Almost half of the world's seabirds have populations that are thought to be in decline, according to a new review.

The study, published in Bird Conservation International, found that 28% of species are considered to be in the highest categories of risk.

Conservationists are particularly concerned by the albatross family.

Threats to the birds include commercial fishing and damage to breeding colonies caused by rats and other invasive species.

Seabirds make up just a small proportion (3.5%) of the world's bird species. But researchers say they are an important indicator of the health of the oceans.

The review, carried out by BirdLife International, found that of 346 species, 47% are known or suspected to be in decline.

It says that seabirds are now more threatened than any other group of birds.

Prof John Croxall, Chair of BirdLife's Global Seabird Programme, told the BBC: "They are top predators in their marine systems. The fact that almost a third are globally threatened should really be telling us something about how we need to look after where they occur to breed on land and where they go to feed in the ocean."

BirdLife assesses the threat status of seabirds on behalf of International Union for Conservation of Nature (IUCN), which compiles the internationally-recognisedRed List. On that list, 5% of seabirds are in the highest category of Critically Endangered.

Image caption Critically endangered Balearic Shearwater

One of those is the Balearic shearwater, which can be found in UK waters in the summer.

The review also found that 17 out of 22 species of the albatross family are threatened with extinction.

Conservationists say commercial fishing is one of the key threats to seabirds with large numbers killed as a bycatch in nets and on lines.

Another is the impact that invasive species such as rats and feral cats have on breeding colonies, either damaging habitats or eating chicks and eggs.

Some of the most important breeding colonies are on remote islands in UK overseas territories. Last year an Royal Society for the Protection of Birds (RSPB) project, part-funded by the UK Government, carried out a programme to eradicate rats on Henderson Island in the South Pacific.

Helicopters guided by GPS dropped rat poison pellets on the island, which is the only known breeding site of the endangered Henderson petrel.

Grahame Madge of the RSPB said: "Without projects like this, these seabirds would have a finite life. On Henderson Island the rats were bringing birds towards extinction."

A pilot study is being carried out to look at the feasibility of removing mice from another important breeding site, Gough Island in the South Atlantic.