The populations of the world's common birds are declining as a result of continued habitat loss, a global assessment has warned.

©Gareth Peacock
Once a familiar sight and sound, the nightingale is becoming less common

The survey by BirdLife International found that 45% of Europe's common birds had seen numbers fall, as had more than 80% of Australia's wading species.

The study's authors said governments were failing to fund their promises to halt biodiversity loss by 2010.

The findings will be presented at the group's World Conference in Argentina.

The State of the World's Birds 2008 report, the first update since 2004, found that common species - ones considered to be familiar in people's everyday lives - were declining in all parts of the world.

In Europe, an analysis of 124 species over a 26-year period revealed that 56 species had declined in 20 countries.

Farmland birds were worst affected, with the number of European turtle-doves (Streptopelia turtur) falling by 79%.

In Africa, birds of prey were experiencing "widespread decline" outside of protected areas. While in Asia, 62% of the continent's migratory water bird species were "declining or already extinct".

Biodiversity barometers

"For decades, people have been focusing their efforts on threatened birds," explained lead editor Ali Stattersfield, BirdLife International's head of science.

"But alongside this, we have been working to try to get a better understanding of what is going on in the countryside as a whole."

By consolidating data from various surveys, the team of researchers were able to identify trends affecting species around the world.

"It tells us that environmental degradation is having a huge impact - not just for birds, but for biodiversity as well," she told BBC News.

While well-known reasons, such as land-use changes and the intensive farming, were causes, Ms Stattersfield said that it was difficult to point the finger of blame at just one activity.

"The reasons are very complex," she explained. "For example, there have been reported declines of migratory species - particularly those on long-distance migrations between Europe and Africa.

"It is not just about understanding what is happening at breeding grounds, but also what is happening at the birds' wintering sites."

She said the findings highlighted the need to tackle conservation in a number of different ways.

"It is not enough to be looking at individual species or individual sites; we need to be looking at some of the policies and practices that affect our wider landscapes."

The global assessment also showed that rare birds were also continuing to be at risk.

One-in-eight of the world's birds - 1,226 species - was listed as being Threatened. Of these, 190 faced an imminent risk of extinction.

The white-rumped vulture, a once common sight in India, has seen its population crash by 99.9% in recent years.

An anti-inflammatory drug for cattle, called diclofenac, has been blamed for poisoning the birds, which eat the carcasses of the dead livestock.

"That has been a really shocking story," Ms Stattersfield said.

"Four years ago, we were not even sure what was responsible for the dramatic declines. It happened so suddenly, people were not prepared for it.

"Since then, the basis for the decline is well understood and measures are being taken to remove diclofenac from veterinary use in India.

"However, it is still available for sale and there still needs to be a lot more work to communicate the problem at a local level.

"But it demonstrates that we can get to the bottom of the reasons behind declines."

The plight of albatrosses becoming entangled in long-line fishing tackle has also been the subject of sustained campaigning, attracting high-profile supporters such as Prince Charles and yachtswoman Dame Ellen MacArthur.

About 100,000 of the slow-breeding birds are estimated to drown each year as a result of being caught on the lines' fish hooks.

But fisheries in a growing number of regions are now introducing measures to minimise the risk to albatrosses.

Ms Stattersfield said these examples showed that concerted effort could investigate and identify what was adversely affecting bird populations.

But she quickly added that prevention was always better than finding a cure.

"We don't want to have to react to problems that come about from bad practice.

"What we are trying to do with this report is to be as clear as possible about what are the underlying causes, and then present a range of conservation measures that can preserve birds and biodiversity."

BirdLife International will use the report, which is being published at its week-long World Conference in Buenos Aires and on the group's website, to call for governments to make more funds available for global conservation.

"Effective biodiversity conservation is easily affordable, requiring relatively trivial sums at the scale of the global economy," said Dr Mike Rands, BirdLife's chief executive.

He estimated that safeguarding 90% of Africa's biodiversity would cost less than US $1bn (ยฃ500m) a year.

"The world is failing in its 2010 pledge to achieve a significant reduction in the current rate of loss of biodiversity," he warned.

"The challenge is to harness international biodiversity commitments and that concrete actions are taken now."