Greenfinches, ptarmigans and even the common blackbird could be among the next British birds to begin the slide towards extinction, according to a report published today.

The birds were highlighted by a new "early warning system" designed to identify species currently abundant but at risk of serious decline in the future. By predicting the next species likely to suffer a fall in numbers researchers hope to give conservationists a head start.

At least a fifth of British birdlife is already on the nation's red list of endangered species, a figure that is expected to rise when the list is revised later this year. According to the Royal Society for the Protection of Birds, the number of red-listed species could hit 50 or more, and include the woodpecker, lapwing and curlew.

Gavin Thomas, a biologist at Imperial College London, used genetic records to create the first detailed family tree of British birdlife. It shows how 249, or 93%, of the nation's bird species are related. When he checked which birds are endangered, he found them grouped on the family tree, suggesting that related birds shared traits that made them vulnerable to population decline. Some, for example, shared low fertility rates while others had similar dietary or habitat needs.

Thomas used the family tree to spot birds likely to become threatened in the future, simply by checking if any of their close relatives have already appeared on the endangered list.

"This study threw up some interesting results," said Thomas, whose research appears in the journal Proceedings of the Royal Society B. The common blackbird had, he said, "several close relatives, including the song thrush, that are experiencing severe levels of population decline. This could mean that populations of blackbirds in the UK are at risk of declining in the future."

Other species at risk include the greenfinch, which is related to the endangered linnet and bullfinch, and the ptarmigan, still found in mountainous regions of Scotland such as the Cairngorms, which is closely related to the black grouse and grey partridge, which have both suffered severe declines in recent years.

This is driven by a variety of factors, including intensive farming, climate change and hunting. This month naturalists disclosed that puffin numbers on the Isle of May, home to the largest puffin colony in the North Sea, had unexpectedly fallen by a third this year. They blamed climate change and overfishing.

Grahame Madge at the RSPB said the study offered conservationists a new tool for assessing risks, but added that a complex mix of reasons was usually to blame. "In a sense, it's another way of seeing the ship sink, and we really have to find a way to keep it buoyant," he said.

Threatened birds

Blackbird (Turdus merula)

A close relative of the dramatically-declining song thrush

Greenfinch (Carduelis chloris)

Relative of threatened linnet and bullfinch

Ptarmigan (Lagopus muta)

Closely related to declining black grouse and grey partridge

Pied wagtail (Motacilla alba)

Grey and yellow wagtail are declining