The finches of the Galapagos Islands, whose beak variations are said to have inspired Charles Darwin's theory of evolution, are being attacked by an invading parasite, scientists say.
Many of the 13 species of Darwin's finches are among the most vulnerable birds in the world.
They are being threatened by the blood-suckinglarvae of alien flies that were accidentally introduced to the islands.
Conservationists are most worried about the mangrove finch, Camarhynchus heliobates, which is represented by fewer than 100 breeding pairs.
Dr Birgit Fessl and Dr Sabine Tebbich, of the Konrad Lorenz Institute in Vienna, analysed 177 nests of a dozen species of Galapagos birds, including seven types of finch.
They discovered that virtually all were heavily infested with the larvae of a warble fly called Philornis downsi.
The fly was unheard of in the Galapagos before 1989.
The larval parasite sucks the blood of fledglings and the fear is that this could seriously weaken young birds.
Drs Fessl and Tebbich found that heavily infested nestlings were more likely to die.
Many dead birds had holes drilled through their flesh.
The scientists analysed the nests of the estimated 30 to 70 breeding pairs of mangrove finch on Isabella, the only island where this critically endangered species exists.
"We confirmed the presence of Philornis at these sites in March 2000," they said.
"A decline in nestling survival related to fly-larvae parasitism would be a severe threat."
Nigel Collar, an adviser to Birdlife International, said the emergence of the parasite in the Galapagos was extremely worrying.
The invader threatened to wipe out "this already critical species" of finches.
