Birds' egg thieves are being forced to ply their trade abroad following a series of successful prosecutions by police and wildlife investigators.

According to the RSPB and the police, the number of illegal collectors has reduced dramatically since prison terms were introduced for the offence.

They believe collectors, including those operating in the North-East and North Yorkshire, may now be working in mainland Europe.

The Countryside and Rights of Way Act 2000 allowed courts to impose a jail sentence of up to six months.

Five years ago, the Northumbria Police area saw the first collector jailed under the new legislation. The man from Blyth, in Northumberland, was sentenced to four months in prison by magistrates. He admitted possessing goshawk and goosander eggs, and possessing, with intent to commit an offence, a tin with compartments for eggs.

Northumbria Police wildlife liaison officer PC Paul Henery said: "Since courts have been able to send egg collectors to prison, the number of rare birds being destroyed by these obsessive criminals has decreased.

"The message is clear; effective investigation leading to prison is a deterrent.

"This doesn't mean egg collecting has ceased, only that offenders are committing offences abroad or using other means to avoid detection.

"Police forces need to be even more vigilant because species of bird on the brink of extinction can be pushed over the edge in one season.

"We cannot allow these selfish, obsessive criminals to limit, or destroy our fabulous natural heritage."

Guy Shorrock, national senior investigative officer for the RSPB, said: "You can lecture people about the immorality of what they do all the time, but the biggest deterrent is being caught.

"There has certainly been an increased effort. There has been more enforcement, prosecutions have been brought and there are less egg collectors operating."

He believes that, because hardcore collectors are now so well-known to law enforcement agencies in the UK, operating in Spain, Portugal, Eastern Europe and Scandinavia is more attractive to them.

Mr Shorrock said: "We are getting less reports of egg collectors nationally each year.

"For us, there are greater concerns, like the shooting of birds of prey, and we want egg collecting to stop so we can get on with them.