Julia Hinde meets the warden of the Queen's swans, Christopher Perrins Swanning around on the river.

The Queen's swan warden is rather looking forward to the next week. Surrounded by publishers' letters and proofs of brightly coloured book covers, Christopher Perrins explains how, for the next five days, he will be swapping his room in the concrete jungle of Oxford University's zoology department for the lure of the Thames and its swans.

Arranging to photograph the university professor carrying out his royal duties counting, marking and checking swans on the Thames is rather like arranging a week away. Monday lunchtime is the Sun at Staines, Tuesday the Little White Hart at Cookham, and so it goes on.

But checking the swans is a serious and deeply traditional business, Perrins explains. Accompanied by representatives of the vintners and the dyers, just as in Victorian times when those allowed to own swans would mark the young and take some away for fattening, the lecturer is on the look-out for swan hazards. Three years ago his research into swan deaths led to the banning of lead weights for fishing and to his elevation to the royal post - for which he has a business card.

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Perrins, professor of ornithology at Oxford University, first got into swans in the early 1960s. Before this much of his work had been involved with tits in Wytham Woods near Oxford. Interested in bird life-cycles, he was concerned that he never knew the exact number of individuals in his study area. "A proportion of birds may not breed in any one year," he explains. "So you cannot find their nests and therefore never know exactly how many there are. So we decided there was one easy bird to count - something big and white. We started looking at swans around Oxford, but it turned out there were an awful lot of small ponds with swans on."

Perrins's passion for natural history was triggered as a young boy living in the Surrey countryside looking to make some extra pocket money. "My first-ever profession was collecting adders for London Zoo," he says. "They wanted striking black and white males for display and would send others away so they could make anti-venom. I lived in an area where there were plenty of snakes. You'd walk around looking for them sunning themselves and put a stick behind their necks so they could not bite and pick them up By the tail.

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"You'd get four shillings, that's about 20p, for them. The real thing was to go up to the zoo where they allowed you to pick the pythons up. It was a hobby, but in those days it was big money."

A notice on a university board during his undergraduate years first brought Perrins, then passionate about butterflies and moths, to switch his attention to birds. "David Lack, my predecessor here at Oxford, started a student bird conference held every January. In my first year at university I went along. Now I run them. We had the 50th one last year. I have missed only one. I guess that is what switched me over. You can follow lifetimes in birds, but in insects it's much harder. They are shortlived and harder to mark. In my third year David Lack offered me a little job here. I came and they never got rid of me."

Perrins extended the university's field site at Wytham Woods, which now forms the most detailed long-term ornithological study area anywhere in the country. Research students help annually with the catching and ringing of young birds and the updating of records, and in return get to use the wealth of information known about the site. "All hell lets loose for two or three weeks in the spring when the birds have to be caught and the chicks ringed. It's bedlam," he explains.

"One of my primary interests is trying to understand why birds lay the number of eggs they do. Very simply, they are doing the best they can. They are laying the number of eggs which will produce the highest number of offspring into the next generation. But it is much more complicated than that. The best number of eggs for some birds is not the same as for others and they can adapt the number of eggs they lay. They do a better job in terms of reproducing than humans."

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Perrins says there are many more questions that need to be answered and some that have not yet even been asked. "Lots of the birds go out of the wood, while others come in. One day I suppose there will be a miniaturisation of radio transmitters so we can follow these birds for their whole lives. It's already being done in bigger birds, such as storks and vultures. I don't suppose it will happen for small birds like tits in my life time, but it will be a technological breakthrough which allows us to ask new biological questions."

Perrins advises the Queen, who officially owns all mute swans, on the management of swan populations and helps to resolve disputes. Diplomacy characterised his approach when trying to outlaw lead pellets for fishing. "We worked at it for 12 years, but we didn't campaign. We met with organisations which, not unreasonably, were not easily persuaded that lead weights were killing swans. They came disbelieving but went away believing. I am never going to be in the front line, throwing bricks through people's windows. But, hopefully, by providing data, I can do something too. It's difficult to admire nature and not to be involved in conservation."

Among ornithologists Perrins is associated with the birdwatchers' bible - the Handbook of the Birds of Europe, the Middle East and North Africa: Birds of the Western Palearctic - a 17-year, nine-volume collection of notes on birds from Europe and the adjoining fringes of western Asia and northern Africa, which he is constantly updating. "It doesn't seem to quite ever go away," he says.