Rural: Where there's muck...there's beaks.

What do these four have in common: Nottingham, Cambridge, Perry Oaks and Wisbech? Not the sort of question for a prime-time TV quiz show, perhaps. Not even a question that will elicit a correct answer from more than a tiny proportion of the million-plus members of the RSPB. But ask a keen birdwatcher of a certain age what these places have in common, and he or she won't hesitate. Sewage works, of course.

Muck is magic for birds. Especially this sort of muck. Many of today's senior ornithologists cut their teeth as birdwatchers - and developed their identification skills - surrounded by percolating filters, humus tanks, settling lagoons and the other paraphernalia that convert the end products of human bodily functions into (almost) pure water. Wagtails making the most of the clouds of tiny black sewage flies; flocks of finches devouring piles of tomato and other pass-through-your-gut seeds; snipe and monotonously churring grasshopper warblers in the damp, lagoon-side vegetation; even occasional rarities such as a lesser yellowlegs visiting from North America, attracted, perhaps, by the all-organic diet.

But, if old birdwatching hands used to check their Ordnance Survey maps for the telltale "wks" symbol (usually denoting a sewage works), most of today's twitchers gain their spurs by tearing about the country having been alerted to the latest rarity on their mobile phones. Not that technology can be blamed exclusively for the demise of the sewage farm birdwatchers' logbooks.

"Sewage farms aren't as popular with young birdwatchers because these have switched from being wetland habitats to something more like industrial plants," comments David Glue, of the British Trust for Ornithology. Glue, with Dennis Bodenham and Frances Bowman, two BTO members, have been visiting Aylesbury works and studying its birds for the last 27 years.

How important sewage farms were for birdwatchers learning their trade, from the Twenties up until the Sixties, can be gauged from the autobiography of one of the founding fathers of British ornithology. In Seventy Years of Birdwatching (Poyser, 1974), the late HG Alexander wrote: "The years between the two Great Wars might be described as the years of reservoirs and sewage farms so far as British field ornithology is concerned."

The attractiveness of the old-fashioned sewage farm, with its extensive muddy settling beds for migrating waders, came later. Probably Norman Joy, of Reading, first made this discovery when he began to report the remarkable waders, including four black-winged stilts hitherto only known as birds of coastal marshes and mudflats, that he had found on an unnamed "marsh" near Reading. Later that year (1923) he revealed that this was Reading sewage farm.

The most attractive sewage farms, such as Northampton and Nottingham, and some of those near London (Perry Oaks being an example) were only discovered in the late Twenties or early Thirties. David Lack, in his Birds of Cambridgeshire (1934) hardly gives any sewage farm records before 1927. By 1934, he was able to reveal that 152 species had been recorded at Cambridge sewage farm.

But if many of the old-fashioned sewage farms have disappeared their modern replacements can still be important oases for birds at all seasons. David Glue points out that birds such as starlings, tree sparrows and linnets - all in decline country-wide - make much use of modern works.

"Some of them could even be nationally important as resting-places for migratory birds arriving in Britain in the spring, allowing them to restock after their lengthy flights," he comments. At Aylesbury, a modern plant covering 10 hectares of land but retaining wet grassland irrigation areas, Glue, Bodenham and Bowman have recorded 61 species that have bred, or attempted to breed - an impressive number by any standard. "They include," says Glue, "water rails, snipe, yellow wagtails, whinchats and grasshopper warblers, all scarce breeding birds in Buckinghamshire."

Ironically, though, sewage treatment is going back to basics, for the final effluent clean-up stage anyway. Irrigation plots - where the partly treated liquid is soaked through wetland vegetation to filter it - are coming back into favour. They are often more effective. Many of the birds once familiar at sewage works could return, too.

Sewage stake-out

The best bits of a sewage works for birds (based on Aylesbury) in descending order of merit. Information from a 27-year study by Glue, Bodenham and Bowman.

Sprinklers and percolating filters: year-round water and food for thousands of starlings and pied wagtails; at other times for yellow wagtails, gulls and meadow pipits.

Wet grassland irrigation areas: nesting spots for reed buntings, warblers, ducks; even snipe and water rails.

Storm and humus tanks: good for wagtails and, at times of shallow water, for wading birds, chiffchaffs, rock pipits and even rare black redstarts.

Lagoons: breeding spots for little grebes, mallard and moorhen; occasionally tufted ducks and kingfishers.

Hedges and scrub: nesting places for blackbirds, wrens, warblers and others. Buildings and machinery: nesting for house sparrows, wagtails, sometimes even kestrels and little owls.

Lawns: feeding for wagtails, green woodpeckers and thrushes.

Rubbish heaps: vegetable wastes and seeds attract pheasants, stock doves, magpies and flocks of finches.