Only about 40,000 breeding pairs of yellow wagtail in the UK

The Royal Society for the Protection of Birds (RSPB) said it was studying why its numbers had fallen dramatically over the past 25 years.

Research into the decline, backed by heritage body English Nature, the University of East Anglia and charity the British Trust for Ornithology, began in April in Lincolnshire and Cambridgeshire.

The results are expected next year.

The delicate-looking birds - Latin name montacilla flava - breed in the UK but fly to Africa to overwinter.

One theory may be new crop types introduced over the years so they are denser earlier in the year, making it difficult for the birds to nest Dr Guy Anderson, of the RSPB

Researchers said it was unclear whether the fall in the number of the birds, which are usually found across central and western England, was due to fewer returning from warmer climes or a failure to breed in the UK.

There has been a 36% decline in the UK population between 1974-99, and only about 40,000 breeding pairs remain.

Dr Guy Anderson, a research biologist with the RSPB, said long-term changes in farming practice could be linked to the decline but it was still unclear what the exact cause was.

Arable farming land is one of the bird's favoured breeding grounds.

He said: "What we do know is that one kind of their favourite breeding habitats - wet grassland - has declined over the last 50 years - but that is not the only type of habitat they will breed on, so we don't know what the drive is for the fall."

Dr Anderson said scientists needed to find out more about the insect-eating birds' habitats, foraging habits and food.

He said: "Currently we are looking at birds breeding in two different habitat types, on wet grassland and arable farmland.

"We are looking to try to identify what areas within these two habitats they select to nest.

Farming methods could be to blame for the decline, say scientists

"One theory may be new crop types introduced over the years so they are denser earlier in the year, making it difficult for the birds to nest."

The RSPB said birds found in the UK were a unique subspecies of the yellow wagtail.

The male bird has distinctive bright yellow underparts and green top feathers, while the female has a paler plumage.

The ground-resting birds usually arrive in the UK in April and leave in September.