Volume 9, Issue 3 p. 4-7
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The Emperor strikes back

Peter Fretwell

Peter Fretwell

Geographer with the British Antarctic Survey.

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First published: 12 June 2012
Citations: 1

Abstract

Studying emperor penguins used to be a job for heroes. But penguins leave traces; thanks to high-resolution satellites they can waddle but they cannot hide. Peter Fretwell comes in from the cold to find that penguin numbers are much higher than we thought.

The emperor penguin is a truly unique bird with unique behavioural characteristics. Not only is it the largest penguin, often standing over 4 feet tall, but its lifestyle is one of the most challenging of any species. Emperors breed exclusively in Antarctica, but unlike the other birds that live on the frozen continent they have adapted to breed in the extreme of the Antarctic winter. Temperatures here at the coast can drop below −50°C, and if you take the wind chill into account this can feel more like below −100°C. Another unusual characteristic of the emperor penguins is that they do not breed on land; their whole breeding cycle takes place on sea ice, the frozen sea surrounding the continent. Each austral autumn, as the sea starts to freeze, the emperors assemble at a few traditional breeding sites. They stay at these sites throughout the winter and spring, when the sea ice is thickest. In early summer, as the ice sea melts, the chicks fledge and the colonies disperse back into the water. These two adaptations, breeding in the depths of winter and on the sea ice, mean that emperors are free from predators throughout their breeding season. It is a reward that more than outweighs the evolutionary adaptations necessary to deal with the extreme environment.

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A medium-resolution Landsat image of an emperor penguin colony in West Antarctica. At this resolution colonies appear as brown stains on the ice. Individuals or groups of birds cannot be differentiated from the surrounding guano, but these satellite images are an ideal tool for searching for undiscovered colony locations as they are freely available and cover large areas. Credit: British Antarctic Survey

But this trait that has served the emperor penguins so well could now be their downfall. With global warming the sea ice in some areas of Antarctica is melting earlier in the season. And predictions of climate change suggest that other areas will soon follow suit. This will mean that many of the emperors’ breeding sites will become unsuitable, leading to chick mortality and a dramatic decrease in future emperor numbers.

Earlier-melting sea ice will lead to a drastic decline in emperor penguin numbers

Until recently it has been extremely hard to quantify the probable effect on the penguins. Scientists knew the species may be in danger, but a general lack of knowledge about the distribution of emperors and their overall numbers has meant that any attempts to model future population change have been limited; how can you estimate future numbers if you do not know the starting figure? Although the species is instantly recognizable to the public (and especially loved by Hollywood directors) our ideas of exactly where they breed and how many there are have been vague in the extreme. The problem relates back to the penguins’ unique breeding cycle. The Antarctic winter is harsh, and any attempt at field work in the extreme conditions is logistically difficult, costly and dangerous. Searching out penguin colonies by traditional methods is expensive, usually involving flying aircraft over huge swaths of sea ice. But few Antarctic operators fly their planes in the winter; the costs and risks are just too high. So up until recently all that we knew about the distribution of emperor penguin was that there was a lot we did not know. The percentage of colonies that remained undiscovered was a mystery.

All that has now changed. In a chance discovery I and biologist Phil Trathan at the British Antarctic Survey found that emperor penguin colonies could be seen in satellite images1. We showed how the stains made by the emperors can be clearly seen in medium-resolution Landsat images. The penguins themselves are too small to be seen by this satellite, but because large groups of birds stay in one place for eight months of the year the surrounding snow gets very dirty, and it is this discoloration that can be picked up in the images. The stains are technically known as guano; colloquially, they are poo. Luckily, data from the Landsat satellite became freely downloadable just after we realized that emperor penguin colonies stood out on the imagery. So we no longer had to traverse huge areas of sea ice, or camp out in tents: the whole continent could be surveyed using suitable Landsat imagery to search for the tell-tale brown stains that identify colony sites. This method led to the discovery of 10 new colonies to add to the 30 that were already known; better still, it has finally given us a good idea of the breeding distribution of the species. No longer is the emperor penguin seen as a species that breeds in a few specific regions; we now know that it is a truly pan-Antarctic species found all the way around the continental coastline. Since then further work using more powerful satellites2 has identified a further six colonies, bringing the total number of known colonies to 46.

All that we knew about the distribution of emperor penguins was that there was a lot we did not know

But that is not all. Now that the distribution is known, the race is on to calculate the population – a much more important piece of scientific information, used in conservation estimates and in the demographic models that assess the species’ vulnerability to climate change. Like the breeding distribution figures, our previous knowledge of the population was rather hazy. It is fair to say that previous estimates were poor. The colonies that had been counted had often been counted at the wrong time of year, in the Antarctic summer rather than the emperors’ breeding season of winter or spring. After all, would you really want to sledge across hundreds of kilometres of fragile sea ice in hurricane-force winds and extreme temperatures in the pitch black of the Antarctic winter to find and count penguins? Two famous explorers who did just that both wrote books about their exploits: Apsley Cherry-Garrard's The Worst Journey in the World3 – the title is not an exaggeration – and Douglas Mawson's The Home of the Blizzard4. (Cherry-Garrard was a member of Scott's expeditions. On a winter trip to collect emperor penguin eggs his tent blew away and his teeth shattered with the cold. Mawson's journey, partially to find penguin rookeries, was not even in the winter, but of his three-man party he alone survived). The titles of these books might give you some clue about the conditions they encountered. These tales come from the heroic age of exploration; both are epics of endurance. More recently, in 1958, in one of the British Antarctic Survey's most tragic incidents, a field party of three men set off across the winter sea ice to count a small emperor colony a couple of days’ sledging away. That night a storm blew up and broke the sea ice. The men were never seen again. Today the only emperor penguin colonies that are usually counted in winter are the two or three sites that exist very close to over-wintering research stations such as the French base of Dumont D'Urville, where the movie March of the Penguins was filmed.

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A high-resolution satellite image of an emperor penguin colony on the sea ice under ice cliffs at Halley Bay. Note the brown guano stains and the small black groups of penguins. Each group can contain over 1000 birds. Credit: British Antarctic Survey/Digital Globe

From The Home of the Blizzard, by Sir Douglas Mawson

There was no sound from behind except a faint, plaintive whine from one of the dogs. When next I looked back it was in response to the anxious gaze of Mertz, who had turned round and halted in his tracks. Behind me nothing met my eye except my own sledge tracks running back in the distance. Where were Ninnis and his sledge?

I hastened back along the trail thinking that a rise in the ground obscured the view. There was no such good fortune, however, for I came to a gaping hole in the surface about eleven feet wide. The lid of the crevasse that had caused me so little thought had broken in; two sledge tracks led up to it on the far side – only one continued beyond.

Frantically waving to Mertz I leaned over and shouted into the dark depths below. No sound came back but the moaning of a dog, caught on a shelf just visible one hundred and fifty feet below. Close by was what appeared in the gloom to be the remains of the tent and a canvas food-sack containing a fortnight's supply.

Secured by a rope we took turns leaning over, calling into the darkness in the hope that our companion might still be alive. For three hours we called unceasingly but no answering sound came back. The dog had ceased to moan and lay without a movement. A chill draught rose out of the abyss. We felt that here was no hope.

All our available rope was tied together but the total length was insufficient to reach the ledge, and any idea of going below to investigate and to secure some of the food had to abandoned. A weight was lowered on a fishing line as far as the dog which had earlier shown some signs of life, but there was no response. All were dead, swallowed up in an instant.

At 9 p.m. we stood by the side of the crevasse and I read the burial service. Then Mertz shook me by the hand with a short “Thank you!” and we turned away to harness up the dogs.

[The two men were 300 miles from their hut, and most of their food had disappeared into the crevasse. Xavier Mertz was to die on the homeward journey, probably poisoned by vitamin A from eating the livers of their remaining huskies; as Mawson reached his hut he saw his ship Aurora sailing away. He had to spend a second winter in Antarctica before it could return. In a cruel and ironic twist of fate, the glacier upon which Xavier Mertz died was one of the locations where in 2009 a new emperor penguin colony was discovered by satellite. Finding such colonies was one of the goals of Mawson's expedition. The explorers had missed the colony by only 20 km.]

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A blizzard at Mawson's winter quarters, Cape Denison, Antarctica, c. 1913. Photo: Frank Hurley (1885–1962)

Occasionally trips are still undertaken to other colony sites in the depths of winter. Here field conditions can be brutal. I have spent a month in a tent in Antarctica and know some of the challenges the White Continent can throw at you. But field work in winter, when the katabatic winds blow off the polar plateau with the strength of an express train and the mercury drops to −50°C, is different. Life in a tent must be sheer hell. It is little wonder that only a handful of emperor colonies are visited in winter on a regular basis.

Even if you do get to a colony, that is only the start. Manually counting 10 000 identical penguins that are continually moving is not an easy task. The usual technique is for several counters to each count the same small group, then to check the accuracy between counts. If there is more than 5–10% difference between counts they are discarded and the count starts again. If they are within this figure then this count is used to extrapolate by estimating what percentage of the colony has been counted. This technique is very prone to scaling errors – the bigger the colony the larger the probable errors. It is easier by far to count penguins from the safety and comfort of your office via a snapshot satellite image.

We also know that many colonies, including those recently found by satellite, had never been counted at all. BirdLife International and the International Union for the Conservation of Nature use a figure of 270 000–350 000 birds. But when you look at the details, this figure was from surveys done before 1984 and so misses over a third of the colonies we now know exist.

Since our first satellite distribution survey, satellite images have improved. In collaboration with other international scientists, we have used higher-resolution satellites to take images of each colony site. The high-resolution images from the QuickBird satellite were procured by the US Antarctic Geospatial Information Center. They give an on-the-ground resolution of 60 cm; this is detailed enough to differentiate penguins from guano or from shadow, and even to count individual penguins when the birds are on their own. When the birds are clustered together it can be used to estimate the area of penguins. A density multiplier is calculated from ground truthing surveys of 11 colonies that were taken simultaneously with the satellite imagery. The figure has been applied to all of the colonies across the continent to give us the first true population figure for the species. This new count comes out at around 600 000 adult birds – about twice the size of the previous estimate. But it has to be remembered that those previous estimates were based on only a limited number of breeding colonies. The number of birds has not increased, just our knowledge of them.

Manually counting 10 000 identical penguins that are continually moving is not an easy task

The work gives us some interesting statistical challenges. Usually population ecologists worry about having a large enough sample size to truly represent the population as a whole, but in our case we have data for every breeding emperor penguin on the planet. I know of no other bird for which this is true. Furthermore, all the data was taken within a two-month window from one breeding season. The question of statistical relevance is therefore not one we have to worry about. What will be a challenge is trying to work out how relevant one snapshot of the population is. We know that the yearly breeding success rate of emperor penguins is highly variable, and scientists do not presently know if the variability is a regional signal, or affects the continent as a whole. Likewise each colony will flux on a seasonal and even daily basis. All these factors have to be taken into account if we are to understand the meaning behind the figures.

Does it mean emperor penguins are less susceptible to climate change? Not necessarily. Of course it is good news that there are more emperors than previously thought, but many of the newly found colonies are in areas that we believe will become susceptible to climatic warming over the coming decades. The rate of decrease in emperor penguin populations will probably not change, but starting from a higher population baseline can only be a good thing for the species. In the future it will be important to try to get as accurate an understanding of the population as possible to use as a baseline to assess population predictions. As the satellite method is repeatable, accurate and relatively inexpensive it give us the opportunity for long-term monitoring to track the dynamics of the whole population. Only then will it be possible to model how climate change predictions for coming decades could affect this charismatic and unusual bird.

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A close-up of the very high-resolution satellite imagery of the Halley Bay colony; individual birds can be clearly differentiated from the surrounding snow and guano. Automated counting procedures are used to count the number of birds or estimate the area of penguins. This area figure is then converted into penguin numbers using ground counts. Credit: British Antarctic Survey/Digital Globe

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High-resolution image of emperors marching from the colony towards the edge of the sea ice to forage in the open ocean. Credit: British Antarctic Survey/Digital Globe