North Brother Island is a desolate place like this - a wooded scrap just off the Bronx - and last Wednesday I went there to see its first renovations for half a century. Only these days, like its brother, South, North Brother isn't being prepared for humans. There is a feeling that we have had our chance. Like many abandoned islands in the city's waterways,

North Brother has become a valuable sanctuary for New York's returning wildlife, and last week it was being readied for herons.

Herons, being fragile, sensible creatures, abandoned New York for much of the twentieth century. It is only since the mid-1970s, after the passing of the Clean Water Act and the gradual improvement of the city's rivers and estuary, that they have returned to breed and forage in the spring and summer.

And even now, the herons remain cautious and their presence is precarious. As they return to make their nests on the city's islands, herons have to contend with invasive trees and vines that have taken over since people started meddling.

So last week, in one of the first active efforts to make the lives of herons more comfortable, a group of New York City Park's Department employees and Audubon Society volunteers went to North Brother Island to chop down an acre of unhelpful Norway maples and replace them with the native trees (hackberry, box elder, American elm) that herons prefer.

"We are building an addition to their house, if you want to think about it that way," said Yigal Gelb, a slim, talkative man who runs the Harbor Herons Project for the New York City Audubon Society.

Even among the peculiar, dark histories of New York's other remote islands, North Brother stands alone. In the 19th century, the 20-acre site was used to house various infectious patients before a full-blown, 332-bed hospital was built in 1885. And for the next 75 years, the Riverside Hospital treated a host of New York's most troubled and unwanted cases - setting up the city's first methadone clinic and a small centre for juvenile delinquents.

But the story of the island is really based around two events: the life of Mary Mallon, or "Typhoid Mary", a seemingly-healthy carrier of typhoid who was isolated on North Brother Island for 26 years until her death in 1938, and the General Slocum ferry disaster. Just a few years before Typhoid Mary began her stay on the island, this overcrowded passenger ferry, equipped only with rotten cork lifejackets, caught fire and ran aground on North Brother with the loss of more than a thousand lives.

Eerie photographs from 1904 show the victims laid out in their Sunday best in the neat hospital grounds.

And so, after all that, over to the herons.

Wednesday was bright and blue. With the Audubon volunteers, I took a speedboat to the island and we tied up against the ruined jetty which, since 1963, has been left to fall into the water. " The current?s crazy, I?m going try and fight it,? said the captain not very reassuringly, leaving us paddle to shore in a canoe.

We were met by Tim Wenskus, the head forester for the New York City Parks Department, who outlined the plan to clear more nesting space for the herons before their breeding season starts in a few weeks.

Starting with an acre around the old hospital tennis courts (area A on the map), the idea was to clear out the tall Norway maples - decorative trees in their day, which have overrun the island - and Kudzu vines which have similarly taken over. "And here's what it's all about," said Mr Wenskus, pointing to an old heron's nest, three quarters of the way up a tree.

As the chainsaws started, I slipped away to explore North Brother's vine-throttled buildings. The ground was thick with the mulch of last year's leaves, making it hard to discern the old paths of the hospital complex, so occasionally I tripped on a ghostly curbstone or came across a fire hydrant or realised that a tree was in fact a streetlamp.

The first building I went into must have housed the hospital's more recreational facilities. In the long dark corridor, the wallpaper was peeling, as thick as bark. There was a creaking basketball court - its floor crunchy with the frozen glass of its mid-century spotlights - and an old auditorium with a small proscenium arch leaning over, stalled in mid-collapse. There were rows of chairs just sitting there, sinking into the ground, as they have done for fifty years, with the initials of patients and adventurers scrawled into them.

The other outlying buildings were more residential, with chests of drawers, sinks and toilets that looked shrunken and exposed where the vines had climbed in through the windows. In the house where the hospital director used to live the floor was covered with old books - telephone directories from 1962, a surgeon's catalogue filled with chilling tools (thoracic rib shears, anyone?), and log of all the island's supplies that came by ferry from Manhattan.

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Of course the hospital is the island's centrepiece, looming with its smashed windows above the trees. The herons apparently have not yet approached it and its only permanent resident is, fittingly, a solitary great horned owl. I went around to the back door and followed a set of goose footprints inside.

I didn't get very far.  According to some of the park rangers who have spent many days on the island, if you're brave enough to make your way past the yellow tiles, enormous kitchens and unhappy profusion of heavy metal doors to the top floor, you come across straitjackets and strange tables. All I had time to find was a bottle of "Grand Old Dad Kentucky Bourbon", nearly empty and many years old.

Then it was time to go. With 500 native trees planted last week and plans to return next year to improve the island further as a bird habitat, North Brother has started the latest, and happiest chapter in its history. Speeding back towards Manhattan's skyline, we kept passing other islands - like Rikers, home to New York's enormous prison, and Wards, home to a psychiatric hospital and the city's largest homeless shelter - that have yet to begin theirs.

Slices of New York