Pirates of the Oceans 12 13 13 14 Skuas Are the Pirates of the Bird World
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1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 SKUAS 9 10 10 11 11 12 Pirates of the oceans 12 13 13 14 Skuas are the pirates of the bird world. Like gulls with a killer 14 15 15 16 instinct,instinct, skuasskuas shareshare manymany characterscharacters withwith terrestrialterrestrial birdsbirds ofof prey,prey, allow-allow- 16 17 17 18 inging interestinginteresting insightsinsights intointo thethe biologybiology ofof birdbird predators.predators. 18 19 19 20 InIn thisthis feature,feature, thethe FitzPatrickFitzPatrick Institute’sInstitute’s PeterPeter RyanRyan exploresexplores thethe 20 21 21 22 varied and often contradictory world of skuas. 22 23 23 24 24 25 25 26 26 27 27 28 28 29 29 30 30 31 31 32 32 33 33 34 34 35 35 36 36 37 37 38 38 39 A pair of Sub-Antarctic 39 40 Skuas giving the 40 41 characteristic long- 41 42 call display with 42 43 raisedraised wings.wings. 43 44 ONNO HUYSER 44 45 45 46 46 47 47 48 48 49 49 50 50 51 51 52 52 53 53 54 54 55 55 56 56 57 57 58 58 59 59 60 60 61 61 62 62 63 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 ONNO HUYSER WARWICK TARBOTON 18 18 19 Above A pair of Sub-Antarctic Skuas sitting peacefully next to a dozing Chinstrap Penguin Pygoscelis antarctica, but other birds 19 20 have to watch out when skuas are around, or they may find themselves losing a meal, their eggs and chicks, or even their lives. 20 21 Above right An intermediate phase Arctic Skua in flight, showing the much more slender build and elongate central tail feather, 21 22 characteristic of Stercorarius. COLIN PATERSON-JONES 22 23 23 24 here are few more stirring sights have captured the imagination of era, but this is controversial. Three 24 25 than the rakish silhouette of a birders around the world. But in Africa small species in the genus Stercorarius 25 26 T skua approaching a flock of we only see these remarkable birds (namely, the Pomarine, Arctic and 26 27 terns, accelerating with deep, effort- during the non-breeding season. To Long-tailed skuas) breed on the Arctic 27 28 less wingbeats. Your pulse quickens as understand skuas you have to travel tundra or, in the case of some Arctic 28 29 the skua pursues its victim relentlessly to their breeding grounds, almost lit- Skuas, in association with northern 29 30 until the tern is forced to regurgitate, erally at the ends of the earth. seabird breeding colonies. After breed- 30 31 and the skua swoops to grab the bolus The skuas are a small family, closely ing they migrate south to winter in 31 32 of food in the air. Skuas are masters of related to the gulls. The consensus is the southern hemisphere. An excep- 32 33 aerial pursuit, and their dogfights to recognize seven species in two gen- tion is the Pomarine Skua, which has 33 34 a more northerly wintering range, 34 35 with many remaining in the tropics. 35 36 By comparison, the four large skuas 36 37 in the genus Catharacta (South Polar, 37 38 Sub-Antarctic, Great, and Chilean) 38 39 tend to have a southerly distribution. 39 ONNO HUYSER 40 Three species breed around the coast of Above A pair of Sub-Antarctic Skuas at their nest site on a rare clear day at Bouvet Island. 40 41 Antarctica, southern South America 41 42 and at sub-Antarctic islands. Only the 42 43 Great Skua breeds in the northern ONE GENUS OR TWO? 43 44 hemisphere, where it is restricted to 44 45 the north-east Atlantic. All four species Historically there was much confusion was its most likely ancestor. However, this Skua and a male Arctic or Long-tailed 45 46 tend to disperse towards warmer areas regarding skua taxonomy, partly because argument has been confounded by recent Skua. The latter explanation accounts for 46 47 during the winter non-breeding sea- of the range of plumages, and partly genetic evidence which shows that the the mosaic of characters exhibited by 47 48 son, but only the most southerly breed- because of regional variation within the three southern species of Catharacta are Pomarine Skuas. Although they closely 48 49 ing species, the South Polar Skua, large skuas. Most authorities accept two more closely related to each other than resemble the small skuas, including hav- 49 50 migrates across the equator. It com- genera, but there is debate about the they are to the Great Skua. ing the diagnostic barred juvenile plum- 50 51 petes with the Arctic Tern Sterna paradi- Pomarine Skua, which is somewhat inter- More surprising is the finding that the age found only among Stercorarius, 51 52 saea for the longest migratory range, mediate between the small Stercorarius Pomarine Skua is very similar genetically detailed analysis of their morphology 52 53 with a bird ringed as a chick on the (termed jaegers in America) and the larger to the Great Skua, much more so than it suggests they are closer to the large 53 54 Antarctic Peninsula being recovered six Catharacta skuas. More contentious has is to either of the other small skuas. This skuas, and they share the wing-raising 54 55 months later north of the Arctic Circle. been the status of the various forms of is based on looking at genes carried in display with the large skuas. 55 56 56 The South Polar Skua also is remark- large skuas, which are rather similar and mitochondria, which are inherited solely Despite this confusion, seven species 57 able for being the only vertebrate other 57 have been combined in various ways. from the female parent. Either Pomarine currently are accepted. All but the 58 than man known to visit the South 58 Researcher Bob Furness has suggested Skuas coincidentally resemble the other Chilean Skua occur at sea off Africa. 59 Pole. It breeds in association with Snow 59 that the Great Skua is a recent colonist of Stercorarius skuas, or the species has aris- 60 Pagodroma nivea and Antarctic Petrel 60 PETER STEYN the North Atlantic, resulting in speculation en from a hybrid between a female Great 61 Above Sub-Antarctic Skua pairs often hunt co-operatively, and help each Thalassoica antarctica colonies, south to 61 as to which of the three southern species 62 other tear up large prey items, such as the remains of this bird. almost 80 ˚S. 62 63 58 SKUAS AFRICA – BIRDS & BIRDING 1997 – VOLUME 2, NUMBER 5 SKUAS 59 1 Ironically, away from its breeding where lemmings are abundant. Males invariably two eggs. Skuas can only lost the innate behaviour found in 1 2 grounds the highly pugnacious Sub- arrive on the breeding grounds first, successfully incubate two eggs at a gulls to retrieve eggs that roll out of the 2 3 Antarctic Skua seldom hassles other and re-establish their territory. When time; adding a third egg results in all nest. Displaced eggs are ignored, which 3 4 birds, feeding largely for itself, and the female arrives, she is initially three failing to hatch. This is because may account for the preference to nest 4 5 often scavenging at fishing vessels. By rebuffed, but after a few days the pair skuas have rather small brood patches, on flat ground! The chicks hatch with a 5 6 comparison, the South Polar Skua, bond is reaffirmed, and she joins the and they incubate their eggs on their thick layer of down, but are closely 6 7 which is generally less aggressive than male in territory defence. feet to supplement the meagre insula- guarded for the first few days. The first- 7 8 the Sub-Antarctic Skua where the two Breeding skuas are intensely territori- tion provided by the nest scrape. The hatched chick, being larger, gains pref- 8 9 species breed together, undergoes a al. Often there are many more potential eggs are laid generally two days apart erential access to food and actively 9 10 Jekyll and Hyde transformation. breeders than available territories. For and, since incubation starts with the excludes the younger sibling from the 10 11 During the non-breeding season it example, Inaccessible Island in the laying of the first egg, the chicks hatch nest. Like many birds of prey, it may 11 12 becomes a fearless pirate, even tackling Tristan group supports fewer than 25 within a day or so of each other. even kill its sibling. This strategy 12 13 the largest of all flying seabirds, the breeding pairs out of a population of Both sexes incubate, although the ensures that at least one chick survives 13 14 Wandering Albatross Diomedea exulans. several hundred skuas. The unsuccessful female spends more time on the eggs when food is scarce, although generally 14 15 I vividly recall the bewilderment of a birds gather in large ‘clubs’ – often more than the male, who does most of the both chicks survive to fledging. 15 16 Wanderer tumbling out of the sky with than 100 strong – on the periphery of foraging. One curious aspect of skua Skuas defend their offspring pugna- 16 17 a South Polar Skua gripping firmly the breeding area. From here, unpaired incubation is that they appear to have ciously, dive-bombing intruders to 17 18 onto its tail, in an unlikely David and birds fly sorties over the breeding terri- 18 19 Goliath rematch.