It was nesting time in London Zoo’s Tropical Bird House, but the Montserrat orioles had ignored the boxes dotted through their free-flying mini-jungle. Instead, they had gone for the top of the “Emergency Exit” sign. The female sat on a bundle of twigs above a running man as if such signs were standard lodging for every Montserrat oriole. Then the male arrived, with a fresh twig. She got up and faced him, beak to beak. He stood awkwardly on the edge of the nest, placed his contribution in the nest and flew away. She tossed it out, settled down again, opened her beak and started laying eggs.

These birds live only in the east Caribbean, on a 40-square-mile island half wrecked by a volcano. They are on the Critically Endangered list. Their choice of the exit sign looked uncomfortably prophetic and those eggs will be crucial to their species’ survival.

Wildlife films are fine, but nothing beats watching wild animals yourself. As I left, the black-and-white colobus monkeys were bedding down for the night in pairs. Draped in gently swinging mantles of snowy fur, they had their arms around each other; each duo looked like silver fruit on the branch with two whitetasselled bell pulls suspended below: their long, plumed tails.

Historically, London Zoo is unique because it was the world’s first scientific zoo, created not for entertainment but by serious zoologists. The Zoological Society of London was founded in 1826, made the zoo for its fellows in 1828, and opened this to the public in 1847. Today, ZSL is a charity dedicated to the conservation of wild animals and the wild places where they live. It uses visitors’ fees to fund three things: increasing public awareness of wild animals and their plight, furthering the science that helps us understand their needs, and working to maintain them in the wild.

The zoo is part of a huge science-based conservation and field-research operation: ZSL also runs the Institute of Zoology, which offers Master’s and PhD programmes in conservation science and wild animal biology, as well as a conservation centre with field teams across the globe. One team is currently in Majorca, inoculating an isolated population of midwife toads to see if the procedure can control a lethal fungal virus threatening the world’s amphibians.

People who are against zoos picture them as prisons where animals live constricted lives for our amusement. This is not what responsible zoos are. In London, animals live as close to wild conditions as possible, with teams of vets, keepers and scientists working to enhance their lives. The otters’ enclosure has glass down one side so you can see them play underwater. I watched one swim with a pebble, scraping it against the glass as if washing a window, pretending to drop it, then diving to catch it before it hit bottom.

Nature deficit

In the 1970s and 1980s, when wild places were being destroyed and animals going extinct, responsible zoos thought hard about their role in society. How animals are kept has changed radically; the reason why they are in zoos has changed, too. ZSL is very clear: zookeeping serves conservation in the wild. Many who work at London Zoo, professionals and volunteers, would never take a job elsewhere: they work passionately, in the words of ZSL’s mission statement, for “a world where wild animals are valued and their conservation is assured”.

With wild nature threatened everywhere,London Zoo tries to show, at every level, what wild animals are, and what they need to survive. There are conservation explanations round every animal enclosure; Keeper for a Day schemes; Zoo Lates on summer evenings, popular with twentysomethings; and Zoo Sleepovers. Children with torches watch sleeping otters, now curled in their holt, twitching, juggling pebbles in their dreams.

London Zoo’s animals are there to support their wild brethren, directly and indirectly. Zoos employ many of the world’s leading conservationists, who speak for wild animals against vested interests aiming to exploit and destroy their habitats. The penguins whose diving delights visitors support conservation in the wild: 10 per cent of the money invested in new exhibits, such as Penguin Beach or the new Tiger Territory (opening next Easter), funds linked projects for wild penguins and tigers. Wild tigers everywhere would be much worse off without zoo tigers, which attract conservation funding.

There’s also the Emergency Exit aspect: zoos breed species such as Montserrat orioles, which are going extinct in the wild. Zoo breeding, now directed internationally by genetic management software, has saved and reintroduced to the wild animals such as the black-footed ferret and European bison. As the wild percentage of the globe shrinks, zoo breeding conserves genetic diversity as a hedge against extinction. London’s Tiger Territory will house two Sumatran tigers: one bred in Australia, the other in America.

For increasingly urban human beings, however, London Zoo’s role is offering us an authentic feel for the wild. A recent National Trust report said children in Britain suffer more from nature deficit disorder than children in other developed countries; their health and education are suffering, too. “What a joy to know,/” wrote Auden in his “Address to the Beasts”, “even when we can’t see or hear you,/that you are around . . .” To stay human, we need that connection with wildness, to feel respect and empathy for the wild Other that is different from, but similar to, ourselves. London Zoo offers a unique glimpse into beings with which we share the planet, and which still – just – exist in the world outside us.

One teacher I know prepared her inner-city seven-year-olds for seeing tigers in the zoo: they read books, saw films, drew pictures, were up to their ears in black and yellow stripes. Then they met the real thing. A girl gazed into a tiger’s eyes and the tiger obligingly gazed back. Shocked, she looked at her teacher. “You didn’t tell us they were real,” she said.

Ruth Padel is a poet. Her books include “The Mara Crossing” (Chatto & Windus, £12.99) and “Tigers in Red Weather” (Abacus, £9.99)

• The Collected "Tales of a City" [2]