By Sallie George
BBC News, England
The theft of eggs belonging to some of Britain's rarest birds is deemed so serious that in some areas, nests are guarded around the clock during the breeding season.
The RSPB believes the bulk of Wheal's collection remains hidden
The RSPB said thieves such as Gregory Wheal were motivated by an overwhelming addiction to the thrill of stealing and possessing rare eggs, with their collections becoming a trophy of their endeavours in the field.
Such is the extent of the compulsion that collectors will risk their lives to satisfy their desire for eggs.
Mark Thomas, investigations officer for the RSPB, said: "It becomes a complete obsession where collectors will risk absolutely everything. They physically can't help themselves.
"We know of two collectors who have fallen to their deaths."
The organisation believes there are currently about 50 egg thieves active in the UK.
Wheal is not the country's most prolific, but he is the one who has been caught and convicted the most times, the RSPB said.
He has now notched up nine convictions for egg theft, dating from 1987, and has been jailed once before, for four months in January 2006.
Wheal is described as the most convicted egg collector in the UK
The RSPB is convinced Wheal's main collection of stolen eggs remains undetected, possibly hidden away in a lock-up hundreds of miles from home - a common practice among ardent collectors.
Mark Thomas, investigations officer for the RSPB, said: "In all of this time we haven't found a substantial collection of eggs, which makes us believe he had got a big collection somewhere hidden away.
"There is no doubt he is prolific, he has been active for a considerable time targeting rare birds and nests which we would spend quite a lot of time and money guarding.
"He would think nothing of travelling vast distances to get to very rare birds' nests."
Collectors 'turned in'
Collecting eggs from wild birds' nests has been illegal since 1954, but since 2001 the offence has carried a maximum six months' jail sentence.
Since then, the number of active egg collectors has dramatically dropped, the RSPB said.
Mr Thomas said: "If you go back 20 years or so there would have been about 200 of these individuals, but now the law has changed.
"About 12 or so individuals have gone to jail for these sorts of offences and that has served as a very good deterrent."
But despite the threat of jail, some collectors find themselves unable to stop stealing from rare birds' nests.
Mr Thomas said: "During the winter months there are no eggs being laid - they will be at home, thinking about the breeding season.
"The thrill is still there, and as soon as the first birds start to nest they have got to go out and do it again.
"These are people who are absolutely obsessed with birds, but whereas a bird watcher will go out and write down their findings, an egg collector has to take something away.
"I have raided houses before and found a drawer full of eggs from one species of bird - but in every shade. They become completely obsessed with minute detail.
"I have been in cases where we have caught an egg collector in the field, and when it comes to interviewing them, they are in floods of tears, they realise it has got completely out of control.
"This is why, a lot of the time, we have collectors' families informing us of what is happening.
"We get wives and girlfriends ringing up to say their partners have got a lock-up somewhere full of eggs.
"At the end of the day their lives are being ruled by a collection of shells."
Meticulous planning goes into the egg theft operation.
Collectors will undertake painstaking research, planning their trips weeks in advance, marking maps and travelling hundreds of miles.
Once the egg has been stolen, the collector may then bury it somewhere to avoid being caught in possession of it while travelling home.
Collectors then return to the site when the breeding season has ended and dig up their find, before putting it into storage.
Some nests, such as that of the Osprey in Loch Garton in Scotland, are deemed so at risk, they are guarded 24 hours a day during the eight-week breeding season.
Mr Thomas said: "The theft of eggs has a very significant impact and poses a very serious threat to many of Britain's rarest birds."