Researchers are trying to find out why the cuckoo is in decline in the UK
The cuckoo - known for its springtime song - has joined a "red list" of the UK's most threatened bird species.
The lapwing, yellow wagtail and herring gull are also deemed to be urgently in need of conservation action.
In the latest assessment of the UK's bird population, the number red-listed has risen by 5% since 2002, to more than one in five.
The RSPB said the growing number of charismatic, widespread and familiar birds now on the list was "scandalous".
RSPB conservation director Mark Avery said the "most shocking" decline was that of summer-visiting birds, like the cuckoo, which has seen numbers fall by 37% in the last 15 years.
Experts are not yet certain why migrant birds are in decline in the UK.
The number of red-listed species has risen to 52 (21%) out of 246 birds assessed, which is up from 40 species (16%) when the last assessment was done in 2002.
Most birds on the red list have seen their range or populations decline by more than half in recent years, or have undergone historical declines since 1800, from which they have not fully recovered.
The house sparrow, starling and song thrush are among the once-common British birds now on the list.
Some 21 red-listed bird species are summer visitors, most of which spend the winter in sub-Saharan Africa.
In the latest assessment, there was good news for the bullfinch, quail, reed bunting, Scottish crossbill, stone curlew and woodlark - which were all downgraded from the red list to "amber".
They had either increased their populations or range - mainly as a result of improvements in management of farmland for the stone curlew, and heathland for the woodlark - or more had been discovered about their numbers, as with the Scottish crossbill.
The assessment results will be published in the June edition of British Birds.
Andy Clements, director of the British Trust for Ornithology (BTO), said: "A gap all of us will notice is the lack of the cuckoo's familiar song. Their decline is emblematic of downward trends in many long-distance migrants from Africa."
Farmland bird the lapwing is also at risk
He said BTO and its partners were working on research to try to understand why migrant birds are in decline.
"As this report shows, when we know what to do, and there are funds to do it, we can improve the fortunes of birds such as stone curlew and woodlark."
Although the cause of the cuckoo's decline is not known, a number of theories have been discussed, said the RSPB's Grahame Madge.
Research will look at whether there are problems with habitat, either in the UK or in Africa, or on their migration route, he said.
The species relies heavily on hairy moth caterpillars for food and, with many species of butterfly and moth are also suffering declines, a diminishing food supply could be having an impact.
In addition, with cuckoos laying their eggs in other birds nests, there may be issues with their key hosts, such as the meadow pipit and dunnock, being in decline.
While no link to climate change has been proved, there are concerns that, with European temperatures rising, migratory birds are losing their ecological advantage over birds which over-winter in Europe.
The addition of five species to the red list - Temminck's stint, ruff, whimbrel, redwing and fieldfare - which are at the southern edge of their spread in the British Isles, could suggest a shift in range brought on by a changing climate.
Seabirds join list
For the first time, two winter visitors, the dunlin and the scaup, have been added to the red list because of declines in their wintering populations.
Rich Hearn, the Wildfowl and Wetlands Trust's head of species monitoring, said their inclusion highlighted an "increasingly widespread phenomenon of climate change-driven shifts in distribution".
Three seabirds have also joined the red list, the Balearic shearwater, which is at a higher risk of global extinction than the giant panda; the Arctic skua, the only bird to go straight from the low-concern "green list" straight to red; and the herring gull.
The assessment, Birds of Conservation Concern 3, is compiled by a group of organisations including the BTO, Countryside Council for Wales, Game and Wildlife Conservation Trust, Joint Nature Conservation Committee, Natural England, Northern Ireland Environment Agency, RSPB, Scottish Natural Heritage and the Wildfowl and Wetlands Trust.