Every year since 1979 there has been a Big Garden Bird Watch, a nationwide survey organised by the RSPB. But I can't help contemplating a survey of a different species: a Big Outdoor Child Watch. I know only too well what it would find. Chicks are now pretty much extinct, outside their own nest areas and a shrinking number of poorly maintained reserves. Juveniles, common in the 1970s, declined in numbers throughout the 1980s and are now rarely seen away from their parents, except in impoverished areas. And adolescents, though not yet endangered, are seen as pests and controlled accordingly. In sum, children are fast disappearing from the outdoor environment, by far their preferred habitat.

The ecology of children apparently being less interesting than that of birds, there is little hard data around. We do have Mayer Hillman's classic One False Move, a study of children's independent mobility. It suggests that, in a single generation, the "home habitat" of a typical eight-year-old - the area in which children are able to travel on their own - has shrunk to one-ninth of its former size. Do not underestimate the significance of this change: for the first time in the 4m-year history of our species, we are effectively trapping children indoors at the very point when their bodies and minds are primed to start getting to grips with the world outside the home.

Taking an ecological perspective not only dramatises these childhood changes, it also sheds light on their consequences. Research from the young of other species shows that captivity causes a fall in physical, cognitive, social and emotional competences. In children, at risk is what naturalists might term survival mechanisms, but what psychologists call resilience.

Sceptics may ask about the relevance of animal findings to our own species. But there's a lot of supporting evidence. Health experts agree that the decline in outdoor play is linked to child obesity - a position supported by recent research on children's activity levels from Professor Roger Mackett at UCL. And Sharon Witherspoon from the Nuffield Foundation was quoted in these pages only last week as saying "we are doing something peculiarly unhelpful for adolescent mental health in Britain".

The decline is, in part, a side effect of wider social changes. Shrinking families, more parents working longer hours and increasingly fragmented communities have left children with fewer friendly faces to look out for them. Many more children have their own rooms, and the entertainment industry makes ever more seductive indoor offers to stave off cabin fever.

Fear plays a key role: parents' fears of traffic (probably justified) and strangers (arguably not), and children's fear of crime and bullying. There is growing hostility to children in public space. Behaviour that would a few years ago have been "larking about" is now labelled antisocial, and parents fear being judged harshly if their kids are seen out of doors unaccompanied.

There is precious little outdoor play space: Audit Commission figures show that each child under 12 has a "ration" of 2.3sq metres - about the size of a kitchen table. As for the quality of facilities, the Children's Play Council quotes one child - surely not unrepresentative of the bigger picture - telling it like it is: "Go to the park - it's rubbish."

Government is not immune from blame. Politicians and Whitehall mandarins insist on hard evidence of the benefits of outdoor play, the implied message to children being, "We'll only let you out if you prove to us it'll do you good." Chris Smith is one exception. In 2001, he secured a Labour pre-election commit ment to spend £200m of National Lottery funding on play. Frank Dobson is another. He chaired the review I worked on to shape the promised programme, and is not afraid to say in public that children need to "lark about". But eight months after our report, Getting Serious About Play, was published, we are still waiting to hear the outcome.

A positive government response to the Dobson report would dramatically raise the profile of playgrounds and play services. And not just children would benefit: many local communities have found new life and energy through playground improvement projects, something the charity Groundwork can confirm.

But as with birds, so with children: their presence all over the place, in diversity and numbers, is a sure sign of a healthy human habitat. To achieve that, our transport and planning priorities need to change, including such measures as 20mph speed limits in the streets where children live. That, in turn, implies a change in adult attitudes - something that pessimists say just isn't going to happen.

The success of home zones - child-friendly streets - suggests the pessimists might be wrong. They show that many local communities want to live in habitats where the noise of children playing outside is not a sign of neglect but a sound to raise the spirits. Like the first cuckoo of spring.

· Tim Gill is outgoing director of the Children's Play Council; he gives his valedictory lecture at the National Children's Bureau, London EC1, at 6.30pm tonight

tim@rethinkingchildhood.com