This summer's cold and wet weather has had a disastrous effect on the breeding season for swifts, the Royal Society for the Protection of Birds says.
The population of the birds that come to the UK every summer to breed is said to be nearly a third lower than it was in the mid-1990s.
The weather has resulted in fewer flying insects for swifts to eat.
Experts say many swifts are returning to Africa early, putting population levels further at risk.
The RSPB says swifts are also facing a lack of nesting sites in the roofs of UK buildings as old properties are renovated and new homes built with no access or space for nests.
RSPB conservation director Martin Harper said: "The last thing this struggling species needed was to be hit hard by the wet weather this year.
"But they are at the mercy of more than just a wet summer. Their ability to nest depends on our buildings having spaces for them.
"They fly as many as 6,000 miles each spring to get here from Africa to breed, only to find that changes in the way we're building and renovating means there are fewer nest spaces."