A tribute to some of the most detested birds in the world from a biologist who studied them in the field for 40 years and came to appreciate their many admirable qualities.
I saw my first south polar skuas, Catharacta maccormicki, immediately after arriving at Scott Base in November 1959. They seemed like rather big, drab, noisy gulls as they scavenged and loitered around the dog lines. Not all that impressive. It wasn’t until I got to Cape Royds, 20 minutes up the coast of Ross Island by helicopter, to work with Rowley Taylor on the skuas and Adélie penguins in that area that I began to appreciate them. There they were in their true element, holding breeding territories, flying about the penguin colony, beating to and fro against the winds. At that time it was pretty much assumed that skuas depended on the penguins for their food over summer, and it was this assumption that made them such villains in the eyes of early expedition members.
After just a day or two at Cape Royds I found this view farcical. There were skuas nesting all over the open ground from Cape Barne to Horseshoe Bay, yet the only ones that had access to the penguins were the six pairs with territories overlapping the penguin colony. All the others had to be feeding at sea, the only other resource available to them (not counting the dumps and dog lines at McMurdo and Scott Bases). The rest of that first summer, and five later summers at Cape Bird, site of a much larger penguin colony near the northern end of Ross Island, were taken up with a study of skua feeding and breeding biology.
Skuas generally get a bad press and are compared unfavourably with penguins. It is true that they kill penguin chicks, which is what upsets most visitors to the colonies, and through ineptitude this is sometimes a drawn-out, gory business. As if that isn’t enough, they’re also uncompromising in defence of their own nests and chicks, and use their legs to deliver stinging fly-by attacks on anyone who walks through their territory. Even so, they don’t deserve the opprobrium heaped on them, as encapsulated by this, from the pen of some early biologists: “The drama of Antarctic bird life is not without its villain. Theft and pillage, murder, cannibalism, infanticide, these crimes are all in the repertoire of the South Polar Skua.”
What did they expect in a predatory seabird? Sweetness and light? These are creatures engaged in a fierce struggle to survive and rear their chicks in an unforgiving habitat. Their breeding success is so low that their very survival on the continent is at all times precarious. And there is much more to them than theft, pillage, murder, cannibalism and infanticide. Their toughness, fierceness in defence, foraging skill at sea, and remarkable powers of flight and navigation must also be considered. These are qualities that make skuas birds to be admired.
Whenever we watched skuas settling onto their territories at the start of the breeding season, they already had a remarkable achievement behind them. Since the autumn of the previous season, they had completed the Great Pacific Circle, flying up past New Zealand, over the equator, past Japan and into the Bering Sea for the northern summer, before returning south along the coast of California, across the equator again and back to Antarctica for the southern summer—some 34,000 km by the shortest route. Every skua can thus rightly be called a 12-to-24-hour bird. They leave Antarctica as the days are shortening, cross the tropics with their 12 hours of daylight, and head north into the increasing day length of the Arctic, before returning through a similar pattern of changing daylight hours to the by now lengthening days of Antarctica. If they did this just once, that would be achievement enough, but they do it in their first year of life, as juveniles, and then every year of their long breeding lives, which for some birds means as many as 30 or 40 times. Adélie penguins venture only as far as the pack ice—achievement enough for a swimming bird, but not to be compared to the hazardous flights of South Polar skuas.
Our Antarctic studies were to entail the close observation of natural populations of skuas and penguins going about their normal lives, with minimal disturbance and interference from us.
There was to be minimal handling of the birds, minimal chasing, and minimal capturing and handling. Treated with care over the several years of our study, the skuas at Cape Bird became increasingly habituated to our working among them. In short, they became quite tame—so tame it was possible to walk among them without birds on all sides abandoning their nests and taking to the air in swirling flights, thereby exposing their eggs and chicks to the risk of seizure and consumption by their own kind. By our last season, the skuas nesting among the penguins along the beach-front were so tame we could lift them off their nests to inspect their eggs. Of all these birds, the one we called Sam- I-Am was the most endearing, taking a close interest in our visits and courting close contact.
We subsequently tried the same approach with brown skuas in the Chatham Islands, but without the slightest success. Despite a much longer study period, these birds never became used to our presence and didn’t become tamer in any respect. Nor did this dedicated approach work with the Adélie penguins. Indeed, no one has yet been able to tame a penguin colony in the way we tamed the Cape Bird skua colony. Our study penguins remained the same 4 kg of aggressive bad temper at the end as they had been at the beginning. With skuas in the Antarctic, you get what you give. Be noisy, wave sticks, throw stones, and they will react aggressively; behave with decorum, and they will repay the compliment. With penguins, no matter how humble your approach, once you are within their nest space they pugnaciously attack you.
I was intrigued rather than surprised by one unusual aspect of the behaviour of skua chicks. Fifty years before, Edward Wilson had recorded: “Only one of the two hatched in a nest survives. This is connected with the tendency of the young to wander and get separated, and with their tendency to fight, and with the instinct that teaches the parent to be chary of giving them too much nursing.” Wilson had been describing siblicide, although this term did not enter the literature until much later. Siblicide—the killing of a brother or sister—is common among several bird groups but rare among sea birds. With skuas, the two chicks are not held in a nest of any consequence, just a bowl in the ground, which allows the younger chick to escape the attacks of its older sibling.
At first sight it seems maladaptive. Surely these skuas are having a hard enough time as it is without their breeding success being halved almost immediately the second chick hatches. In fact, research shows that it is facultative, and occurs to varying degrees in different colonies. Where food is usually sufficient to raise both chicks, it is absent or infrequent. But where food is hard to find later in the season, it may occur in every nest. Better to raise one chick than lose both is the evolutionary rule, and the instinct of any chick is to chase away any other it finds in the nest with it. Invariably this means the first-born evicting its younger sibling, and the sooner it sets about this, the easier the task, for the first chick’s advantage in strength and maturity is greatest immediately the second chick hatches. Siblicidal behaviour is usually characterised as fighting, but in this case the difference in strength and maturity between the two “combatants” is so marked that it is simply attack by one and flight by the other. At Capes Royds and Bird, almost no second chicks survive. At Cape Crozier, a third of pairs rear both chicks.
Which brings us to Napoleon. It isn’t known what domestic ructions in his parents’ territory set this chick off on his perilous journey. At nine days old and weighing just 230 g—a mere handful of fluffy down—Napoleon left the safety of his natal territory and travelled 145 m uphill over rough scoria before finding sanctuary in the nest of pair 12. Perilous? Certainly! Undefended skua chicks are ready prey for hungry adults, and Napoleon passed through two foreign territories before insinuating himself into his new home two days after setting out. He then proceeded to evict the two chicks already there. The larger (100 g) and older was missing 12 hours later, the younger, having been badly mauled, a day later. A comprehensive victory and takeover. Not too many skua chicks could embark on a journey like this and survive. We had great expectations for Napoleon’s future, but although banded he didn’t feature in any later records as a local breeding bird.
Long periods of observation at the very large northern colony of Adélie penguins and skuas at Cape Bird confirmed our impression that, in spite of their reputation as the hyenas of the Antarctic, skuas were rather incompetent predators of penguins and that most of their penguin consumption was the result of scavenging. We tested this hypothesis by fencing off some penguin nests from the skuas while leaving others accessible. In the first season, the protected penguins were more successful at producing chicks, with only 24 per cent of eggs failing to hatch compared with 43 per cent in the unprotected nests. The following year, however, with more difficult breeding conditions in which only the most experienced penguins were successful, there was less of a difference, 36 per cent of protected eggs failing versus 45 per cent of unprotected eggs.
Most nesting failures occurred on the margins of breeding groups. In the first season, 31 per cent of eggs in central nests failed to produce chicks, compared with 59 per cent in marginal nests, and central nests in the unprotected areas met with similar success to those in the protected areas. The margins of breeding groups are usually where the least experienced birds are concentrated and the areas most subject to skua harassment. Yet even within the protected groups, 34 per cent of eggs and chicks were lost in the first season, and 39 per cent in the second. These losses represented food that, in the absence of protective fencing, could have been scavenged by skuas.
South Polar skuas are generally too timid, too impatient or too lacking in skill to attack nesting Adélie penguins— which are much larger than themselves—to get at their eggs or chicks. Some develop the skill over the seasons, others never show much interest. One would have thought that the gang of a hundred or so young birds that loitered among the penguins on the lower beach would have caused mayhem. In fact they were so inept, so lacking skill and determination, that it is doubtful they ever took a single egg or chick from among the hundreds of penguin nests that surrounded them. Even for the best of skuas, penguin intake was usually limited to the unhatched and the very young. Most were quite unable to overcome the maturing chicks when they congregated later in the season while both parents were at sea getting krill for them.
However, there were exceptions to this pervasive indifference and incompetence. Easily the most effective predators among the 80 or so skuas at Cape Bird that had access to penguin nests in their territories were the males of pairs 15 and 153. Their antics enlivened the many hours of otherwise tedious observation and recording of skua–penguin interaction at this colony.
Pair 15 had 100 penguin nests in its territory, below the Cape Bird hut. Pair 153 had over 500 nests. The two males adopted quite different tactics when attacking penguins. Male 15 made rapid jump flights to and fro across the penguins, pulling their tails as he passed, in an attempt to unsettle them. As they tried to counter his rapid movements by shuffling round on their nests, sooner or later one would expose eggs or chicks, or, even better, fling them out, whereupon the skua would snatch the meal on offer. Male 153, with so many more penguins at its disposal, and therefore a much greater chance of opportunistically picking up eggs or chicks from beneath inattentive parents or abandoned on nests after a penguin fight, favoured a far more relaxed approach. He usually flew slowly across the penguins in regular circuits or stood near the margins of the subcolonies. He was nevertheless quite capable of dauntless assaults when excited, and carried out most “category 12” attacks we witnessed at the colony—crash flights into defended nests on the margins in an attempt to knock eggs or chicks from beneath sitting parents.
The wide difference in skill between male 153 and the other skuas was graphically demonstrated by an incident involving his mate. She could be classed only as a hopeful predator. She would parade along the margins making ineffectual forays at the nesting penguins and seldom managed to retrieve anything worth eating—dead, abandoned or from under a parent. However, on one occasion she surprised herself by managing to grab a half-grown chick, which was then so disoriented that it kept pushing under her for protection. The more it pushed, the more alarmed she became, proving herself quite unable to kill it. The male had been watching this encounter from the nest with rising interest and, one couldn’t help feeling, not a little disgust. Eventually, as if with a resigned shrug of the shoulders, he lifted off, leaving nest and eggs untended, flew the 60 m down to his fumbling mate, killed the chick with a few practised jabs of his bill and promptly returned to base, in a clear demonstration of how prey should be taken.
During the many hours we spent watching this bird, he was never caught by the penguins, which, had he been in the middle of a group, could have been fatal. The risk skuas ran by preying on penguins or scavenging eggs or carrion was clearly illustrated by the experience of his neighbour, the male of pair 152, which was caught by the wing when attempting to retrieve a dead chick from an abandoned nest. In a moment he was surrounded by a seething mass of very angry penguins. Not only did he have to struggle past the sitting birds, with their fierce bills and ferocious flippers, he also had to contend with those other penguins in the area not sitting on nests, which all flocked to join the mêlée. It was a badly beaten and exhausted skua that finally reached the safety of the colony margin 7 m and 11 penguin nests from the discarded chick. Not that he was safe even then, because he was now in pair 153’s territory, and skuas defend their patches and mates to the death. He was too valuable to the study to be lost in this way so I picked him up and took him back to his own territory. This horrific experience should have been enough make him drop penguin from his diet and join the sea foragers instead, but not so: next day he was back to his aggressive best.
Pair 153 commanded our affection and respect for another reason. During the five years of the study at Cape Bird, they were the only pair that managed to raise both chicks to fledging every breeding season. In contrast to other pairs, not only did they feed both chicks throughout the season, they also intervened in any fighting that began between them, rushing at the aggressor quite fiercely and forcing it away from its sibling. The usual breeding-success rate in the colony was a bare 0.2–0.4 of a chick raised per pair, and siblicide was virtually obligatory.
For most skuas on Ross Island, food comes exclusively from the surrounding ocean in the form of small pelagic fish, mostly Pleuragramma antarcticum. Only those skuas with penguin nests in their territories can feed on penguins; all the rest have to forage at sea, and even those with penguins to draw on need to fish at the start and end of the season, when there is little to be had in the penguin colonies.
How good are skuas at fishing? A test at Cape Royds in 1959 indicated they were very good indeed. It involved the male of pair 3 at Green Lake and was one of the few occasions when skuas were experimentally manipulated. On a rather dismally cold, overcast afternoon, each time this bird returned from foraging, the fish he regurgitated onto the ground for his mate and chick was retrieved before they could pick it up. If he thought this extraordinary or unfair, he didn’t show it, and when begged again by his dependents he merely took off on another foray. During the afternoon he undertook three trips. The first, of 93 minutes’ duration, returned 75 g of fish; the second, of 33 minutes’ duration, returned 105 g; and the third, of 52 minutes’ duration, returned 140 g: a total of 320 g, comprising 10 small fish. These were good to average flight times and returns. In perfect conditions—open sea and light winds—flights were about 30 minutes in duration; in adverse conditions—dense pack-ice and bad weather—they could extend to several hours. When conditions were unfavourable, therefore, the attraction of penguin eggs and plump penguin chicks must have been almost irresistible, even though the risks involved in obtaining them were much higher than those of flying out to sea.
Although some skuas on the Antarctic Peninsula have access to grass and moss with which to line their nests, everywhere else in Antarctica a nest is, of necessity, merely a shallow bowl in the gravel, perhaps with the occasional few feathers or pieces of bone for lining. However, pair B1E at Cape Crozier carried nest-lining to an extreme. Debris from an old hut seemed to have triggered a long-dormant impulse in the two birds as they collected over 90 rusty nails, plus screws, nuts and bolts, and incorporated them in their nest bowl. More comfortable than stones? Apparently.
Cooperative breeding is the sharing of parental duties by more than just a pair of birds. It is rare enough among land birds and especially uncommon among sea birds. Yet, for unknown reasons, cooperative breeding by brown skuas Catharacta lonnbergi in the New Zealand region is relatively common. It was first described by farmer and naturalist William Herbert Guthrie- Smith on the small islands fringing Stewart Island. In the summer of 1974 I visited the Chatham Islands to see if skuas there practised siblicide (which they didn’t), but stumbled across this much more intriguing behaviour.
We found a high proportion of nests on Rangitira Island had three birds in attendance, and in later seasons found nests with four or more. Over the following years we worked on Rangitira, Mangere and Star Keys Islands to see whether these cooperatives comprised family members—either parents and offspring or siblings—how many of the birds were male and how many female, and which were the parents of the chicks. To answer the first question, we banded the chicks each year so we could track how they subsequently formed pairs and groups, and established that the non-parent members of a cooperative were not family. Questions two and three could be answered only through DNA fingerprinting, a technique that didn’t become available until quite late in the programme. What it revealed was that there was just one female in each cooperative, and that although one of the males was usually dominant, the other(s) might also father a chick or chicks.
There were some quite wonderful skuas in this population. Group 10’s territory was on a grassy flat in the middle of Skua Gully, on Rangitira Island. In 1974 it was occupied by a single pair of birds. In 1978 it was frequented by, variously, six to nine birds. Were they a cooperative? As no nesting took place, this seemed rather unlikely. In the following seasons, however, nesting did take place, and all the birds were captured and banded. It was clear they made up an unusually large cooperative.
To begin with there were at least two large, heavy birds, which on first sight appeared to be female, but in later years just one remained, known, on account of her colour band, as Yellow. She was of a different personality from all the others, found by DNA fingerprinting to be males. Whereas they presented a maelstrom of swirling aggression whenever we appeared, she merely flew round in gentle circuits calling forlornly. At the first opportunity she would return to the nest, even while the others were still attacking us.
I say “nest”, but in the early seasons we found to our surprise that the group had two nests, with a single egg in each, and the eggs were sometimes moved between them. Yellow sat on both at different times.
However, this group was really an example of too many cooks spoiling the broth. It enjoyed only very modest breeding success, raising on average just 0.7 of a chick to fledging per season over the 19 seasons it held together, when most other groups and pairs were averaging twice that.
What was remarkable about it was that a single female held together so large a group of rambunctious males over so many seasons and in the end began to turn them into a successful breeding unit. Also exceptional was Yellow’s great gentleness amid so much avian bad temper. Even when handled she was extraordinary, lying quietly in our arms while we weighed and measured her.
The bird we nicknamed Red Baron was impossible to overlook. It liked to remind us that a breeding bird was not to be trifled with, swooping down over us from its nest more than 300 m away when we were on the shore platform, on supposedly neutral ground. Its presence was even more inescapable whenever we walked round the island: the closer we approached its nest, the more determined its swoops, so that in the end it was prudent to seek cover in the bush or to run, waving a soft flax stick overhead in an attempt, invariably vain, to ensure it didn’t hit us with its legs. Checking the nest was a test of one’s determination and had to be done as quickly as possible, while any work with the chicks, such as banding and measuring, could only be carried out in the protection of the bush, and that while keeping one’s head down the whole time. A 2 kg bird in your face moving at 80 kph is no joke.
When we caught this ace of skuas for banding, its great weight—actually 1930 g but still rather more than that of its mate, which tipped the scales at 1700 g—indicated it wasn’t Red Baron after all but Red Baroness!
In the first three seasons the baroness and her mate raised five fledglings—a good record for the island. However, in the 1982–83 season a completely new pair took over the territory, and the baroness was nowhere to be seen. These new birds held sway for two seasons before Red Baroness returned and ousted the female, an unprecedented development. In all other recorded instances, once a breeding bird was displaced from a territory, it was never found there again.
The four birds of cooperative group 21A, in Black Gully, Rangitira Island, seized their territory from a pair in the 1985–86 season, killing the male in the process and displacing the female. Over the following 10 seasons they were steady rather than spectacular breeders, raising 13 fledglings. During routine observation in 1993–94 we noticed the female was unable to put any weight on one leg. We caught her to have a look and found she had a 40 mm slash in the lower leg, the bone of which was splintered, with pieces sticking through the skin. This was such a serious injury that it was unlikely she would recover from it unaided, so we removed the bands from the broken leg, cut back the projecting bone and released her.
Not only did she survive, but when we came across her the following season she was breeding again, with two chicks in the nest. On inspecting her we found it hard to discern where the injury had been, the scar being scarcely visible among the scutes covering the front of the leg. This demonstrated to us that, although a broken wing bone was invariably fatal, even a serious leg injury could be overcome.
Sudden progress in science often comes from the transfer of techniques from one discipline to another. This is as true in the biological sciences as in any other, and our use of DNA fingerprinting to determine paternity and relationships among skuas is a good example. When, in 1987, Terry Burke and Michael Bruford of Leicester University adapted for birds the human DNA fingerprinting techniques developed by Alec Jeffreys, they opened up a whole new sphere of research opportunities. And they provided us with the perfect solution to our problem of determining which of the males in skua cooperatives the parents of the chicks were and which merely helpers were.
Chick #44857 has its place in skua history as the first of its kind to give a blood sample for DNA fingerprinting. The diary for November 1987 records:
“The first chick was one of 4’s, a nail-biting experiment— it worked so easily didn’t really believe it so then did the two chicks of 5’s. Bringing chicks to the lab—no kicking, no squealing and able to do it on my own with the chick lightly wrapped in a cloth.”
The blood was taken from a vein running along the inside of the lower leg. The vein couldn’t be seen through the darkly pigmented skin but was easily found by palpation. The samples were stored in liquid nitrogen.
However, it wasn’t easy to produce quality fingerprints, and only once research student Craig Millar had taken up the problem in his PhD study did it become routine. Over the following years, samples were taken from all adults and chicks in cooperatives, and from many pairs and their chicks, and fingerprints produced and analysed. Generally the dominant male in a cooperative was the father of the chicks, but non-dominant males did father chicks in some seasons, and in some exceptional groups two males each fathered one of the two chicks in the same year. The results also showed these skuas to be faithful partners. There was no copulation outside the group or pair, and no egg dumping in another group or pair’s nest. In all cases, the chicks belonged to the birds of the territory. Besides determining which males were parents, Millar discovered that female fingerprints had unique, sex-specific bands. For the first time it was possible to sex all the birds— hatchlings, fledglings, immatures and breeders. It was this development that allowed us to determine that the cooperatives on our three islands comprised a single female and two or more males.
On most counts, chick #32703—the first of our banded fledglings to begin breeding—was the single most significant skua of our research. She was raised in the 1974–75 season by a pair of skuas that nested on barren Landing Point, on the north of Rangitira Island, and in 1982 she was found breeding in a cooperative group 800 m away, at the southern end of Skua Gully in the middle of the island—a common enough progression. So what was remarkable about her?
First, although only a small bird, she replaced a big, heavy bird that, on account of her size and weight, we considered at the time a typical female. Any bird over 1900 g was automatically identified as female, yet here was a very light, small bird, at 1640 g no heavier or bigger than the two known males in the group it had joined, proving to be female. This was a blow to our confidence concerning the sexing of skuas. Females needn’t be large, heavy birds after all. We had to find other ways to sex them—something we managed only with the advent of DNA fingerprinting.
Second, she was clearly not an offspring of the birds already in the cooperative she had joined. She had moved from her natal territory to join the group. What she showed was that a cooperative need not comprise family members, whether parents plus mature offspring or a group of siblings. The birds in a group could be unrelated. This raised new questions concerning the evolutionary purpose of cooperatives. Family cooperatives are easy to understand: all members are promoting the survival of their own genes. In cooperatives of unrelated individuals, this cannot be the case.
On her own, therefore, #32703 overturned two important misconceptions: that females are invariably big birds and that cooperatives are family groups.
In any measure of devotion to offspring, Caren Lyders, the female of pair 24, which nested at a spot on Rangitira Island known as The Clears, would stand out. While swoop attacks in defence of nest and brood are a fact of life for anyone who chooses to work in a skua colony, what distinguished Caren Lyders was that, whenever we picked up her chick to weigh and measure it, she would march across the ground towards us, chest puffed out, head high, her 1950 g radiating intense indignation, in an attempt to secure its escape. She made a very angry matron, and an extremely disconcerting one for those used only to airborne attacks. No other skua ever behaved in this way.
The hut on Mangere Island is on a low peninsula that extends towards Little Mangere Island. To do any work on Mangere involves first climbing up to the spine of the peninsula, then descending steeply to the narrow neck at its base, and then climbing again beyond the neck to a track that sidles along the southern flank of the island’s main bulk towards Robin Bush in the east.
The Friendlies, as we called them, took a great interest in anyone toiling up this trail, flying over from their nesting area and accompanying them up the track in a series of little jump flights, landing ahead of them, watching them pass, then flying ahead again. They were frequently within reach, although no one ever managed to touch them. Were they seeking company, or were they merely puzzled by the human inability to fly up the slope as effortlessly as they themselves could? Whatever the explanation, their friendly attitude lasted only as long as one stayed on the track. Any deviation towards their nesting area prompted standard nest-defence behaviour.
Territory 1, in the north-eastern corner of Rangitira, is an idyllic spot with gently rounded slopes clothed in long grasses in summer and with views to the south into Thinornis Bay and over the sea to the north. The original trio of birds here was replaced in 1980–81 by a pair which was itself replaced by a new trio that included the later-to-become-notorious female B-59. Come the following season, B-59 had decamped from this territory and joined a male to form a new pair down the hill at the north end of Thinornis Bay. This left the two males on their own, although not entirely so, as she would fly up from time to time to visit them.
How long could this unusual arrangement continue? More importantly, how long would the two males maintain their lovelorn bachelor state? For eight long years, as it turned out. DNA fingerprinting established that they were indeed both males and not simply a failing pair, a finding confirmed in 1995–96, when they were found with a new female, a nest and chicks. One could only feel delighted for them. As for B-59, she lived happily in her new territory for 10 years and raised 13 fledglings there
We used many different methods for catching skuas: cannon netting over baits; a clap net over a nest or bait; spot lighting on the ground at night. Others have used cages and snares. All methods have their uses and all carry some risk, but the most effective means of catching breeding birds—those that will swoop at you in defence of eggs or chicks—is a large soft net on a springy frame attached to a light alloy pole. When we were trying to catch skuas was the only time they were in danger from us.
It was in 1980–81 that we banded the Rangitira pair 9A, when they took up their territory on cliffs at the southern end of Skua Gully. We caught the male regularly over the following years, finding his weight to range from 1680 g to 1780 g, which was fairly normal for a male on the island.
In December 1995, late in the afternoon after a long day in the field, I netted him easily enough on his first pass but then dropped him carelessly—too heavily—to the ground. When I picked him up from beneath the net, he was unnaturally flaccid in my hands, head hanging, a trickle of blood running from the beak, and eyes slowly closing. Dead—after 16 breeding seasons and raising 21 chicks. I felt like a murderer. I threw him down and left the hill, inconsolable, refusing to be reassured by Andrew Given, who was helping me that day. The single consolation was that the dead bird’s mate of 10 seasons was not bereft for too long, re-mating and bearing chicks the following year.
This wasn’t the first bird we had killed. One perished early in the programme from overheating in a holding bag after we’d been cannon netting in the “roosting club”. It did, however, have a stellar “after-life” as a mounted specimen paraded in front of many crowded first-year university classes on evolution.
If you ask a field ecologist what would most upset them in their research, they would answer, “To kill a study animal needlessly, carelessly or negligently.” It was because I did just this with male 9A that I was so upset that day.
Skuas in the Chatham Islands get most of their food by catching small and medium-sized petrels, mostly white-faced storm petrels and broad-billed prions, on the ground at night. Their night sight, however, isn’t much better than ours, as is readily demonstrated by sitting quietly near a roost on a dark night, when a skua might alight on your shoulder, thinking it a convenient place to rest. They’re very reluctant to fly on dark nights, something I exploited at the roosting club on the end of Mangere Island by crawling among the closely grouped birds and reading their bands by pencil torch as they shuffled and edged around me. How, therefore, do they manage to catch petrels in the dark not only of night but also of the bush, where the petrels have their burrows? I think they do this by following the noise the petrels make as they crash down through the canopy foliage and scuttle across the ground, sometimes through grass and fern, on their way home. If I could grab a petrel in the dark, which I could, and dark so intense that I literally couldn’t see a hand in front of my face, so could a skua. Of course, around the coastline and in the open there was always a little light, and on moonlit nights there was an abundance of it.
But are skuas clever in the way they search for petrels? The petrels aren’t distributed evenly over the islands. On Rangitira, burrows are found in some bush areas but not in others. Why? I’d heard many learned discussions of this question over the years but the explanation, provided in 1997 by Nicolette Was, a Lincoln University student, turned out to be very simple. Although petrels can crash down through the canopy, they cannot fly back up through it. If an area has no “climbing trees”—trees, that is, with sloping trunks that petrels can flutter and claw their way up to the canopy top—it remains unfrequented.
At the bush edge, of course, the birds can scuttle out along tracks to apparently well-established take-off points. Now, if we could find these, and watch in astonishment the unending stream of birds pouring out from them in the early morning just before the sky began to lighten, why couldn’t the skuas? They fly around on most nights when there is some moon and should have discovered them. Just imagine how easy it would be for them to stand at one of these take-off points and grab petrels at will as they passed before them like helpings in a sushi bar. It would be even easier, actually, as many petrels fluff their first attempt at take-off and fall back into the low ground cover. Consideration of this has somewhat dulled my earlier impression of skuas as really clever birds. They’re still clever, I believe—much more so than Adélie penguins, for instance—but not anywhere near as clever as a mammalian predator.
If they survive general harassment and shooting, these island skuas will have more food but fewer nesting places than before, as the bush, cleared decades ago to make way for pasture, once more covers the land. In the end they’ll be able to nest only on the few rocky headlands and outcrops: about 16 pairs or cooperative groups on Rangitira and perhaps six on Mangere and three on Little Mangere. Considered a common nuisance species a few decades ago, they are now in need of protection. And despite their irascibility, I wish them well.