Birdwatchers and ornithologists are not at all the same. To the latter, everything about birds is of interest - how they migrate, where they breed, or what they eat. Birdwatchers have a single concern, which is to see as many kinds as they can. Once seen, as soon forgotten, or, at least, ticked off and added to the Life List that is the basis of their self-esteem.

I went through the same phase. After the usual interest in stamps and an eccentric deviation into cheese labels, I was given a pair of binoculars. At the age of twelve I eavesdropped on a group of excited amateurs (twitchers, as they call themselves nowadays) as they peered at some gulls bobbing, on a dim winter day, on the then filthy waters of the River Mersey. All agreed: one of the birds was not an ordinary herring gull, but the much scarcer glaucous gull, seldom seen so far south. My problem was that I could not see any difference. A member of the flock was a rarity, but which was it? Did it count? Could I check the box in my bird book? It was my introduction to the ethics of science. I admit it: I made the tick, but then I rubbed it out.

Twitchers, like scientists, belong to a fellowship of faith. They play cards against Nature. It is possible to win every time by faking one's hand, but to do so removes the point of the game. That is the strength of science, and its greatest weakness. Without collective trust it could not work. Instead there would be the dismal apparatus of mutual suspicion familiar to every accountant.

Birding is refreshingly free from fraud. It has had its scandals, such as the notorious case of the Hastings Rarities (a set of bizarre sightings on the South Coast in Edwardian times), the dubious claim of a Dalmatian pelican in Colchester in the 1960s, and more. Even so - and whatever the rivalry amongst the twitching fanatics - most of those involved in the sport play by the rules.

One problem baffles the most ethical birdwatcher. Stamps (or cheese labels) are easy. The 1853 One Shilling Cape of Good Hope Triangular (or the 1954 Vache qui Rit) is, or is not, genuine. Fakery apart, there is no reason to question the object itself. But what if it is ambiguous? Then opinion, the enemy of science, creeps in. Is one kind of bird really unlike another? How different does it have to be to count as distinct? What, indeed, is a 'species' in the first place? Does it have a scientific definition, or is it all in the eye of the beholder?

US Fish and Wildlife Service/ Stefan Dobert Spot the difference? Only an expert can tell the difference between an Iceland and a glaucous gull.

Most people can tell gulls from terns. Many can separate the herring gull from the lesser black-backed (look at the back, which is pale in the first and dark in the other). My friend the glaucous gull is more subtle - as large as a greater black-backed, but as pale as a herring gull, without its black wingtips. A real expert can even sort out the Iceland gull (like a small glaucous, with longer wings). Birders still argue about the existence of the yellow-legged gull as a distinct entity, but its 'bold, confident look' is said to be diagnostic.

But how to deal with variation within each bird? Herring gulls from Estonian bogs had, some say, yellow legs, but these have now disappeared in favour of the pink legs found everywhere else. Those from the Atlantic islands are on occasion blown to Britain. Their backs are almost as black as those of lesser black-backeds. A 'generally stouter bill' might help, but what use is a generalization when an individual must be sorted out? If the despondent twitcher were to travel to Iceland he would be frustrated by hybrids between the Iceland gull and its commonplace relatives. Where does he put his tick?

Species and nations have a lot in common. What, for example, is a German? The tribe has a shared and guttural means of communication that interrupts intercourse of most kinds, but the character is equivocal, for Austrians speak the same language. Since 1913, the country has defined its own citizens by descent, by German blood (whatever that might be). It includes within the realm the remnants of the Saxon diaspora (many of whom - Romanians included - cannot speak German at all), but cuts out children born in Berlin of Turkish parents. One badly behaved teenager was deported to Turkey even though he was born in Germany; while at the same time a hundred thousand Romanian-speakers of approved blood were allowed in. Until the fall of the Berlin wall, indeed, a geographical barrier made many German citizens more alien to each other than Westerners or Easterners were to the French or the Poles. A century ago German identity itself meant little, as there were only Prussians, Bavarians and Rhinelanders, political entities of their own, each now reduced to variants in some greater Teutonic whole.

University Southern California Charles Darwin: he described the process of natural selection, the theory of evolution waited for an advance in genetic knowledge.

The problem of how to define Germans, or any nation, arises because the question is ambiguous: does it turn on shared appearance and behaviour, on geography, or on descent? Is a country an historical entity, or should it be identified only on criteria that apply today? How much can frontiers be allowed to leak before a nation loses its essence? When will Germans be seen as Europeans, as Prussians have become German?

Such problems of identity turn on natural variation, the raw material of evolutionary change. Like a politician, the twitcher has to deal not just with differences among individuals but with the subtle distinctions that separate each kind. The difficulty of how to define domestic breeds has been magnified and transferred to the world as a whole. Twitchers are asking a question older than the theory of evolution. How should they deal with forms that possess in some considerable degree the nature of species but are not classified as such?

Taxonomy, the science of ordering life, has to worry about that problem. Needless to say, many animals and plants are easy to tell apart. If they were not, birdwatching and natural history museums would each go out of business. One tribe in New Guinea recognizes a hundred and thirty-six kinds of bird, just one fewer than that accepted by the experts. Experts and tribesmen have the same philosophy. Each needs an archetype, a gold standard, to allow their specimens to be put in the correct cabinet.

Cracow University of Technology The scientific world didn't acknowledge its existence until they saw a stuffed one in 1928.

Once all taxonomists worked in the same way. An animal was killed and its remains stuffed, pinned or bottled. Then, it was described in the scientific literature. The cadaver was the 'type' against which others could be checked. In 1868, in China the French missionary Pere Armand David saw the skin of a black-legged white bear. It resembled animals shown in ancient works of art and until then assumed to be polar bears brought back from the north by hunters. The first specimen of the mysterious beast was collected in 1929 by Theodore and Kermit Roosevelt, the sons of the President. They shot a giant panda asleep under a tree. Its body gave the animal entry to the pantheon of mammals as Ailuropoda melanoleuca. It joined the world of science as had all its relatives, as a corpse.

Now, fewer than a thousand pandas are left. In China, to kill one means the death penalty. Taxonomists. too, are more careful with their material than once they were. The essence of a species can now be (or so the museum-keepers believe) be preserved not in its bones but in its genes. The Bulo Burti boubou shrike of Somalia was recognized in 1991 on the basis of the DNA sequence in a feather shed by a captive bird. The type specimen - the very substance - of this new form is a set of dark bands on a photographic plate. The rest of the bird was released.

Not all pandas - or Bulo Burti boubou shrikes - are alike. They may look the same but are, like whales, dogs, or viruses, full of diversity. Classifiers hence face a fatal temptation: to split their animals into too many groups. As in the Kennel club or the United Nations, quarrels break out between those who like to subdivide the world and those who hope to unify it.

Amersham Pharmacia Biotech Ltd 1998 DNA fingerprints - can these genetic blueprints define a species?

A rich nineteenth-century collector, Isaac Leigh, was interested in the freshwater mussels of North America. He named more than a thousand kinds on the basis of tiny variations in shell-shape and size. Now the number has been reduced by two-thirds. A hundred and two of his types are classified as one. Isaac Leigh was too enthusiastic about his varieties. His cherished diversity was no more than that between people with brown or blue eyes or between the pink- and yellow-legged herring gulls that once infested Estonian marshes. He had, nevertheless, put his finger on a problem that still plagues museums. How should they fix the frontiers between supposed entities when each is filled with variation?

Genetics, the science of differences, has not made their job any easier. Before it began it had often been asserted - but the assertion was quite incapable of proof - that the amount of variation under nature is strictly limited in quantity. Now, the claim can be tested, and it fails.

Most members of most species do not look much different one from the next. Any fruit-fly is much like another, and even their best friends find it hard to tell mice apart. In spite of some exceptions - the colourful snails or butterflies that come in dozens of forms and are still studied by a few outmoded naturalists - to share a Latin name imposes, almost by definition, a certain uniformity upon those who bear it. That comforts both creationists and experts on taxonomy. They like to see existence as a set of neat ideals, each filled with some pure Platonic essence. However, a great deal is hidden within even the most uniform creature. Genetics shows that no one - not even the glorified chemists which most biologists have become - can any longer suppose that all the individuals of same species are cast in the very same mould.