Tuatara: Volume 15, Issue 3, December 1967
Studies On The Skua by members of the University of Canterbury have been at two of the major concentrations of skuas on Ross Island, at Cape Royds, occupied each year from 1959 until the summer 1965/66, and at Cape Bird for the past two summers. The study shifted to Bird to allow examination of a larger, more typical, grouping of skuas about penguins than at Royds.
The South Polar Skua (Catharacta maccormicki) is similar in size and appearance to the juvenile of the largest of the New Zealand gulls (Larus dominicanus). The plumage is more variable, ranging from fawn to dark brown. Spellerberg (1966) has suggested that these skuas may be polymorphic for colour, with three separate forms (light, mid and dark brown) genetically determined. The head and neck are usually more lightly pigmented than the rest of the body and all birds have a white band on each wing that shows during flight and in some of the displays. Skuas are noisy, intelligent and aggressive birds and able to fly much better than their rather heavy build would suggest. Their flights from point to point average around thirty miles an hour and are marked by a steady wing-beat and purposeful, stolid attitude. As a guide to their abilities in the air these flight characteristics are misleading and around the breeding areas and penguin rookeries they appear different birds — indulging in fantastic flights, wheeling, twisting and diving at breakneck speeds.
These birds spend most of the year away from the Antarctic, and although few have been seen at this time it has been suggested (a) that they live in the pack-ice zone ringing the continent and/or (b) tour the main oceans of the world, even beyond the equator. Studies on skua at the moment all have a common starting point with the return of the first birds in spring. These appear at Ross Island in late October but take little interest in their breeding areas until about the second week in November. The Ross Island birds seem now to gather first at the McMurdo-Scott rubbish dumps and disperse later. The birds nest singly or in loose colonies around the entire continent wherever there is ice-free land near the sea, both with and away from breeding penguins. A few nest far inland where they seem to be existing on food taken from breeding petrels.page 143
Some 600-700 pairs of skua breed on the western side of Ross Island. Roughly, there are three breeding areas: on the Dellbridge Islands and Cape Evans, about Cape Royds, from Cape Barne in the south to Rocky Point in the north, and a single very large group much further north about the Cape Bird rookeries (Cape Bird itself is five miles further north again and is ice-covered). The only other major breeding area on the island is at Cape Crozier, forty miles to the east. Spellerberg (1966) found fewer breeding birds on the western side of McMurdo Sound during a recent survey there. The skuas in three of these areas have been studied in some detail recently; those at Royds by Young in 1959/60 and Spellerberg in 1963-1965, those at Bird by Young in 1965/66 and Young and Procter in 1966/67 and those at Cape Crozier for a number of years by Robert Wood for the Johns Hopkins University. [In case this seems a lot of people for one bird, I mention that there is also a vigorous programme at Cape Hallett operated by the New Zealand D.S.I.R., and at least two other studies by other countries elsewhere in the Antarctic.]
The University's programme has gone through a common enough evolution, starting with the general study of nearly all aspects of the breeding biology and maturing into more detailed work on single topics. These are considered below.
Begun by Young at Royds and Reid at Hallet and continued by Spellerberg at Royds these involve recording the success of the various phases leading to the fledging of juveniles at the end of summer. Pairs and nests are identified, egg laying, hatching and survival recorded and chick growth followed to fledging. Spellerberg has added considerably to the earlier work by following the same pairs through several years. He has found firstly that most skuas are faithful both to their mates and breeding territories and secondly, that there are wide differences in the survival of eggs and chicks in the different years. He is at present sorting out the factors determining the survival differences but they seem mainly to be climatic, depending on the timing and severity of the storms in each year. All these studies have shown that the skuas have a very low breeding success—only about a quarter of the eggs laid produce chicks to fly from the territories at the end of summer.
Studies of nesting birds invariably generate studies of breeding behaviour, and skua studies are no exception. The most detailed work is that of Spellerberg (1966) using as a pattern, and for comparison, the work of Perdeck (1960) on the northern hemisphere page 144 Great Skua. The two birds appear to have closely similar behaviour. A study comparing skua behaviour to that of the more sophisticated gulls is now needed, and this really requires an experienced ethologist rather than a field zoologist, especially as in many places the behaviour of the two groups is similar. [We can note in passing that experienced workers in this field are most rare.]
Behavioural studies involve precise descriptions of activities (preferably into a tape recorder and backed by cine filming) from long periods of alert observation on undisturbed birds. This last condition may be more difficult to realise than previously thought. Procter (1967) found that birds acting apparently normally on the first day of a watching period became steadily more distracted as the days passed and never became used to the observer. Hides will need to be rigid and unobtrusive to be accepted at all readily and even with these doubt will remain of their effect. For most studies it is necessary to decide at the outset whether ‘normal’ behaviour is what is required or whether weights, length measurements and so on are needed. From sad experience it appears that only one of these can be studied safely with any one group of birds. Disturbed nesting skua neither leave the territory nor feed each other or the chicks—accounting for the few times feeding is seen by those walking or working in skua colonies. Birds should be handled as little as possible and even banding, though necessary, causes great distress and can change a placid group into one that is nervous and violently aggressive. In this marked sensitivity to disturbance skuas differ tremendously from penguins, which as far as it is possible to tell, settle down much more quickly following the turmoil and bedlam of nest-checking. Few penguins desert their nests during routine working.
[In the past two years I have attempted to reduce disturbance of penguins by mapping the nests in the colonies and thereafter checking the nests as far as possible from the periphery. As a working policy I have handled and disturbed skuas as little as possible, by restricting banding to a single bird of a pair, by never intentionally flushing birds from their nests and by cutting out the widespread, and repetitive, measuring and weighing of eggs and chicks. I hope through these precautions that my study of the penguin - skua association is of a “normal” situation and that the birds will continue breeding in their present numbers and distribution.]
Since the earliest studies at the beginning of the century, it has been known that although skuas lay and often hatch two eggs they seldom rear two chicks. Young (1963a) sorted out that it was the older chick of the two that invariably survived (and few exceptions page 145 have since been found) and that the fighting between the two chicks soon after hatching resulted in the younger and weaker chick being chased from the nest area, and often into starvation. This work merely described what took place without providing any sort of explanation. Last season Procter worked specifically on this aspect of chick survival and gathered evidence suggesting that fighting begins when the older chick is becoming hungry. He could set it off by stopping chick-feeding for an hour or so. By moving chicks of different ages around a series of nests he also established that it is the stronger and more robust chick, whether a first or second hatched chick, that wins an encounter. The problem of parental response to chick-fighting and the separation of the two chicks on the territory will be looked into in the coming summer.
Thermoregulation and Body Temperatures
Spellerberg's (1966) work has produced a most detailed account of this phenomenon in the skua. Using thermocouples, temperatures were obtained from incubating eggs through the incubation period, in which they showed a steady temperature rise with time, and in chicks and adults over a period long enough to establish the onset of temperature stability in the former (about 50 hours from hatching) and a diurnal variation in the latter. Experiments established that the feet and legs were important areas for radiation of heat from the body when the animal was active. Such radiating areas are important in birds which have the body efficiently insulated with feathers.
Feeding and the Association with Penguins
The early, casual, observations suggested that skua preyed exclusively on penguins, were dependent on them entirely and nested near them because of this. [These conclusions no doubt account at least in part for the poor reputation skua enjoy.] This view was challenged by Young (1963b) and has now given way largely to the view that skua are really a fish-feeding species simply taking advantage of the abundant food to be found at times on the penguin rookeries. It is, of course, much easier to establish the skuas’ independence of penguins in areas, as at Royds, where a handful of penguins is surrounded by several hundred pairs of skuas, than when the opposite occurs.
Indeed, it is not yet certain that skuas at the large rookeries at Hallet and Crozier do need to forage at sea during summer. Certainly Maher (1966) thought that the skuas fed only from the rookery at Hallet, but the studies resulting in this paper were done in 1960/61 before their nature as fishing birds was widely page 146 appreciated. [Looking back it seems that work done by the University on feeding and the association between the two birds followed from our going first to Royds. If we had begun at Hallet or Crozier probably a far different sort of programme would have resulted. Similarly, my preoccupation with skua also came about by chance as I went to Royds in 1959 expecting to work with penguins but found the study pre-empted and on the barren wastes could see a single alternative, the skua—a switch of animal never since regretted.]
It is clear from the studies at Royds and Bird that skuas are unable to find enough food at the rookery to feed them throughout the summer. Only for a short period, from late December to mid January, is food more or less abundant in the form of penguin chicks and at this time skua pairs with more than a hundred or so penguin nests in their territory are able to feed largely from them. For the remainder of the year, and especially before December and after January, pickings are very lean and virtually all pairs feed at sea on most days. Last year a long period of northerly winds packed ice into McMurdo Sound, preventing skua from fishing there, and nearly all eggs laid were deserted as the birds began starving—in pairs without penguins and those with 500 or more penguin nests to forage amongst. The incubation period is a great strain on the male bird. He has not only to take his part in incubating but also to provide all food for the two birds of the pair. Little wonder that nests were deserted when the male was away at sea for up to 20 hours at a spell instead of an hour or two.
The general observations on the association between penguins and skuas made at Royds have been taken much further during the past two summers at Cape Bird. This study area contains 26,000 pairs of penguins and 200 skua pairs near them and provides opportunity for experimentation not possible at Royds. Three aspects of the skua-penguin association have been examined. One study has been to determine precisely what effect the skuas have on penguin breeding—the number of eggs and chicks taken. This seems simple enough to determine but is complicated by the fact that the penguins cause a considerable loss themselves, through egg infertility and desertion, and mismanagement and fighting among the breeding birds. It has been possible to apportion losses on the colonies between penguins and skuas by comparing survival in colonies fenced off from skuas with those in which they have a free run. Comparisons of this sort are useful also in sorting out whether the sudden upsurge in egg and chick mortality that occurs about the time the penguin eggs are hatching results from increased interest in the penguins by the skua, chicks being better value for risk and easier to take than eggs anyway, or from disturbance to breeding birds by penguins that had lost their nests earlier returning to the colonies.page 147
The skuas have a number of less immediate effects on penguin breeding and these form a second study. Firstly, at least in part, they are responsible both for the colonial nesting habit of penguins — few isolated penguins rear chicks — and the close synchronisation of breeding dates within a rookery. Secondly, by taking relatively more of the progeny of both the early and late breeding birds the variation among penguin chicks becomes steadily reduced through the season.
The third topic concerns the effect of the relation on the skua. In general they are much less affected by this association. They have not lost their ability to forage at sea and there is as yet no certain evidence that they are competing for areas containing penguins instead of those that are most suitable for breeding. However, pairs nesting among the penguins fared very badly and reared few chicks. Some had their nests destroyed by penguins. All of them tended to rely on penguin food rather than forage at sea so that the nest was often deserted, allowing the eggs to cool, or to be taken by other skuas, when food was obtained after a period of starvation. Skuas breed far more successfully away from penguins except when conditions are so bad that fishing is prevented.
Direct observation involving as it does long periods of boredom to mull over things has furnished two other conclusions in this vein. As skuas are so poor at killing chicks, and feeding from them, and as birds coming newly into a rookery territory are almost totally incompetent, it seems a reasonable conclusion that they are not doing what they were designed for; which presumably is feeding at sea. Also, the best way to protect penguins from skuas, short of fencing over the colonies in cherry-orchard style, is to disturb the breeding skua pairs in the rookery as little as possible. These birds keep all others away from the penguins and their requirements are far less than for a flock of non-breeders.
As a general conclusion it is fair to say that the University's studies both at Royds and Bird have been directed at specific problems in which some answers could be expected within a season or two. This contrasts with the other approach of long term banding and recording, stretching over a number of years, that has characterised the studies at Hallet or Crozier. The two sorts of project are complementary, rather than competitive, and provide different information. Most studies have been on topics peculiar to the Antarctic, thermoregulation for example, or on general problems that are more easily investigated there than elsewhere, such as the predator/prey relation, in this case of skua and penguin. The topic “Conservation” has not loomed largely in the programme even though the skua deserves sympathy because of its low breeding success (about 1/4 to 1/3 that of the penguin), its sensitivity to page 148 human disturbance when breeding and its role of penguin protector, albeit unconscious, at many adelie rookeries.
This sort of work is expected to continue for at least another three or four seasons at Cape Bird: there are more problems now than answers and the situation worsens year by year.
Maher, W. J., 1966. Predations impact on penguins. Nat. Hist. 75(1): 42-51.
Perdeck, A. C., 1960. Observations on the reproductive behaviour of the Great Skua or Bonxie, Stercorarius skua skua (Brünn) in Shetland. Ardea 48:111-136.
Procter, D. L. C., 1967. Causes of chick mortality in the South Polar Skua. Unpublished report.
Spellerberg, I. F., 1966. Ecology of the McCormick Skua Catharacta maccormicki (Saunders) in Southern McMurdo Sound, Antarctica. Unpublished thesis. University of Canterbury.
Young. E. C., 1963a. The breeding behaviour of the South Polar Skua. Ibis 105: 203-233.
—— 1963b. Feeding habits of the South Polar Skua. Ibis 105: 301-318.