 The
Skuas are a distinctive group of larid-like kleptoparasitic predators.
British and Old World observers call all 7 species "skuas" while Americans
restrict the term "skua" to four species of very closely related 'Catharacta'
skuas, leaving the three Holarctic breeding species, the original Stercorarius
group, as "jaegers." The three jaegers nest on the Holarctic tundra; a
light-morph adult Parasitic Jaeger (left) is shown here at
its nest site in the boggy terrain at Churchill, Manitoba, Canada. In the
Old World, this same species is known as Arctic Skua.
Recently, biochemical evidence (e.g., Cohen et al. 1997) has shown that Pomarine Jaeger is more closely related to the other skuas than it is to the two other jaegers; they suggested that the genera be merged (into Stercorarius), a suggestion followed by the A.O.U. (2002). Yet it is easy and convenient to separate the seven species of Stercorarius into 4 skuas and 3 jaegers, and I do so here. I live on the shores of Monterey Bay and take pelagic trips regularly. The three jaegers are regular migrants and (in season) are routine, but the shout "SKUA !" means an encounter with something much rare — South Polar Skua — that deserves immediate attention. Although it is also regular in autumn, numbers are minuscule compared to the three jaegers. Here, at least, the difference between "jaeger" and "skua" is important. Skuas are closely related to gulls, terns, skimmers, and alcids; indeed, some authors follows Sibley & Monroe (1990) in considering them simply a tribe (not even a subfamily) of an expanded Laridae family. I prefer the traditional approach that considers skuas to constitute a family, an approach used in the on-going Handbook of the Birds of the World series (Furness 1996). This approach emphasizes their distinctiveness while recognizing that they evolved from larid-like ancestors. Further, new biochemical evidence (Paton et al. 2003) shows that skuas are the sister group of alcids, and should therefore be considered a family even if the gulls, terns, and skimmers are lumped together. |
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 Although
related to gulls, all the skuas show adaptations associated with their
piratical and predatory way of life, and with their breeding at high latitudes
close to both poles. The plumage is particularly dense to limit heat loss,
and the heavily armored legs and feet are thought to serve a similar purpose.
Strong bills and claws aid the predatory lifestyle, as to short wings and
powerful musculature for piracy.
Kleptoparasitism or piracy — the stealing of food from other birds — is highly developed in skuas. Some species or populations take almost all their food at sea by stealing from other birds. It is thought that the big white wing patches in the outer primaries serve to emphasize to other birds that a pirate is attacking, and that they best disgorge their prey! Here a South Polar Skua (right) shows its white wing patches as it maneuvers on Monterey Bay. It is interacting with a smaller Heermann's Gull Larus heermanni; oddly enough, a small percentage of this dark gull have developed white wing patches (but most, like this one, have not), presumably as a mimic. And, on the theory that 'turnabout-is-fair-play,' this Heermann's is actually chasing the skua! |
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While any particular jaeger or skua may chase most any bird that has
food, here on Monterey Bay the migrations of the three jaegers are closely
tied with the migrations of three smaller seabirds. The large Pomarine
Jaeger (below left), adults of which have long but broad-tipped
tails (as shown), is generally found in a band ~2-20 miles offshore where
the migration of Sabine's Gull Xema sabini occurs. Or at least this
pattern is obvious in August-September; some Pomarine Jaegers linger around
Monterey Bay into fall and winter, and switch their piratic efforts to
Black-legged Kittiwakes Rissa tridactyla and California Gulls Larus
californicus. The mid-sized Parasitic Jaeger is most often encountered
within a mile or two of shore, and sometimes in estuaries, as it chases
Elegant Terns Sterna elegans that have moved north from nesting
grounds in Mexico or southern California. The small and lovely Long-tailed
Jaeger (below right) kleptoparastizes the dainty Arctic Terns
Sterna arctica that head south far offshore. Thus Long-tailed Jaeger is
usually found 20-40 miles offshore on longer pelagic trips.
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 All
the photos so far have been of adults with characteristic tail patterns.
Jaegers typically do not breed until 3 years old, and the larger skuas
not until they are 5-6 years of age (Furness 1996, Olsen & Larsson
1997). Immature plumages are complex and not well understood. But we do
now have a good handle on the identification of juvenal-plumaged jaegers
in their first flight south. The 'cold-colored,' short-billed juvenal jaeger
in flight (right) is Long-tailed Jaeger. And yes, that is
something different that the ocean in the background! This juvenal Long-tailed
strayed inland to the Salinas wastewater ponds in September 1990. It stayed
a couple days and then, a few days later, a buffy-colored juvenal Parasitic
Jaeger appeared. I photographed it as well. Years later a rumor got back
to me that "local Monterey birds had screwed up their jaeger i.d." because
some out-of-town birder, chasing the Long-tailed that was on the Rare Bird
Alert but visiting several days later, located the Parasitic. I was told
that the rumor had spread throughout the eastern U.S. that we had messed
up our jaegers.... All going to show that the ignorant should not start
rumors! Little did the out-of-towner know that both birds had been extensively
studied and photographed. [Can you tell that I am still annoyed about those
rumors?] |
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The big 'Catharacta' skuas are even more of an i.d. challenge
(but see Devillers (1977, 1978; Olsen & Larsson 1997). Their kleptoparasticism
can accelerate into predation. The three southern species nest around colonies
of other seabirds, and are major predators and scavengers on penguins,
terns, gulls, and baby seals and sea-lions.
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| Photos: The nesting Parasitic Jaeger
Stercorarius parasiticus was on the tundra at Churchill, Manitoba,
Canada, in July 1988. The chasee South Polar Skua
S. maccormicki was harassed by the Heermann's Gull on Monterey Bay,
California, on 22 Oct 1998. The adult Pomarine
Jaeger S. pomarinus was well offshore Pt. Arguello, Santa
Barbara Co, California, on 5 Feb 1994. The adult
Long-tailed Jaeger S. longicaudus was offshore
of the Monterey Peninsula, California, on 22 Aug 2004. The juvenal Long-tailed
Jaeger was at the Salinas wastewater ponds, California, where it
was a vagrant on 2 Sep 1990. John Marchant photographed the adult
Great Skua S. skua at the Hermaness, Shetland, U.K.,
on 10 June 1975. Greg W. Lasley photographed the adult
Brown Skua S. antarctica at Copper Bay, South Georgia Island,
on 27 Jan 1999. Photos © 2004 Don Roberson except
those attributed to John Marchant and Greg W. Lasley, and used with permission;
all rights reserved.
Bibliographic essay Olsen, K.M., and H. Larsson. 1997. Skuas and Jaegers: a Guide to the Skuas and Jaegers of the World. Yale Univ. Press, New Haven, CT This book is primarily an identification guide, although it does briefly summarize breeding biology, kleptoparasitism, and polymorphism in the introductory pages. In truth, I have not yet had occasion to review this book in any detail. There are color plates, color and black-and-white photos, maps showing migration routes and the like; it looks nicely done. There is a lot of detail about molts and ageing that appears to be of great use. But I lack the expertise to have much of an opinion about how to separate the 'Catharacta' skuas in the southern Hemisphere. This book does have a lot of text aimed at that topic. So, perfunctorily, this book looks quite promising.
Literature cited: American Ornithologists' Union. 2002. Forty-third supplement to A.O.U. Check-list of North American Birds. Auk 119: 897-906. |
