Mistle thrushes have disappeared from UK gardens at a "staggering" rate, according to the Royal Society for the Protection of Birds.
Results from the charity's annual survey suggest the birds are seen in fewer than half the number of gardens they were 10 years ago.
Population estimates published at the end of last year confirm there are now just 170,000 breeding pairs.
The warning comes on the eve of the RSPB's annual Big Garden Birdwatch.
Experts compare the decline to that of the closely related song thrush.
Both thrush species have become rarer sights in UK gardens, with populations falling by more than half since the 1970s, according to the ongoing Breeding Bird Survey carried out by the British Trust for Ornithology (BTO) and partners.
But the continuing trend of decline among mistle thrushes is a key concern.
"If you look at the decline in the short term, from 1995 to 2010, we see that mistle thrushes have declined by 28%," said the RSPB's Graham Madge.
Over the same period, the song thrush, which has been recognised as a species of serious conservation concern, increased by 13%, according to figures from the The State of UK Birds report jointly published last November by the RSPB, BTO and Wildfowl and Wetlands Trust.
"We've still got a lot to learn about why the [mistle thrush] is declining," said Mr Madge.
"It's reaching rates where as a community we should begin to develop some concerns because its population does seem to be haemorrhaging from our gardens, woodlands and countryside."
He described reports from the annual Big Garden Birdwatch as "startling".
"Over the last six or seven years of Big Garden Birdwatch, the participants have been telling us that mistle thrushes have been declining - even over that short period."
The survey, described as the largest citizen science project in the world, sees volunteers count the bird visitors to their garden for an hour over the last weekend in January.
Mr Madge describes the "snapshot" of UK bird populations provided by the survey as an "effective early warning system" to highlight future trends.
Mistle thrushes are named for their fondness for berries, particularly those of the mistletoe plant.
How to spot a mistle thrush
A mistle thrush sings from the top of a post
Listen out for their distinctive rattling call
They are a cold, grey brown whereas song thrushes are a warm, earthy brown
Spots are "scaly" and join up unlike the perfect round spots of the song thrush
You can see white tips to the feathers of a mistle thrush flying away from you
Song thrushes are blackbird sized; mistle thrushes are 10-15% bigger
The birds sing throughout the winter to defend territories that are rich in fruit, making their song the easiest way to recognise them.
"It's a species that very often people will mistake for a song thrush. They're both medium sized, brown and white birds with large spots," said Mr Madge.
"But mistle thrushes are great characters and they will very often give their presence away by their call."
The birds are early nesters and their mournful songs can be heard in February. They are also known as stormcocks due to country legends that the birds are the last to be heard before a storm.
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