sign in sign up
<<
Home , Chilean skua, Great skua, Skua, South polar skua

1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 Pirates of the oceans 12 13 13 14 Skuas are the pirates of the world. Like 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 Pygoscelis , but other 19 20 have to watch out when skuas are around, or they may find themselves losing a meal, their and chicks, or even their lives. 20 21 Above right An intermediate phase Arctic 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 in the Stercorarius 25 26 T skua approaching a flock of we only see these remarkable birds (namely, the Pomarine, Arctic and 26 27 , 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 is forced to regurgitate, erally at the ends of the earth. 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 . 40 41 Antarctica, southern South America 41 42 and at sub-Antarctic islands. Only the 42 43 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 , 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 , 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 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 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 60 PETER STEYN the North Atlantic, resulting in speculation en from a 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 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 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 , the breeding pairs out of a population of Both sexes incubate, although the ensures that at least one chick survives 13 14 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. tories looking for vacancies. In a some- Below An endearing Sub-Antarctic Skua chick with its well-developed 19 20 Although most birds hunt singly, what callous experiment on Gough legs which allow it to run and hide when threatened. 20 21 pairs team up to kill birds almost the Island, researcher Bob Furness removed ONNO HUYSER 21 nigel adams 22 Above Sub-Antartic Skuas have extremely catholic diets, including carrion same size as themselves, such as one member of a pair from a territory 22 23 such as this young penguin. At the breeding islands they are displaced from White-chinned Procellaria aequi- each night for 10 successive nights. In 23 24 carcasses by the larger giant petrels, Macronectes spp. noctialis. Other pairs work together to each case, the bird was replaced within 24 25 steal eggs and chicks from large, sur- a few hours the next day by a new 25 26 on piracy to feed their chicks. During face-nesting birds such as recruit from the club, irrespective of the 26 27 Specialist generalists the non-breeding season, Arctic and and albatrosses. One bird distracts the sex of the bird removed. 27 28 Skuas differ from gulls in a number of Pomarine skuas parasitize seabirds in guarding parent while the other raids Although monogamy is the rule, 28 29 ways. Perhaps the most remarkable dif- coastal waters, although Pomarines are the nest. Small prey, such as storm some birds, notably certain populations 29 30 ferences are the larger size of females not averse to snacking on the odd petrels, are swallowed whole, but larger of Sub-Antarctic Skuas, form breeding 30 31 and the presence of pale and dark phalarope or other small bird. By com- birds are torn apart. Skuas don’t use groups. These almost invariably involve 31 32 plumage varieties independent of age parison, Long-tailed Skuas occur far their feet to hold prey, complicating one female and two or more males, and 32 33 or sex. Both these features are unusual offshore and have seldom been seen the handling of large prey. Pairs often may be stable for several years. The 33 34 among birds, but are found in other stealing food from other birds. combine to tear up food items, and exact reason why some birds opt for a 34 35 birds of prey, which suggests that skuas The large skuas use a wide array of newly hatched skua chicks may starve group breeding strategy is unclear, but it 35 36 evolved these characters in response to feeding techniques. Some Great Skuas if one parent dies, leaving the other doesn’t appear to be related to a prob- 36 37 their predatory lifestyle (see box, page kleptoparasitize Northern incapable of reducing the meal into lem with feeding the chicks: pairs have 37 38 63). Other adaptations to a life of skul- Sula bassana and other seabirds, but bite-sized pieces for the chicks. the same or slightly better breeding suc- 38 39 duggery and piracy include a powerful few of the other large skuas indulge in One characteristic of skua diet is the cess than groups. Nor is it a case of 39 40 bill for tearing flesh, and long, recurved piracy during the breeding season. considerable variation between individ- groups of related birds working togeth- 40 41 claws. Also, relative to gulls, skuas have are important in the diets of uals. They are supreme opportunists, er: generally the males involved are not 41 42 larger, more efficient flight muscles and some, notably South Polar and Great willing to try any new gastronomic closely related. Intriguingly, there is no 42 43 longer primaries to allow them to accel- skuas. Other birds steal eggs and possibilities, even bars of soap left out strict hierarchy in terms of access to the 43 44 erate rapidly – useful when chasing chicks from penguins and other sea- by unwary visitors! Once an individual female, with all males copulating and 44 45 other birds. birds, or compete with giant petrels finds a successful foraging technique, it fathering offspring. 45 46 Skuas are renowned for kleptopara- Macronectes for carrion. may exploit it to the virtual exclusion Once the pair bond is established, 46 47 sitism – stealing food from other birds. However, the most impressive forag- of other methods, often destroying the the female becomes almost totally reli- 47 48 They are arguably the bird world’s ing behaviour is the predation of bur- resource base. Single birds have been ant on the male for food. She remains 48 49 greatest pirates. Frigatebirds, the other rowing petrels and other seabirds. On recorded to cause the breeding failure on the territory and begs for food, 49 50 contenders for the piracy crown, are the breeding grounds Sub-Antarctic of entire seabird colonies by developing inducing the male to regurgitate. 50 51 certainly efficient at robbing boobies, Skuas are every bit as dashing as fal- a particularly effective new foraging When faced with a begging female, 51 52 but stealing is relatively unimportant cons, pursuing petrels in high-speed technique. Such methods can also males have the choice between regurgi- 52 53 in their diet, and their morphology chases where the goal is more than spread through the population because, tating or soliciting copulation. Perhaps 53 54 has evolved to grab flying fish out of just robbing the petrel of its last meal. like gulls, skuas have the ability to learn not surprisingly, the appropriate 54 55 the air. But most petrels are killed at night on new behaviours. response is to feed the female when 55 56 the ground, pounced on as they stum- 56 However, not all skuas live by piracy. Skua home life her begging is especially vociferous! 57 Small skuas breeding on the Arctic tun- ble to and from their burrows. Skuas All skuas nest on the ground, select- 57 58 dra feed mostly on lemmings and other also dig up burrows to reach petrels, Like almost all seabirds, skuas breed in ing a level area which is often slightly 58 59 , as well as birds, insects, berries often showing considerable persever- the same area and tend to retain the raised to afford all-round visibility. Both 59 60 and carrion. Only Arctic Skuas breeding ance as they wrestle for up to an hour same mate in successive years. The only sexes contribute to building the nest 60 61 in association with seabird colonies rely to clear roots and other debris from exception is the Pomarine Skua, which scrape, which tends to be rather rudi- 61 62 nest entrances. is nomadic, moving around to breed  mentary. The clutch size is almost 62 63

60 skuas africa – birds & birding 1997 – volume 2, number 5 SKUAS 61

 1 the territory, be they foxes, humans or However, if this measure fails, Arctic food and persecuted because they 1 2 sheep. The ferocity of these attacks var- Skuas are unique among skuas, gulls occasionally attack livestock, but for- 2 3 ies between birds and with the stage of and terns in also having a ‘broken tunately such practices have largely Convergence with raptors 3 4 the breeding season, peaking shortly wing’ distraction display to lure away ceased. One threat skuas face currently 4 5 after the eggs hatch. Interestingly, Sub- particularly persistent predators such as is another consequence of their preda- Large females and plumage polymorphism 5 6 Antarctic Skuas breeding on predator- Arctic foxes. tory lifestyle: like terrestrial birds of jim enticott 6 7 free Sub-Antarctic islands show the Once the chicks fledge, they disperse prey, skuas tend to accumulate high the care and guarding of the eggs and chicks, while the 7 8 same behaviour, although they only and, in the case of long-distance levels of pollutants, such as PCBs and male does most of the hunting. Large size in females has 8 9 become aggressive towards the end of migrants, many don’t return to the DDT derivatives, that are concentrated been favoured to afford maximum protection at the nest, 9 10 the incubation period. This mobbing breeding grounds until they are two through food chains. Another problem especially in species where nest-robbing by adjacent territo- 10 11 behaviour is effective, as anyone who years old. Some individuals start breed- is that changes in fishing practices, ry holders is not unknown. 11 12 has searched for skua chicks can attest. ing as early as one to two years of age, notably the reduction in waste dump- The other unusual character found in skuas and many 12 raptors is plumage polymorphism – the presence of a num- 13 The birds dive-bomb you from behind, but most take considerably longer, ing, is reducing this food source for 13 ber of plumage types irrespective of age or sex. One possi- 14 often striking you on the head with especially in areas where competition skuas and other seabirds. In Europe 14 ble explanation for this feature is that prey species learn to 15 their feet. Should you choose to ignore for territories is intense. Adult survival there is concern that this is causing 15 recognize predators. Colour morphs make life harder for 16 16 this warning, Sub-Antarctic Skuas will is fairly high: some 90 per cent return Great Skuas to increase their predation prey species, and favour the persistence of rare varieties in 17 17 even land on your head and try to pull to the breeding grounds each year. The on other seabirds. On the whole, the predator population by enhancing their feeding 18 out beaksful of hair! longest lived individual known from though, the remote areas where skuas success. Among small owls, polymorphism may reduce 18 19 The small skuas are especially good at ringing studies is a 34-year-old Great breed and their pelagic existence when Above A rather dark pale-phase South Polar Skua on the detection during the day, when they are subject to mob- 19 20 synchronizing their attacks, so that as Skua from . away from the breeding grounds, Antarctic ice. This is the only Catharacta species to show bing and predation. 20 21 one bird whizzes over your head, the In the past, skuas breeding in areas should ensure the survival of this fas- well marked plumage polymorphism. The pale phase is Pale and dark forms are found in the small Stercorarius 21 22 other is lining up for its next pass. frequented by man were exploited for cinating family of ocean pirates.  readily distinguished from other Catharacta skuas, but the skuas, although dark morphs are rare among Pomarine and 22 23 dark phases pose serious identification problems. especially Long-tailed Skuas. Pale and dark forms also occur 23 24 Below A Sub-Antarctic Skua giving the long-call display with raised wings. in South Polar Skuas, and there are at least vestiges of poly- 24 25 nigel adams n most birds the male is the larger sex, presumably morphism among some of the other large skuas. 25 26 because sexual selection favours larger males in the com- Is there evidence to support the idea that polymorphism 26 27 Ipetition for mates. Skuas are unusual in that females aver- enhances feeding success? The generally dark plumage of 27 28 age 10–15 per cent heavier and some three per cent larger skuas (relative to gulls and many other seabirds) is suggest- 28 29 than males. They share this ‘reverse size dimorphism’ with ed to be a form of aggressive camouflage, allowing skuas to 29 30 inter alia frigatebirds, owls and raptors, which suggests that sneak up on potential victims. Among colonies of Arctic 30 31 it is a characteristic of a predatory lifestyle. It has been Skuas that feed almost exclusively by robbing other sea- 31 32 argued that smaller males are favoured because of the exten- birds, dark birds are preferred as mates and are the most 32 33 sive use of aerial chases and displays in skua territoriality. common form. However, they do not have a higher attack 33 34 Smaller birds are more agile, and can outcompete larger success rate than pale birds, presumably because many 34 prey species fail to recognize the threat posed by the much 35 males in the air. There is some validity to this argument; 35 male skuas have proportionately longer tails than females, rarer pale-morph skuas. Interestingly, dark-morph Arctic 36 36 enhancing their aerial abilities. However, it doesn’t explain Skuas become increasingly rare farther north, correspond- 37 37 reverse dimorphism among owls, which are not renowned ing with a switch from bird to mammal prey. 38 38 for aerial displays! Colour variation in skuas is thought to result from switch- 39 39 Behavioural differences during breeding provide a more ing in a single gene. However, males tend to have a higher 40 plausible explanation for the larger size of females. Among proportion of dark individuals than females, suggesting that 40 41 skuas and other predators, the female undertakes most of additional factors may play a role.  41 42 42 43 Below A dark-morph Arctic Skua on its nest. Plumage polymorphism has been studied extensively in this species, 43 44 which shows geographic and sexual differences in the frequency of colour phases. 44 45 peter steyn 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

62 skuas africa – birds & birding 1997 – volume 2, number 5 SKUAS 63