by Simon Barnes
These birds of the sea have as little to do with the land as possible. Land is only useful to them as a place to breed: if they could build a nest and lay an egg on the surface of the sea, like the halcyon, they would certainly do so.I They come to land reluctantly, and for as short a time as possible: and it’s not what they’re good at. A cliff ledge a couple of inches wide with the restless sea a hundred feet below: that’s as far away as a guillemot ever wishes to get from the ocean. These seabirds are creatures of the deep sea, and when not involved in making more gannets or guillemots, they spend all their time out on the sea, feeding beneath it when hungry and roosting on its ever-shifting surfaces when not.
The birds’ power of flight has given them not just lakes and rivers and inland waters but the entire ocean as well: all that three-quarters of the earth’s surface that isn’t covered by land is accessible to the birds, and most of those that exploit it do so by combining the ability to fly with their skills as swimmers. Flight gives them mobility, the ability to move from one food source to the next, and of course, the ability to reach a suitably inaccessible nesting site: most seabirds like rocky places, precipitous cliffs that beetle o’er their base into the sea, preferably on a flyspeck island that no human ever visits. Just as some mammals have adapted to take advantage of the riches of sea, so have many species of birds. Like seals, they are tied to the land for breeding, but their power of flight gives them great versatility and a wider choice of breeding ground. The five species of frigate birds have the largest wingspan relative to body weight of all birds. They can’t swim, they can’t take off from a flat surface and they can hardly walk. That means they have to roost on land rather than on the surface of the sea, and they do so on cliffs and trees, where they also breed.
The masters of this ocean/airways double are the albatrosses, 21 species of them, with the wandering albatross supreme among them with a wingspan of nearly 12 feet, more than 3.5 m. They are the champions of glide: no bird uses the wind more efficiently; they can cover 1,000 km, more than 600 miles, a day without so much as a flap. They use two techniques: dynamic soaring and slope soaring. In the first, they gain height by turning into the wind, rising until they are right on the point of stalling out of the sky, and then they regain speed by turning downwind. They can carry on with this rising and falling perpetually, or as long as the wind supplies them with energy. In slope soaring, they exploit the updrafts that are created as the wind hits the big waves and is deflected upwards. The wings of an albatross are equipped with a shoulder lock, which means they can keep their wings outstretched with no muscular effort: their heart rate when gliding is not much higher than their resting heart rate. They have a glide ratio of 22:1 or better: that is to say, they can gain 22 m horizontally for every one dropped vertically, which is staggeringly efficient. They manage this with the help of the tubes in their beaks – they come from an order called Procellariiformes or tube-noses – which measure their airspeed very accurately indeed, like the pitot tubes on an aircraft.
There is naturally a payback: they aren’t much good at powered flight and calm days leave them grounded, or rather, sitting on the surface of the sea. But still days are as rare as albatross teeth in the Southern Ocean, where most albatrosses live, and in the northern Pacific. They aren’t found around the equator, where the doldrums and the frequency of dead-calm days would make their lives impossible. There is an exception: the waved albatross breeds on the Galapagos on the equator and uses the winds that are caused locally by the cold Humboldt Current. Albatrosses are phenomenally long-lived birds: the oldest living bird, as I write, is a Laysan albatross that was first ringed on Midway Island in 1956. Of the 21 species, 19 are classified as threatened.
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I. The halcyon is a mythical bird that was able to calm the wind and the waves in order to nest on the surface of the sea, hence of course halcyon days. A genus of kingfishers bears the name Halcyon.
Fearful the death of the diver must be
There are five orders in the phylum of molluscs. We’ve met the cephalopods, which comprise the octopuses and squids and nautiluses; there are the minor orders of chitons and tusk shells, and the rest are divided into two big ones. There are the gastropods, which we’ll come to in a moment, and the bivalves. Bivalves have hinged shells, shells like a game of two halves: two doors, the meaning of the name. Oysters, mussels and clams and many others: this is a highly successful design, and there are getting on for 10,000 known species. The larva produces a single uncalcified shell, which is then enveloped by two mantles, the shell-creating tissues of the molluscs. Each mantle then becomes a separate centre for calcification, each making a shell that is the mirror image of the other. You can see this with asymmetrical oysters: if the two shells weren’t equally asymmetrical they wouldn’t make a pair, wouldn’t close and wouldn’t be any use.
The most celebrated bivalve is the giant clam, which is famous for grasping the hands or feet of divers and holding them helpless until they drown: a classic example of the hostility of nature to humankind. We live in a world in which you can’t even trust a shellfish not to kill you. There are versions of the US Navy Diving Manual that tell you how to escape from the clutches of a giant clam; you do so by cutting the living creature from inside the twin shells. All this is fascinating because it isn’t true. Not even a bit. Giant clams don’t attack and drown humans, even inadvertently. True, they are pretty big, and true, they open and close, but they are not hostile. They close as a defensive, not an aggressive, measure, and they do it so slowly that you put your hand in and out several times over while they are doing so. And even when they have closed, there is usually plenty of room to slide a hand or even a foot out again: they don’t close tight like a gin trap. The giant clam as the static slayer of the deep is a human fantasy about the hostility of the wild world, that’s all. I’m reminded of a poem, set to music, that was a great favourite of Uncle Matthew in Nancy Mitford’s The Pursuit of Love:
Fearful the death of the diver must be,
Sleeping alone in the de-he-he-he-epths of the sea.I
Giant clams are spectacular creatures despite this disappointing lack of lethal tendencies. They do something much cleverer than killing humans. They have become farmers. But first the size of them: the giant clam fantasy wants them to be at least as big as Volkswagens, so the exact dimensions may come as a bit of a disappointment. All the same, they’re pretty big, certainly when compared with the shellfish that turn up on plates, and they get big relatively quickly because of their skills as farmers. A respectable size for a giant clam is around the 4-foot mark or 120 cm across and weighing in at 440 pounds, 200 kg. Only the shells are left of the largest specimen ever found. It’s now in a museum in Northern Ireland and measures 4 feet 8 inches, 137 cm, across and weighs 510 pounds, 230 kg. Add another 20 kg for the animal itself and you have a seriously impressive creature.
Giant clams are found in warm shallow seas to a depth of about 20 m, 66 feet, in the Pacific and Indian Oceans, South China Sea and off the Philippines, and they can live to be 100. They are scarce and have gone extinct in places where they were once common, for they have been overexploited for food. The truth is that giant clams need to beware of divers. Their preference for shallow seas makes them accessible to humans. They live in shallow water because they need the sunlight, not for themselves but for the algae they farm. The giant clam’s evolutionary breakthrough is a symbiotic relationship with single-celled algae that it grows in the tissues of its mantle. The metabolic products of the algae are added to the food the clam takes in by filter-feeding, which means that the animal is well supplied with nutrients even in nutrient-poor water – so it can grow fast and to a splendid size. Once it has gone through a free-swimming larval phase, it settles down and can’t move at all, which makes sex a little difficult, at least if you think in human terms. With clam sex, both partners lie back and think of whatever it is that inspires a clam to sexual performance. They are hermaphrodites but they can’t se
lf-fertilise. They release both sperm and eggs and neighbouring clams are alerted to this by a trigger substance that readies them for reception. A little like telephone sex, then.
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I. Misquoted as “walking alone” in the novel; the poem is by G Douglas Thompson, set to music by Edward James Loder.
No flying, please, we’re birds
Flight is the honour and glory of a bird. So what is the first thing that a bird does when given the opportunity? It gives up flight. New Zealand went through 20 million years without mammalian predators, and so the birds started to become mammals. Or to be more accurate, they filled ecological niches that are normally filled by mammals: a kiwi has feathers that look fur, it lives on the ground, it can’t fly, it uses scent to track down invertebrate prey, and unique among birds, its nostrils are at the end of its beak. New Zealand is the world centre for flightless birds, with getting on for 20 species. These includes the kakapo, a parrot that has become a forest browser, the takahe, a rail that looks like a chunky moorhen and eats grass, and wrens that creep across the forest floor like feathered mice. New Zealand has five species of kiwi and six of penguin.
Flightlessness is not an evolutionary or taxonomic category. Species from many groups have shed flight and evolved flightlessness. All it needs is the right circumstances. The obvious explanation is that flight, being highly expensive in terms of energy, is dropped as soon it becomes a luxury rather than a necessity. It has also been suggested that flightlessness is a survival adaptation: you are less likely to get blown off your island home into the limitless ocean if you don’t take to the air. It is also obvious that if circumstances change, a flightless bird becomes horribly vulnerable. The long list of extinct birds has an impressive subsection of flightless species, and they encompass most avian evolutionary possibilities: great auk, Finsch’s duck, long-tailed wren, Jamaican ibis, New Zealand owlet-nightjar, Ascension night heron, long-legged bunting.
With flightlessness, there is no limit to size: so a bird can get far bigger than the kori bustard and the mute swan. The ostrich is the biggest surviving bird; the moas of New Zealand were bigger and the elephant bird of Madagascar bigger still. Some birds developed flightlessness despite the presence of predators: ostriches did so in the face of the most fearsome animals in the world. They can cope because they have speed on the ground, size and weaponry: their clawed feet are seriously intimidating. In Australasia, cassowaries have a reputation for attacking dogs and even humans; there have even been fatalities, though these attacks were made by birds that had previously been fed by humans.
The readiness with which birds shed the use of their wings tells us two stories at the same time. The first is the miraculous nature of every bird that retains flight: the high energy demands of that lifestyle clearly put every flying bird on the far edge of evolutionary possibilities. Every sparrow is a small miracle. It is this intuitive sense of the miraculous that has made birds the most studied group of animals on earth: birds, more than any other form of life, are the miracle on your doorstep: the marvel in the back garden. You see a moorhen’s labouring, leg-trailing flight as it escapes from the apparent threat of your arrival and realise that it was not a great step for the related takahe or the Inaccessible Island rail to give up flight. Evolution always looks for the easiest option, the most economical way of life. The great extravagance of the albatross or the tiger is, when all the results come in, a piece of precisely calculated parsimony. So it is with flightless birds, which have often evolved on small islands where flight would be a positive disadvantage – so long as the island remained safe.
That was the dodo: standing a metre tall, 3 feet 3 inches, and weighing as much as 18 kg, 40 pounds: perfectly adapted to the gentle life of Mauritius and fearing not a thing in the world. When the environment changes radically, the most perfectly evolved creatures in history will often fail to adapt. As the dinosaurs failed to survive the meteor-strike 65 million years ago, so the dodos failed to survive the arrival of humankind. And so did many birds that had evolved flightlessness. If the humans didn’t get them on purpose, the dogs, cats and rats they brought with them would often do the job. On many a flyspeck island, humans had the effect of a meteor and changed everything. Twenty million years of evolution in New Zealand were changed in the course of a millennium – an evolutionary instant – when humans started arriving, a process that accelerated when Europeans joined in.
The biggest bird of prey of them all lived on New Zealand. It could fly all right, fly with immense power and speed – and it evolved to tackle the massive and flightless herbivorous moas which stood 6 feet tall at the highest point of their backs. Female Haast’s eagles had a wingspan of up to 10 feet, 3 m (female birds of prey being normally larger than males) which is about the same as the biggest birds of prey still living, like martial eagles and golden eagles. But they were up to 40 per cent heavier, top weight being 15 kg, 33 pounds. That meant that when they built up speed they could hit the enormous moas with massive force. Haast’s eagle went extinct around the start of the 15th century because humans had by then killed off most of the moas. The eagles evolved to kill bipedal prey standing 6 foot high: did they also prey on humans? Impossible to believe they didn’t give it a go.
But some species of flightless birds, like the kakapo of Zealand, live on, sometimes thanks to massive conservation efforts, sometimes because the conditions they evolved for have not been too radically changed. Stare across the Kalahari, a place where it seems at times you can see forever. That bush in the far, far distance is not a bush: it’s an ostrich, lying on the ground and lowering its neck. Ostriches don’t hide their heads in the sand in that famously comic way (if non-human life is not portrayed as malevolent, it is generally shown as stupid): but they will drop to the ground to rest up and become extremely hard to see. And watch them going about their business: a male, deep black picked out with white, guarding a dozen chicks fussing around his ankles; standing as tall as 9 feet 2 inches, 2.8 m, weighing 145 kg or 320 pounds and capable of running at 70 kph, 43 mph, and with a kick that can disembowel a lion. Don’t call him an evolutionary mistake: at least not to his face.
Valuing oysters
The world is not only weirder than we can imagine; it is unimaginably more finely detailed. So many terms seem perfectly adequate, only to be found wanting after the most cursory look. Monkey? There are about 260 species of monkeys divided into two radically different kinds, those from the old world and those from the new. There are, as we have seen, 21 species of albatross. We think that a term is pretty specific, and then we start to look closer and find that it is far too vague and loose even to be considered a generic term. Oysters are like that. The term is bandied recklessly across the taxonomic groupings of oysters, sweeping up families that are not really all that closely related to each other: pilgrim oysters, saddle oysters and thorny oysters have never had much to do with each other.
The creatures we are most used to calling oysters belong to two quite distinct families. The edible oysters – in itself a loose term, for all oysters are capable of being ingested, but one generally used to refer to the oysters humans most like to eat – are in a quite separate family from the pearl oysters. But that’s another confusing term, because practically all shell-wearing molluscs are capable of making pearls.I The pearls we are talking about here are those prized and valued by humans.
The edible oysters come from the various species in the family Ostreidae; the pearl-bearers from the family Pteriidae. The pearl-making process starts not with a grain of sand, as the legend has it, but with the invasion of a parasite. The oyster covers the parasite with nacre or mother-of-pearl, and continues adding layer after layer. The creation of cultivated pearls is triggered by the introduction of a piece of polished mussel shell into the mantle of an oyster. And in only three to six years, you will have your perfect pearl. In the wild, you can go through four tons of oysters and perhaps find three or four perfect pearls. Perfection is of course another human con
cept: to an oyster, all pearls that relieve the irritation of a parasite are perfect. A pearl is the way an oyster scratches an itch.
Adult oysters generally start off as males producing sperm and then after a couple of years they become egg-producing females; a female can produce as many as 100 million eggs annually, using the scatter-gun reproductive strategy that is operated by many species. (Remember all that it takes for a population to remain stable is for each oyster to produce a single reproducing adult.) Oysters spend two or three weeks as free-swimming larvae before they settle down and become filter-feeders. One oyster can process five litres of water an hour: it follows that a bank of oysters has a powerful effect on the waters around it. Oysters help to maintain water quality, which is a benefit for most living things: it makes sense then, to keep a few of them out there in the sea.II
Oysters often gather in phenomenal numbers, forming reefs, which create habitats for many other species. Oysters are at their most remarkable not on a plate or as producers of jewellery but as an environment.
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I. The endangered freshwater pearl mussel can live for 250 years.
II. This crucial maintenance work on our planet was once considered important but incalculable, and therefore irrelevant. It was not a notion capable of slowing down the rate of destruction. A new way of thinking brings together economics and ecology to assess the worth of ecoservices, and attempts to put a serious financial value on them.
Do I contradict myself?
Sometimes I think evolution is just showing off: demonstrating its virtuosity without any sound reason for doing so, like a lead guitarist shredding in a heavy metal band.I
