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Recognizable Forms

Morphs of the Parasitic Jaegerby

Ron Pittaway and Peter Burke

IntroductionParasitic Jaegers (Stercorariusparasiticus ) are seagoing piratesduring the nonbreeding season,making their living by robbing gulls,terns and other seabirds by forcingthem to disgorge or drop their prey.Seeing a Parasitic Jaeger acceleratingPeregrine-like, swiftly pursuing a ternuntil it drops its fish, then catchingthe fish in mid-air before it strikesthe water, is an unforgettable sight.On their tundra breeding grounds,they also prey on small birds, eggsand young birds, lemmings, andinvertebrates such as insects andspiders. Skuas and jaegers, subfamilyStercorariinae, are unique amongbirds in having the combination ofstrong, sharply hooked claws andfully webbed feet.

In Canada, the Parasitic Jaegerbreeds in the Arctic south to northernOntario (Godfrey 1986). It is a I I raresummer resident along the HudsonBay coast" of Ontario (James 1991).The Parasitic Jaeger is a rare tolocally uncommon migrant insouthern Ontario, mainly in the fallon the Great Lakes from late Augustto mid-November. Two of the bestplaces to see jaegers in southernOntario are Lake Ontario from VanWagner's Beach at Hamilton duringeast or northeast winds, and LakeHuron at Sarnia during north ornorthwest winds. Directions to VanWagner's Beach and Sarnia are in

Goodwin (1995), and there is anexcellent site guide to seeing jaegersat Sarnia in OFO NEWS (Rupert1995). Parasitic Jaegers are casual inspring in southern Ontario.

Adult Parasitic Jaegers arevariable in appearance, but generallyoccur in three colour morphs(phases): light, intermediate anddark. See Figure 1. Our classificationof adult morphs is based on thedescriptions in the genetic studies ofO'Donald (1983) and O'Donald inCooke and Buckley (1987) . Juvenilemorphs are also variable inappearance; see the light,intermediate and dark morphjuveniles illustrated in Figure 2.

In this article, we discuss thedistinguishing features, frequencyand distribution, and genetics of thethree morphs of the Parasitic Jaegerin Ontario. Discussion is restricted toadults in breeding plumage andjuveniles because these are the twoage classes normally seen in Ontario.The best references on theidentification of all three jaegerspecies are Olsen (1989), Kaufman(1990), Harrison (1983), Cramp(1983) and Harris et al. (1989).

TaxonomyNo subspecies (races) of the ParasiticJaeger are recognized becausedifferences between populations areslight. However, there is a markedgeographical variation in the

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frequency of the morphs betweenpopulations. For example, in Canadathe dark morph makes up about 40per cent of the breeding population inLabrador and less than one per centin the Canadian High Arctic (Cramp1983). See the discussion on thedistribution and frequency of themorphs under Morph Genetics.

Plumage, Molts, Ages and SexesSexes are alike in all plumages.Almost all birds seen in Ontario arein juvenile (juvenal) or adult breeding(definitive alternate) plumage. Jaegersmolt twice a year. The postjuvenile(first prebasic), postbreeding (secondand later prebasic) and prebreeding(prealternate) molts take place mainlyon the oceanic wintering grounds farsouth of Ontario (Cramp 1983). Somevery limited postbreeding molt(scattered pin feathers) begins on thebreeding grounds (Parmelee et al.1967). Parasitic Jaegers molt theirpair of central tail feathers only oncea year, during the completepostbreeding molt. However, theother two jaeger species molt theircentral tail feathers twice a year,replacing them in the postbreedingmolt and again during the partialprebreeding molt (Cramp 1983).Olsen's (1989) statement that all threejaeger species molt their central tailfeathers twice a year is questionableregarding the Parasitic Jaeger. Likelarge gulls, Parasitic Jaegers probablyattain their first adult breedingplumage at four years of age (Todd1963, Cramp 1983). Other agesbetween juvenile and adult breedingplumages are normally not seen inOntario because most ParasiticJaegers spend at least their first twoyears at sea (Cramp 1983). Subadultbirds, probably in second sumnler

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plumage, are casual in southernOntario in late spring. As well, thereis a subadult (exact age not reported)in the Buffalo Museum of Sciencethat was collected by Harold Axtellon 26 November 1960 at Fort Erie,Niagara (Beardslee and Mitchell1965). See the illustration of a lightmorph subadult on page 143 of theNational Geographic Guide (Scotti987). Third summer birds areessentially like adults, but at closerange show some barring on the winglinings and have shorter tailprojections. For a full discussion ofplumages, see Cramp (1983).

Morph GeneticsO'Donald in Cooke and Buckley(1987) provided strong evidence thatthe three morphs of adult ParasiticJaegers are under the control of asingle gene. This gene has two alleles(forms), one for light coloration andone for dark coloration. Light morphbirds have two alleles for lightcoloration, inheriting one light allelefrom each parent. Similarly, darkmorph birds inherit two dark alleles.Intermediate morph birds have onelight allele and one dark allele. Theallele for dark coloration isincompletely dominant over the allelefor light coloration. Therefore, mostintermediate morph birds(heterozygotes) are more like darkbirds in appearance. Compare theintermediate and dark morph birds inFigure 1. A similar situation occurs inthe Snow Goose where mostintermediate morph birds(heterozygotes) are closer inappearance to blue morph than whiteInorph birds (Pittaway 1992).

A pure dark bird nlated to a purelight bird should produce onlyinternlediate nlorphs. A pair

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Figure 1: Adult Parasitic Jaegers: light morph (top), intermediate morph(middle), at:ld dark morph (bottom) at Van Wagner's Beach, Hamilton.Drawing by Peter Burke.

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comprising a dark and anintermediate morph should produceonly dark and intermediate morphs ina ratio of one dark to oneintermediate. Similarly, a pair of lightand intermediate birds shouldproduce only light and intermediatemorphs in a ratio of one to one.

Genetic studies by O'Donald inCooke and Buckley (1987) indicatethat all light morph adults arehomozygous (pure), vvhereasthedarkest intermediate birds often arehomozygous, and some dark morphbirds are heterozygous (alleles forboth dark and light coloration). Asvvell, colour changes betvveen thedarkest intermediates and darkmorphs "occur from one breedingseason to the next", shovving thatmorph coloration is "partlydevelopmental in origin" .

The frequency of the dark morphvaries betvveen populations andshovvs an interesting pattern.Generally, dark birds are common incoastal and southern parts of thebreeding range (for example, 40 percent dark in Labrador, 60 per centdark in Britain, 89 per cent dark insouthern Iceland). Light birdspredominate at inland continentalsites (for example, about 100 per centlight in central Russia) and in thehigh Arctic (for example, about 100per cent light in the Canadian Arcticislands), sometimes almost to theexclusion of the dark birds (Cramp1983). The distribution of the morphsagrees vvith Gloger's Rule. The rulesays that dark pigments in feathersincrease in humid parts of thebreeding range, vvhereas lighterpigments prevail in dryer areas(Terres 1982).

In addition, O'Donald in Cookeand Buckley (1987) shovved that

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polymorphism in the Parasitic Jaegervvas maintained by a combination ofnatural selection favouring lightmorphs and sexual selectionfavouring dark morphs. Females,especially dark females, prefer tomate vvith dark males (assortativemating). Therefore, dark males takeless time to find a mate and theybreed earlier than light birds, gaininga selective advantage of /I earlierbreeding and increased reproductivesuccess".

Light Morph AdultsMost adult Parasitic Jaegers seen inOntario are light morph birds. Forexample, in northern Ontario, BruceDi Labio (pers. comm.) observed ninelight morph adults on 15 September1994 at Shipsands Island (nearMoosonee) at the south end of JamesBay during strong northeast vvinds. Insouthern Ontario, Rupert (1995)reported that 90 per cent of the adultsseen at Sarnia at the south end ofLake Huron vvere light morphs.

Typical light morph adultParasitic Jaegers are mainly vvhite onthe throat, breast and belly; they mayhave a gray band across the breastlike the bird in Figure 1. Theundertail coverts are usually darkerbut may be vvhitish. See theillustration of a light morph adult onPlate 39 in Godfrey (1986).

Dark Morph AdultsDark morph adult Parasitic Jaegersare much rarer than light morphs inOntario. See the bottom bird inFigure 1 and the illustration of a darkmorph adult in flight on Plate 39 inGodfrey (1986). Many dark morphadults are uniformly dark brovvn vviththe cap being only slightly darker.Dark morph birds have not been

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Figure 2: Juvenile Parasitic Jaegers: light morph (top), intermediate morph(middle), and dark morph (bottom) at Van Wagner's Beach, Hamilton.Drawing by Peter Burke.

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reported breeding in northern Ontarioto our knowledge. At Churchill,Manitoba, Jehl and Smith (1970) statethat the breeding population "consistsentirely of light- or intermediate­phased birds, but a few dark-phasedindividuals occur in migration".

Describing the 1974 Hamiltonboat trip on 22 September, Curry(1974) wrote, "About twenty minutesout from the Canal, the familiarexcited cry of 'Jaeger' rang out. Alleyes peered to port. It was a darkbird flying left - probably animmature Parasitic? But a closer lookrevealed a ramrod straight thin tailand a dark cap against smooth, darkbrown underparts, revealing it as thefirst dark-phase adult Parasitic Jaegermost of us had ever seen locally" .Rupert (1995) reports that 10 per centof the adults seen at Sarnia are darkmorph birds. This percentage ishigher than at Hamilton andelsewhere on the Great Lakes.

Intermediate Morph AdultsIntermediate morph adult ParasiticJaegers are variable, but most arecloser in appearance to dark morphadults because the gene controllingdark coloration is incompletelydominant over the gene for lightcoloration. See Figure 1. A fewintermediates show a distinct junco­like hood and dark undertail covertscontrasting with a white belly. Plate55 in Harrison (1983) illustrates apale intermediate showing a verydark chest band contrasting with awhite throat and belly. Note also thatthe intermediate morph adultillustrated on Plate 64 in Cramp(1983) is a pale extreme in ourclassification, being much paler thanmost intermediates.

Based on his genetic studies,

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0'Donald in Cooke and Buckley(1987) describes intermediate adultsas "dark with a variable amount oflighter plumage around the cheeks,collar and breast; the bases of thebreast and belly feathers are white.Those intermediates with a verywhite base to their belly feathersshow a distinctly lighter bellycompared to the dark birds, whohave no white base to their bellyfeathers. But the darkest intermediatebirds cannot always be distinguishedfrom the dark birds, except byexamination of the belly feathers;even then, dark intermediates mergeinto darks in a continuous sequence".

Intermediate morphs are rarerthan light morphs in Ontario. Theirstatus is unknown because manyintermediates were probably calleddark morphs in the past.

JuvenilesParasitic Jaegers retain their fulljuvenile plumage during fallmigration in southern Ontario. Themolt to first winter plumage takesplace on the wintering grounds.Figure 2 shows typical light,intermediate and dark morphjuveniles. See also the perchedintermediate morph juvenile on Plate39 in Godfrey (1986). Most juvenilesseen in Ontario are light orintermediate in coloration, but oftenappear dark at a distance over water.

Juveniles usually outnumberadults on the Great Lakes. Forexample, Rupert (1995) reported overan 18 year period that two-thirds ofthe Parasitic Jaegers seen at Sarniawere juveniles. On the 23 September1973 Hamilton boat trip, Curry(1974) reported that 23 jaegers wereseen, "none of which was adult" .Bob Curry (in litt.) noted that "Most

years juveniles outnumber adultsconsiderably but a closer look at thedata reveals a more interestingtemporal pattern. The majority ofadult Parasitics at Hamilton occur inSeptember and very early Octoberwhile most birds seen later than thisare juveniles. Thus, in some yearsadults outnumber juveniles inSeptember in the ratio of 3:2, buttaking into consideration the entireseason juveniles will almost alwaysgreatly outnumber adults" .Interestingly, more adults thanjuveniles have been seen in Ottawaover the years (Bruce Di Labia, pers.comm.), perhaps indicating thatOttawa is nearer the main route ofadults from James Bay to the Atlantic(Michel Gosselin, pers. comm.).

Morphs of the juvenile are highlyvariable, ranging from light to dark,the main difference being the extentof barring on the undersides. Cramp(1983) describes three morphs: light,barred (intermediate) and dark.Distinguishing juveniles from adultsat a distance is difficult to impossible.At close range, typical light andintermediate morph juveniles arestreaked on the head and neck,barred below, and edged with rufousabove and below. The pointed centraltail feathers are short and projectonly slightly beyond the others.Extremely dark morph juveniles areuniformly sooty-black and aredifficult to separate from dark morphadults unless the length of the tailprojection or fresh juvenile plumagecan be seen.

The seqence of colour changesfrom juvenile to adult is complex andpoorly understood. Olsen (1989) notesthat dark juveniles may become lightadults! Readers are referred to Cramp(1983) and Olsen (1989) for more

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information on plumages andmorphs. See also the excellent articleand illustrations by Jonsson (1984) onthe identification of juvenilePomarine (P. pomarinus) and ParasiticJaegers.

SummaryAdult Parasitic Jaegers occur in threecolour morphs: light, intermediateand dark. Most adult Parasitic Jaegersseen in Ontario are light morph birds,whereas intermediate and darkmorph adults are decidedly rarer.Genetic studies indicate thatintermediate morph adults are darkerand more like dark morph birds inappearance than the intermediatesdescribed and illustrated by mostauthors. Our illustration of anintermediate morph adult is thereforemore typical of a geneticallyintermediate bird. Juveniles alsooccur in three morphs. Juveniles seenin Ontario tend to be light orintermediate in coloration, but oftenappear dark at a distance. Juvenilesusually outnumber adults on theGreat Lakes in fall.

AcknowledgementsThe following people provided uswith information and helpfulcomments on the first draft of thisarticle: Dawn Brenner, Bill Crins, JimDick, Rob Dobos, Jon Dunn, EarlGodfrey, Michel Gosselin, Jean Iron,Ross James, Alvaro Jaramillo, KevinMcLaughlin, Dennis Rupert, RonTozer and Mike Turner.

Literature citedBeardslee, C.S. and H.D. Mitchell. 1965. Birds

of the Niagara Frontier Region. Bulletin of theBuffalo Society of Natural Sciences,Volume 22.

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Cooke, F. and P.A. Buckley {editors} 1987.Avian Genetics: A Population and EcologicalApproach. Academic Press, London.

Cramp, S. {editor} 1983. Birds of the WesternPalearctic: The Handbook of the Birds ofEurope, the Middle East and North Mrica.Volume 3: Waders to Gulls. OxfordUniversity Press, London.

Curry, R. 1974. The 1974 Hamilton boat trip.The Wood Duck 28 (4): 65-66.

Godfrey, W.E. 1986. The Birds of Canada.Revised Edition. National Museums ofCanada, Ottawa.

Goodwin, C. 1995. A Bird-Finding Guide toOntario. Revised Edition. University ofToronto Press, Toronto.

Harris, A., L. Tucker, and K. Vinicombe. 1989.The MacMillan Field Guide to BirdIdentification. The MacMillan Press Ltd.,London.

Harrison, P. 1983. Seabirds: An IdentificationGuide. Houghton Mifflin, Boston.

James, R. D. 1991. Annotated Checklist of theBirds of Ontario. Second Edition. LifeSciences Miscellaneous Publications, RoyalOntario Museum, Toronto.

Jehl, J.R. Jr. and B.A. Smith. 1970. Birds ofthe Churchill Region, Manitoba. SpecialPublication Number 1, Manitoba Museumof Man and Nature, Winnipeg.

Jonsson, L. 1984. Identification of juvenilePomarine and Arctic Skuas. British Birds 77:443-446.

Kaufman, K. 1990. A Field Guide to AdvancedBirding. Houghton Mifflin, Boston.

O'Donald, P. 1983. The Arctic Skua. A Studyof the Ecology and Evolution of a Seabird.Cambridge University Press.

Olsen, K.M. 1989. Field identification of thesmaller skuas. British Birds 82: 143-176.

Parmelee, D.F., H.A. Stephens andR.H. Schmidt. 1967. The birds ofsoutheastern Victoria Island and adjacentsmall islands. Bulletin 222. National Museumof Canada.

Pittaway, R. 1992. Subspecies and morphs ofthe Snow Goose. Ontario Birds 10: 72-76.

Rupert, D. 1995. Favourite Birding Hotspots,Sarnia Waterworks. OFO NEWS 13 (1): 2-3.

Scott, S.L. {editor} 1987. Field Guide to theBirds of North America. National GeographicSociety, Washington, D.C.

Terres, J.K. 1982. The Audubon SocietyEncyclopedia of North American Birds.Alfred A. Knopf, New York.

Todd, W.E.C. 1963. Birds of the LabradorPeninsula and Adjacent Areas. Universityof Toronto Press, Toronto.

Ron Pittaway, Box 619, Minden, Ontario KOM 2KO.Peter Burke, Box 1092, Lakefield, Ontario KOL 2HO.

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