Mark Thomas (left) and Guy Shorrock keep watch on Britain’s egg obsessives. “These are not normal criminals,” Shorrock says. Photographs by Richard Barnes

On the afternoon of May 31, 2011, Charlie Everitt, an investigator for the National Wildlife Crime Unit in Edinburgh, Scotland, received an urgent call from a colleague in the Northern Constabulary, the regional police department whose jurisdiction includes the islands off the country’s western coast. The officer told Everitt that a nature-reserve warden on the Isle of Rum, twenty miles offshore, had reported seeing a man “dancing about” in a gull colony. Everitt looked at the clock. It was 4 P.M., too late to catch the last ferry, so he drove halfway to Mallaig, a tiny port town four hours away, where he could take the first boat out in the morning.

The Isle of Rum, a forty-one-square-mile rock, is inhabited by about forty people, many of them employed by the Scottish Natural Heritage, an environmental organization charged with protecting wildlife. Rum has red deer and an assortment of rare birds, including merlins and white-tailed sea eagles, and it is a principal breeding ground for the Manx shearwater, a seabird with a distinctly eerie call. For this reason, as Everitt knew, the place was also a target for egg collectors, a secretive network of men obsessed with accumulating and cataloguing the eggs of rare birds.

The ferry ride the next morning was choppy; clouds hung almost to the water. Everitt, a moonfaced man of forty-six, wondered what kind of day lay ahead. In Victorian times, egg collecting in England was the quaint province of natural historians, but, as laws protecting endangered birds were passed, the activity became a criminal act. Collectors had gone underground; some communicated with one another using code numbers as aliases. In his ten years on the job, Everitt had never encountered a collector.

Halfway across the sound, Everitt was contacted by radio. It was the Northern Constabulary: the suspect was on the jetty. As Everitt disembarked, he saw a rangy man dressed in camouflage leaning against a bulging rucksack. He looked to be in his late forties, with sunken brown eyes. Everitt approached and asked his name. “As soon as he said it, I thought, We’ve found our person.”

The man was Matthew Gonshaw, the most notorious egg collector in Britain. An unemployed Londoner, Gonshaw had already served three prison terms on egg-collecting charges. When he was last apprehended, in 2004, investigators had seized nearly six hundred eggs, a hundred and four of them hidden inside a secret compartment in his bed frame.

There is no police station on Rum, so Everitt took Gonshaw to the Scottish Natural Heritage office, where Gonshaw consented to a search of his rucksack. It held several small syringes, which collectors use to forcibly blow out the contents of eggs; topographical maps of the area; a loop of rope; and a military survival guide. Everitt had also noticed shredded newspaper sticking out of some food containers. Inside them were twenty eggs, including eight of the Manx shearwater.

Gonshaw refused to answer questions; he knew the police would raid his apartment and asked that they get the key from the landlord instead of breaking down the door. Everitt called his Edinburgh office, which informed the Metropolitan Police, in London. Within hours, a joint special-forces team from the police and the Royal Society for the Protection of Birds, the U.K.’s largest conservation organization, were preparing to search Gonshaw’s flat. Mark Thomas, a senior investigator for the R.S.P.B. who had become a minor celebrity on the environmental-crime circuit for his work on egg-collecting cases, told me recently that, when he got the call, “I put down the phone and literally ran to my car.”

Most of the bird eggs collected by Britain’s preëminent natural historians are housed at the Natural History Museum at Tring, forty-five minutes north of London. The Tring museum has the largest zoological collection ever amassed by a single person and is situated on the former estate of its founder, Lord Walter Rothschild, the banker and zoologist, who was famous for driving a zebra-drawn carriage. Rothschild died in 1937; more than two hundred animal species, including a worm, bear his name.

Today, the museum at Tring comprises six galleries, housing thousands of taxidermy specimens, among them crocodiles, domestic dogs, wild asses, and every known species of zebra, including the now extinct quagga, the last of which died in Amsterdam in 1883. It is also home to the largest bird-egg collection in the world, with around two million specimens, which has made it a leading institution for researchers. In the nineteen-sixties, the collection figured prominently in a study by the conservationist Derek Ratcliffe, who wanted to know why peregrine falcons were failing to produce offspring. By comparing recent eggs with older ones at the museum, Ratcliffe discovered that the shells had become perilously thin, likely owing to pesticides that the birds were eating. The attention led to a Europe-wide ban on DDT.

Oology—the study of eggs—is “one of the most exciting areas of ornithology and, in many respects, one of the least known,” Douglas Russell, the curator of the egg collection at Tring, told me. But Russell, a small, serious man with an orange goatee, was not eager to show me the collection. It has been, he said, “completely locked down” since 1979, after Mervyn Shorthouse, a regular visitor posing as a wheelchair-bound invalid, stole ten thousand eggs in a three-year period. I’d been directed to a side entrance next to a dumpster, where a guard took my bag and identification and escorted me to Russell, who held my passport to my face and gave me a hard stare. “That will be kept on file for five years,” he said, and led me down an echoing corridor to a photocopier, where he scanned my documents.

The museum’s eggs are kept in an acrid basement filled with row after row of temperature-controlled steel cabinets, some arranged by collector. Russell grabbed a key off his belt loop and took me to one of the cabinets, which opened like a bank vault. Inside a glass-covered drawer, like rare jewels, were groups of small eggs, speckled blue, beige, and gray, each with a small round hole where its contents had been extracted. “These are Stuart Baker’s cuckoos,” Russell said. “Look how much variety there is.” I followed him to another cabinet; inside was a single white egg from a Samoan wood rail, collected in 1873. Russell paused to wipe his eyes. “That is the only egg there ever will be,” he said. “Because it’s extinct. But at least we have that one window.”

At the turn of the twentieth century, as the conservation movement began raising awareness of endangered species, the collecting of wild-bird eggs came under scrutiny. In 1922 in London, Earl Buxton, addressing the annual meeting of the Royal Society for the Protection of Birds, warned of the “distinct menace” posed by egg-collecting members of the British Ornithologists’ Union, of which Lord Rothschild was a member. Indignant, Rothschild split off and, with the Reverend Francis Charles Robert Jourdain, a cantankerous Oxford-educated ornithologist who bore a scar across his forehead from falling off a cliff in search of an eagle’s nest, formed the British Oological Association. The group, which renamed itself the Jourdain Society after Jourdain died, in 1940, proclaimed that it was the only organization in the country dedicated to egg collecting.

It has not fared well. In 1954, the Protection of Birds Act outlawed the taking of most wild-bird eggs in the U.K. In 1981, some ninety species were declared Schedule 1; possession of their eggs, unless they were taken before 1954, is a crime. Meetings of the Jourdain Society, to which members wore formal attire and carried display cabinets full of eggs, became the target of spectacular raids and stings. By the nineteen-nineties, more than half of Jourdain Society members had egg-collecting convictions, according to the R.S.P.B. One member recently agreed to a radio interview only after insuring that his voice would be disguised.