123 Recognizable Forms

Morphs of the Parasitic Jaeger by Ron Pittaway and Peter Burke

Introduction Goodwin (1995). and there is an Parasitic Jaegers (Stercorarius excellent site guide to seeing jaegers parasiticus) are seagoing pirates at Sarnia in OFO NEWS (Rupert during the nonbreeding season, 1995). Parasitic Jaegers are casual in making their living by robbing gulls, spring in southern Ontario. terns and other seabirds by forcing Adult Parasitic Jaegers are them to disgorge or drop their prey. variable in appearance, but generally Seeing a Parasitic Jaeger accelerating occur in three colour morphs Peregrine-like, swiftly pursuing a tern (phases): light, intermediate and until it drops its fish, then catching dark. See Figure 1. Our classification the fish in mid-air before it strikes of adult morphs is based on the the water, is an unforgettable sight. descriptions in the genetic studies of On their tundra breeding grounds, O'Donald (1983) and O'Donald in they also prey on small birds, eggs Cooke and Buckley (1987). Juvenile and young birds, lemmings, and morphs are also variable in invertebrates such as insects and appearance; see the light, spiders. Skuas and jaegers, subfamily intermediate and dark morph Stercorariinae, are unique among juveniles illustrated in Figure 2. birds in having the combination of In this article, we discuss the strong, sharply hooked claws and distinguishing features, frequency fully webbed feet. and distribution, and genetics of the In Canada, the Parasitic Jaeger three morphs of the Parasitic Jaeger breeds in the Arctic south to northern in Ontario. Discussion is restricted to Ontario (Godfrey 1986). It is a "rare adults in breeding plumage and summer resident along the Hudson juveniles because these are the two Bay coast" of Ontario (James 1991). age classes normally seen in Ontario. The Parasitic Jaeger is a rare to The best references on the locally uncommon migrant in identification of all three jaeger southern Ontario, mainly in the fall species are Olsen (1989), Kaufman on the Great Lakes from late August (1990), Harrison (1983), Cramp to mid-November. Two of the best (1983) and Harris et al. (1989). places to see jaegers in southern Ontario are Lake Ontario from Van Taxonomy Wagner's Beach at Hamilton during No subspecies (races) of the Parasitic east or northeast winds, and Lake Jaeger are recognized because Huron at Sarnia during north or differences between populations are northwest winds. Directions to Van slight. However, there is a marked Wagner's Beach and Sarnia are in geographical variation in the

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frequency of the morphs between plumage, are casual in southern populations. For example, in Canada Ontario in late spring. As well, there the dark morph makes up about 40 is a subadult (exact age not reported) per cent of the breeding population in in the Buffalo Museum of Science Labrador and less than one per cent that was collected by Harold Axtell in the Canadian High Arctic (Cramp on 26 November 1960 at Fort Erie, 1983). See the discussion on the Niagara [Beardslee and Mitchell distribution and frequency of the 1965). See the illustration of a light morphs under Morph Genetics. morph subadult on page 143 of the National Geographic Guide [Scott Plumage, Molts, Ages and Sexes i987). Third summer birds are Sexes are alike in all plumages. essentially like adults, but at close Almost all birds seen in Ontario are range show some barring on the wing in juvenile [juvenal] or adult breeding linings and have shorter tail (definitive alternate) plumage. Jaegers projections. For a full discussion of molt twice a year. The postjuvenile plumages, see Cramp (1983). (first prebasic), postbreeding (second and later prebasic) and prebreeding Morph Genetics (prealternate) molts take place mainly O'Donald in Cooke and Buckley on the oceanic wintering grounds far (1987) provided strong evidence that south of Ontario (Cramp 1983). Some the three morphs of adult Parasitic very limited postbreeding molt Jaegers are under the control of a (scattered pin feathers) begins on the single gene. This gene has two alleles breeding grounds [Parrnelee et al. (forms), one for light coloration and 1967). Parasitic Jaegers molt their one for dark coloration. Light morph pair of central tail feathers only once birds have two alleles for light a year, during the complete coloration, inheriting one light allele postbreeding molt. However, the from each parent. Similarly, dark other two jaeger species molt their morph birds inherit two dark alleles. central tail feathers twice a year, Intermediate morph birds have one replacing them in the postbreeding light allele and one dark allele. The molt and again during the partial allele for dark coloration is prebreeding molt (Cramp 1983). incompletely dominant over the allele Olsen's (1989) statement that all three for light coloration. Therefore, most jaeger species molt their central tail intermediate morph birds feathers twice a year is questionable (heterozygotes) are more like dark regarding the Parasitic Jaeger. Like birds in appearance. Compare the large gulls, Parasitic Jaegers probably intermediate and dark morph birds in attain their first adult breeding Figure 1. A similar situation occurs in plumage at four years of age [Todd the Snow Goose where most 1963, Cramp 1983). Other ages intermediate morph birds between juvenile and adult breeding (heterozygotes) are closer in plumages are normally not seen in appearance to blue morph than white Ontario because most Parasitic morph birds (Pittaway 1992). Jaegers spend at least their first two A pure dark bird mated to a pure years at sea (Cramp 1983). Subadult light bird should produce only birds, probably in second summer intermediate morphs. A pair

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Figure 1: Adult 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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comprising a dark and an polymorphism in the Parasitic Jaeger intermediate morph should produce was maintained by a combination of only dark and intermediate morphs in natural selection favouring light a ratio of one dark to one morphs and sexual selection intermediate. Similarly, a pair of light favouring dark morphs. Females, and intermediate birds should especially dark females, prefer to produce only light and intermediate mate with dark males (assortative morphs in a ratio of one to one. mating]. Therefore, dark males take Genetic studies by O'Donald in less time to find a mate and they Cooke and Buckley (19871 indicate breed earlier than light birds, gaining that all light morph adults are a selective advantage of "earlier homozygous [pure], whereas the breeding and increased reproductive darkest intermediate birds often are success" . homozygous, and some dark morph birds are heterozygous (alleles for Light Morph Adults both dark and light coloration]. As Most adult Parasitic Jaegers seen in well, colour changes between the Ontario are light morph birds. For darkest intermediates and dark example, in northern Ontario, Bruce morphs' 'occur from one breeding Di Labio (pers. cornm.] observed nine season to the next", showing that light morph adults on 15 September morph coloration is "partly 1994 at Shipsands Island (near developmental in origin". Moosonee] at the south end of James The frequency of the dark morph Bay during strong northeast winds. In varies between populations and southern Ontario, Rupert (19951 shows an interesting pattern. reported that 90 per cent of the adults Generally, dark birds are common in seen at Sarnia at the south end of coastal and southern parts of the Lake Huron were light morphs. breeding range (for example, 40 per Typical light morph adult cent dark in Labrador, 60 per cent Parasitic Jaegers are mainly white on dark in Britain, 89 per cent dark in the throat, breast and belly; they may southern Iceland], Light birds have a gray band across the breast predominate at inland continental like the bird in Figure 1. The sites (for example, about 100 per cent undertail coverts are usually darker light in central Russia] and in the but may be whitish. See the high Arctic (for example, about 100 illustration of a light morph adult on per cent light in the Canadian Arctic Plate 39 in Godfrey (19861. islands], sometimes almost to the exclusion of the dark birds (Cramp Dark Morph Adults 19831. The distribution of the morphs Dark morph adult Parasitic Jaegers agrees with Gloger's Rule. The rule are much rarer than light morphs in says that dark pigments in feathers Ontario. See the bottom bird in increase in humid parts of the Figure 1 and the illustration of a dark breeding range, whereas lighter morph adult in flight on Plate 39 in pigments prevail in dryer areas Godfrey (19861. Many dark morph (Terres 19821. adults are uniformly dark brown with In addition, O'Donald in Cooke the cap being only slightly darker. and Buckley (19871 showed that 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 Ontario O'Donald in Cooke and Buckley to our knowledge. At Churchill, (19871 describes intermediate adults Manitoba, Jehl and Smith (19701 state as "dark with a variable amount of that the breeding population "consists lighter plumage around the cheeks, entirely of light- or intermediate- collar and breast; the bases of the phased birds, but a few dark-phased breast and belly feathers are white. individuals occur in migration". Those intermediates with a very Describing the 1974 Hamilton white base to their belly feathers boat trip on 22 September, Curry show a distinctly lighter belly (19741 wrote, "About twenty minutes compared to the dark birds, who out from the Canal, the familiar have no white base to their belly excited cry of '[aeger' rang out. All feathers. But the darkest intermediate eyes peered to port. It was a dark birds cannot always be distinguished bird flying left - probably an from the dark birds, except by immature Parasitic? But a closer look examination of the belly feathers; revealed a ramrod straight thin tail even then, dark intermediates merge and a dark cap against smooth, dark into darks in a continuous sequence". brown underparts, revealing it as the Intermediate morphs are rarer first dark-phase adult Parasitic Jaeger than light morphs in Ontario. Their most of us had ever seen locally". status is unknown because many Rupert (19951 reports that 10 per cent intermediates were probably called of the adults seen at Sarnia are dark dark morphs in the past. morph birds. This percentage is higher than at Hamilton and Juveniles elsewhere on the Great Lakes. Parasitic Jaegers retain their full juvenile plumage during fall Intermediate Morph Adults migration in southern Ontario. The Intermediate morph adult Parasitic molt to first winter plumage takes Jaegers are variable, but most are place on the wintering grounds. closer in appearance to dark morph Figure 2 shows typical light, adults because the gene controlling intermediate and dark morph dark coloration is incompletely juveniles. See also the perched dominant over the gene for light intermediate morph juvenile on Plate coloration. See Figure 1. A few 39 in Godfrey (19861. Most juveniles intermediates show a distinct junco- seen in Ontario are light or like hood and dark undertail coverts intermediate in coloration, but often contrasting with a white belly. Plate appear dark at a distance over water. 55 in Harrison (19831 illustrates a Juveniles usually outnumber pale intermediate showing a very adults on the Great Lakes. For dark chest band contrasting with a example, Rupert (19951 reported over white throat and belly. Note also that an 18 year period that two-thirds of the intermediate morph adult the Parasitic Jaegers seen at Sarnia illustrated on Plate 64 in Cramp were juveniles. On the 23 September (19831 is a pale extreme in our 1973 Hamilton boat trip, Curry classification, being much paler than (19741 reported that 23 jaegers were most intermediates. seen, "none of which was adult". Based on his genetic studies, Bob Curry (in litt.l noted that "Most

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years juveniles outnumber adults information on plumages and considerably but a closer look at the morphs. See also the excellent article data reveals a more interesting and illustrations by Jonsson (1984) on temporal pattern. The majority of the identification of juvenile adult Parasitics at Hamilton occur in Pomarine (P. pomarinus) and Parasitic September and very early October Jaegers. while most birds seen later than this are juveniles. Thus, in some years Summary adults outnumber juveniles in Adult Parasitic Jaegers occur in three September in the ratio of 3:2, but colour morphs: light, intermediate taking into consideration the entire season juveniles will almost always and dark. Most adult Parasitic Jaegers greatly outnumber adults". seen in Ontario are light morph birds, whereas intermediate and dark Interestingly, more adults than morph adults are decidedly rarer. juveniles have been seen in Ottawa Genetic studies indicate that over the years (Bruce Di Labio, pers. intermediate morph adults are darker comm.], perhaps indicating that and more like dark morph birds in Ottawa is nearer the main route of appearance than the intermediates adults from James Bay to the Atlantic described and illustrated by most (Michel Gosselin, pers. comm.). authors. Our illustration of an Morphs of the juvenile are highly intermediate morph adult is therefore variable, ranging from light to dark, more typical of a genetically the main difference being the extent intermediate bird. Juveniles also of barring on the undersides. Cramp occur in three morphs. Juveniles seen (1983) describes three morphs: light, in Ontario tend to be light or barred (intermediate) and dark. intermediate in coloration, but often Distinguishing juveniles from adults appear dark at a distance. Juveniles at a distance is difficult to impossible. usually outnumber adults on the At close range, typical light and Great Lakes in fall. intermediate morph juveniles are streaked on the head and neck, Acknowledgements barred below, and edged with rufous The following people provided us above and below. The pointed central with information and helpful tail feathers are short and project comments on the first draft of this only slightly beyond the others. article: Dawn Brenner, Bill Crins, Jim Extremely dark morph juveniles are Dick, Rob Dobos, Jon Dunn, Earl uniformly sooty-black and are Godfrey, Michel Gosselin, Jean Iron, difficult to separate from dark morph Ross James, Alvaro jaramillo, Kevin adults unless the length of the tail McLaughlin, Dennis Rupert, Ron projection or fresh juvenile plumage Tozer and Mike Turner. can be seen. The seqence of colour changes from juvenile to adult is complex and Literature cited poorly understood. Olsen (1989) notes Beardslee, C.S. and H.D. Mitchell. 1965. Birds that dark juveniles may become light of the Niagara Frontier Region. Bulletin of the adults! Readers are referred to Cramp Buffalo Society of Natural Sciences. (1983) and Olsen (1989) for more Volume 22.

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Cooke, F. and P.A. Buckley (editors) 1987. [onsson, L. 1984. Identification of juvenile Avian Genetics: A Population and Ecological Pomarine and Arctic Skuas. British Birds 77: Approach. Academic Press, London. 443-446. Cramp, S. (editor) 1983. Birds of the Western Kaufman, K. 1990. A Field Guide to Advanced Palearctic: The Handbook of the Birds of Birding. Houghton Mifflin, Boston. Europe, the Middle East and North Africa. O'Donald, P. 1983. The Arctic Skua. A Study Volume 3: Waders to Gulls. Oxford of the Ecology and Evolution of a Seabird. University Press, London. Cambridge University Press. Curry, R. 1974. The 1974 Hamilton boat trip. Olsen, K.M. 1989. Field identification of the The Wood Duck 28141: 65·66. smaller skuas. British Birds 82: 143-176. Godfrey, W.E. 1986. The Birds of Canada. Parmelee, D.F., H.A. Stephens and Revised Edition. National Museums of R.H. Schmidt. 1967. The birds of Canada, Ottawa. southeastern Victoria Island and adjacent Goodwin, C. 1995. A Bird-Finding Guide to small islands. Bulletin 222. National Museum Ontario. Revised Edition. University of of Canada. Toronto Press, Toronto. Pittaway, R. 1992. Subspecies and morphs of Hams, A., L. Tucker, and K. Vinicombe. 1989. the Snow Goose. Ontario Birds 10: 72-76. The MacMillan Field Guide to Bird Rupert, D. 1995. Favourite Birding Hotspots, Identification. The MacMillan Press Ltd., Sarnia Waterworks. OFO NEWS 13 Ill: 2-3. London. Scott, S.L. (editor) 1987. Field Guide to the Harrison, P. 1983. Seabirds: An Identification Birds of North America. National Geographic Guide. Houghton Mifflin, Boston. Society, Washington, D.C. [ames, R.D. 1991. Annotated Checklist of the Terres, j.K. 1982. The Audubon Society Birds of Ontario. Second Edition. Life Encyclopedia of North American Birds. Sciences Miscellaneous Publications, Royal Alfred A. Knopf, New York. Ontario Museum, Toronto. Todd, W.E. C. 1963. Birds of the Labrador jehl, J.R. Jr. and B.A. Smith. 1970. Birds of Peninsula and Adjacent Areas. University the Churchill Region, Manitoba. Special of Toronto Press, Toronto. Publication Number I, Manitoba Museum of Man and Nature, Winnipeg.

Ron Pittaway, Box 619, Minden, Ontario KOM 2KO. Peter Burke, Box 1092, Lakefield, Ontario KOL2HO.