falklands) Molts of all flight feathers and body regions in with adventitious white feathering scattered on the Rock Shagare like those of other cormorants the head and neck (Fig. 4A) were not molting studied (Potts 197 1; Berry 1976; Bernstein and any feathersin those areas(Table 4). In addition, Maxson 198 1; Ginn and Melville 1983; Ras- the adventitious white feathering cannot be ex- mussen,in press).No more than three molt waves plained as a result of molt pauses,because many occur per primary row; in this respectthe Rock of the feathers involved are parti-colored black Shag is more similar to the , P. and white. I examined 16 adult specimens col- carbo,and the European Shag,P. aristotelis(Ginn lected during the austral fall and winter (April and Melville 1983) than to the Blue-eyed Shag, through August); 14 of thesehad black necks and P. atriceps (Rasmussen, in press), which often throats as in the breeding plumage (Fig. 4B). The has four molt waves per primary row. exceptions(LSUMZ nos. 69633 and 69634) had I found no evidence indicating that there are scatteredwhite featherson the throat and ventral two immature plumages attained by molt, even ROCK SHAG MOLTS AND PLUMAGES 163

FIGURE 2. Ventral plumage feathering types into which juvenal Rock Shageswere classified.A = nearly or completely white; B = about ‘14white; C = about half white; D = about ‘14dark, E = nearly or completely dark. White chin feathering varies independently of ventral plumage type.

though Murphy’s (1936) description of the im- represent intermediate plumage stages. P. S. mature plumage sequencestates that the first im- Humphrey and I have seen only two mostly mature plumage, which is white-bellied, molts white-bellied juveniles (as in Fig. 2B) among the into a dark-bellied stage; then this second im- scoresof nondowny nestlings and fledglings ob- mature plumage molts into another white-bellied served at each of the following Argentine local- stage, the adult plumage. In actuality, there is ities: Puerto Melo, Chubut Province, and Puerto only one predefinitive (immature) plumage, Deseado and Monte Leon, Santa Cruz Province which varies from totally white on the venter to (the latter is where the two Atlantic coast spec- totally black (Table 2). Many specimensin classes imens in class B were collected). The only ju- ED (Fig. 2) were not molting venter feathers venile specimenswith totally white bellies (Fig. (Table 2) and in addition, had some individual 2A) are those from non-Fuegian Chile, where feathersof the venter parti-colored dark and white most juveniles of the sample are white-bellied or (giving the appearance of a mixture of old and nearly so (Table 2). The Atlantic and Pacific pop- new feathers); therefore these specimens cannot ulations thus differ greatly in frequency of the 164 PAMELA C. RASMUSSEN

SUMMER FALL KEY

P S R H N B P S R H N 13 V 100 0 old basic

75 n old alternate

molting

25 new basic 2 g 0 I u n 17 17 16 I8 I8 18 I6 m new alternate

rk m juvenal

B WINTER SPRING c pause in first PSRHNBV prebasic molt

25

0 ” 2 2 2 2 2 2 2 6 6 5 6 6 6 6

FIGURE 3. Percentof subadultand adult specimensof the Rock Shagin molt and in variousplumages for eachseason. P = primaries,S = secondaries,R = rectrices,H = head,N = neck,B = back,V = venter. juvenal plumage classes,and in the Fuegian re- cer 1976). In this vicariance event, the marine gion juveniles tend to be intermediate and more littoral was uninhabitable from just south of variable. ChiloC Island, Chile (Flint 1971), to Tierra de1 The Rock Shag is placed in the clade Sticto- Fuego, excluding the eastern Fuegian region turbo (Siegel-Causey 1986), in which the ju- (Porter et al. 1984). During this time the pre- venal plumage of other clade members (the Pe- sumably small, isolated Pacific population of the lagic Cormorant, P. pelagicus, and the Red-faced Rock Shag occupying the ChiloC refugium may Cormorant) is dark below, as it is in most other have been affectedby the founder effector genetic cormorants. Another member, the Red-legged drift. Following glacial retreat, repopulation of Shag (P. gaimardi), often has a dark gray venter southern Chile and subsequent mixing in the in the juvenal plumage (Rasmussen, unpubl. Fuegian region of the two formerly isolated pop- data). Thus the white-bellied juvenal plumage is ulations may explain the intermediacy in juvenal best considered derived where it occursin Rock plumage classesfrom that area relative to Atlan- Shags.It is likely that this plumage variant arose tic and Pacific populations. The only two nearly during separation of the Atlantic and Pacific pop- white-bellied juvenal specimens (Fig. 2B) from ulations of this species caused by Llanquihue the Atlantic coast were collected at Monte Leon; Glaciation from 20,000 ybp to 10,000 ybp (Mer- this and preliminary evidence from P. atriceps

TABLE 4. Molt conditionand extent ofwhite in RockShags from the Falkland Islands (AMNH Beckcollection).

Specimen no. Date Molt on head, neck Adventitious white feathering 443219 10 Jan None, none Numerouswhite feathersin lowerthroat. 443209 10 Dee None, none Entire throatand crestmostly white. 443208 16 Dee None, none Ventral necklargely white. 729946 2 Jan None, none Somewhite feathersin lowerthroat. 443218 10 Jan None, none Entire throatmostly white, scatteredwhite featherselsewhere. 443202 17 Dee None, none Entire throatmostly white, crestmostly white. 446191 10 Dee None, none Entire throatmostly white, crestmostly white. 443206 4 Nov None, none Entire throatmixed blackand white. 443207 4 Nov None, none Few white feathersin centralthroat. ROCK SHAG MOLTS AND PLUMAGES 765

PC+? ‘86

FIGURE 4. Head and neck of A: typical mottled breeding plumage specimen (AMNH 443202; 17 December; head and neck not molting); B: winter-plumaged specimen (SDNHM 38273; 7 August). A is in basic plumage, B is in combined alternate and basic plumages,

(specimens spanning the size range of this oth- The adventitious white feathering around the erwise highly clinal speciesoccur at Monte Leon head and neck found on many breeding speci- in the breeding season;Rasmussen, unpubl. data) mens of the Rock Shagwas explained by Murphy suggestthat the Monte Leon area is currently a (1936) as an intermediate stagebetween breeding zone of mixing between Fuegian and more north- and nonbreeding plumages. However, I found erly Patagonian cormorant populations. that none of the AMNH Beck collection speci- Other cormorants known to have polymorphic mens on which he based this opinion were molt- juvenal plumages are the Pied Cormorant, P. ing feathers of the head or neck. The lack of varius, the , P. melanoleu- white-necked winter museum specimensand the cus, the Stewart Island Cormorant, P. chalcono- occurrence of at least 15 black-necked winter- tus (adults also polymorphic in the latter two collected specimens (and two with white speck- forms), and the , P. verrucosus ling) in U.S. museums also argue against Mur- (Falla 1937, Falla et al. 1966, Voisin 1973, Har- phy’s interpretation. Harrison (pers. comm.) rison 1983). The Rock Shag is not usually con- based his 1983 illustration of the “winter-plum- sidered closely related to any of these species aged” Rock Shag on Murphy’s description, as (Siegel-Causey 1986), although Falla (1937) and apparently have others. All evidence showsthat Voisin (1973) postulated relationship between there is no phenotypically distinct winter plum- Rock and Kerguelen cormorants on the basis of age in the Rock Shag, and that adults with ad- plumage characters. ventitious white feathering are merely plumage

TABLE 5. Numbers of specimensexamined and sight recordsof individuals with adventitiouswhite feathering on head and neck.

Sightingsand photosof No. adult phenotype No. specimenswith adventitious individuals with adventi- Locality specimensexamined white feathering tious white feathering

Pto. Melo, Argentina 16 2 several Pto. Deseado, Argentina 17 1 several Monte Leon, Argentina 1 2 several Falkland Islands 11 8 several Tierra de1Fuego 28 0 none Non-Fuegian Chile 20 2 (very few whites) none 766 PAMELA C. RASMUSSEN variants similar to those that occur in the Bank and R. Cerezani, Direcci6n de Recursos Naturales, Cormorant, P. neglectus (Harrison 1983), rather Territory of Tierra de1 Fuego, Argentina. This study than a transitional stagebetween plumages.There was supported by the Frank M. Chapman Memorial Fund and the American Museum of Natural History apparently is an alternate plumage of the head CollectionsStudy Grant Program;the Museum of Nat- and neck in the Rock Shag, but more specimens ural History and the Department of Systematicsand are needed to verify this. The Blue-eyed Shag, Ecology, University of Kansas, and the Kansas Uni- the European Shag,and the North American cor- versity Endowment Association; the Humphrey fam- morants all have an alternate plumage of the ily; and National Science Foundation Grant BSR- 8407365 to Humphrey and Siegel-Causey. head and neck (Palmer, in litt.). Winter Rock Shag specimens(Fig. 4B) are very similar in as- LITERATURE CITED pect to summer specimensbut have more highly BERNSTEIN,N. P., AND S. J. MAXSON. 1981. Notes developed filoplumes on the head, back, wing on moult and seasonablyvariable characters of coverts, and thighs; these filoplumes are proba- the Antarctic Blue-eyed Shag Phalacrocoraxatri- bly the supplemental feather generation, as in cepsbransjieldensis. Notom& 28:35-39. other cormorants (Palmer, in litt.). BERRY.H. H. 1976. Phvsiological and behavioural e&logy of the Cape C&mor&t Phalacrocoraxca- Rock Shags with adventitious white feather- pensis.Madoqua 9(4):5-55. ing, while frequent in non-Fuegian Argentina and FALLA,R. A. 1937. . B. A. N. Z. Antarctic Re- the Falkland Islands, are unknown from Fuegia searchExpedition 1929-l 93 1. Reports(B)II: l-304. and rare in non-Fuegian Chile. This, in addition FALLA,R. A., R. B. SIBSON,AND E. G. TURBOTT. 1966. A field guide to the birds of and to differential distribution of juvenal plumage outlying islands. Houghton-Mifflin, Boston. classes,supports evolution in isolation by this FLINT, R. F. 1971. Glacial and Quaternary geology. relatively sedentary species.Because of lowered John Wilev and Sons. New York. sealevels, the unglaciated Falkland Islands (Mul- FRAY, C., AN; M. EWING. 1963. Pleistocene sedi- ler 1973) were separatedfrom mainland Atlantic mentation and fauna of the Argentine shelf. Proc. Acad. Nat. Sci. Philadelohia 115:113-l 52. Patagonia by much narrower straits during the GINN, H. B., AND D. S. MELVILLE. 1983. Moult in Llanquihue Glaciation than at present (Fray and birds. B. T. 0. Guide 19, British Trust for Omi- Ewing 1963, Flint 1971), and thus Rock Shags thology, Beech Grove, Tring, Hertfordshire, En- in the Falklands would not have been as isolated gland. HARRISON,P. 1983. :ar! identification guide. from the main Atlantic population as was the Houghton Mifflin Co., Boston. population in the ChiloC refugium. In this species, MERCER,J. H. 1976. Glacial history of southernmost therefore, geographic proximity during a major . Quaternary Res. 6: 125-166. vicariance event is congruent with degree of MULLER,P. 1973. The dispersalcentres of terrestrial plumage differentiation. vertebrates in the Neotropic realm. Biogeogra- phica 2: l-244. ACKNOWLEDGMENTS MURPHY,R. C. 1936. Oceanic birds of South Amer- ica. American Museum of Natural History, New This study would not have been possiblewithout many York. kinds of assistancefrom P. S. Humphrey and D. Siegel- PORTER,S. C., M. STUIVER,AND C. J. HEUSSER.1984. Causey.For information, loan of specimens,and access Holocene sea-levelchanges along the Strait of Ma- to collections I thank G. E. Barrowclough,W. E. Lan- gellan and , southernmost South yon, and M. LeCroy (American Museum of Natural America. Quatemary Res. 22:59-67. History [AMNH]); K. C. Parkes(Carnegie Museum of POTTS,G. R. 1971. Moult in the ShagPhalacrocorax Natural History [CMNH]); R. Schreiber (Los Angeles aristotelis,and the ontogeny of the “Staffelmau- County Museum); J. V. Remsen, Jr. (Louisiana State ser.” Ibis 113:298-305. Museum of Zoology [LSUMZ]); R. Paynter (Museum RASMUSSEN,P.C. In press. Stepwise molt of remiges of Comparative Zoology [MCZ]); J. Navas (Muse0 Ar- in Blue-eyed and King shags.Condor. gentino de Ciencias Naturales); R. L. Zusi (National SIEGEL-CAUSEY,D. 1986. The behaviour and affini- Museum of Natural History); A. Rea (San Diego Nat- ties of the Magellanic Cormorant (Phalacrocorax ural History Museum [SDNHM]), andE. Stickney(Yale Manellanicus). Notomis 331249-257. Peabody Museum). P. Harrison provided helpful in- SOKAL,ii. R., ANDF. J. ROHLF. 1981. Biometry. 2nd formation. R. S. Palmer and M. Gottfiied read and ed. W. H. Freeman and Co.. New York. improved the manuscript. For permits, lodging, and STRESEMANN,E., AND V. STRESEMANN.1966. Die other assistance,I thank B. Mayer and F. Fauring, Mauser der Vogel. J. Omithol. 107:Sonderheft. PatagoniaComercial S. R. L.; C. 0. Perez, Ministerio TUCK, G. S. 1978. A field guide to the seabirds of de Economia, Chubut; J. Vinuesa and C. Schroeder, Britain and the world. Collins, London. Centro Australde InvestigacionesCientificas; R. Clarke, VOISIN,J.-F. 1973. Notes on the blue-eyed shags(Ge- Dpto. Conservacibnde la Fauna, SantaCruz Province: nus LatcocarboBonaparte). Notomis 201262-271.