One of Britain's biggest bird surveys gets under way this weekend. In four years, when the British Trust for Ornithology's survey is completed, we should have a clearer idea about this country's bird population and popular nesting spots.

Such a comprehensive survey is overdue. The last bird census on this scale was conducted 20 years ago. More than 250 species of birds populate our islands, and we need reliable figures to judge how their numbers are being affected by the changing climate and the decline of suitable habitats.

The survey also offers nature-lovers the opportunity to take part in a major conservation project. More than 50,000 regular birdwatchers will be signed up, but everyone is encouraged to report sightings of birds in their back gardens and where they live. We would encourage all our readers to take part in a project that will inform conservation work for a generation.