Thieves have stolen a priceless collection of tropical birds from the Natural History Museum.
Curators said almost 300 brightly-coloured specimens were taken from a collection in Tring, Hertfordshire.
They said the birds, some of which are more than a century old, are a priceless part of the world's ornithological heritage.
Detectives said the artefacts may have been stolen to order for a collector or for their valuable plumage.
They fear the irreplaceable birds may be ripped apart for use as fishing lures, in dressmaking or costume jewellery.
Detective Inspector Fraser Wylie, who is leading the inquiry, said the birds were clearly deliberately targeted, possibly by an expert.
He said: "This is a very unusual crime and we are keen to recover the bird skins, which are part of our national heritage.
"Some of these may be irreplaceable and have been part of our UK heritage for years. People have studied these for generations.
"We are appealing for anyone who may have seen any suspicious activity around the museum at the time of the break-in.
"Also, we would ask any collectors of such specimens to keep a watchful eye out in case they are offered anything resembling them."
Professor Richard Lane, director of science at the Natural History Museum, said his staff were extremely upset by the theft.
He said the birds play a key role in studying the history of their species and may prove impossible to replace.
Prof Lane added: "It is quite hard for us to express just how tragic this is to members of the museum. This is the nation's collection.
"These birds are extremely scarce: they are scarce in collections and even more scarce in the wild.
"Our utmost priority is working with the police to return these specimens to the national collections so that they can be used by future generations of scientists."
A break-in at the ornithological collection in Akeman Street, Tring, was discovered on 24 June.
At first museum bosses were relieved to discover none of their most valuable birds had been taken from the private research area.
More than 8,000 "specimen types", perfect examples of bird species, and finches collected by Charles Darwin in the Galapagos islands were untouched.
But several days later an academic making a routine inquiry discovered several brightly-coloured bird skins were missing.
Experts then spent several weeks examining the full extent of the theft as they checked more than 500 cabinets over three floors.
Eventually they realised 299 birds had been stolen from drawers in three cabinets.
The thieves targeted Quetzal and Cotinga birds, from central and south America, and Birds of Paradise, from Papua New Guinea.
Birds of Paradise featured prominently in the recent BBC Planet Earth series narrated by David Attenborough.
They are known for clearing spaces in tropical forests and putting on extremely elaborate displays to attract a mate.
All of the missing tropical birds had brightly-coloured plumage, while brown-feathered female specimens were left behind.
Police believe whoever is responsible for the theft had detailed knowledge of the missing birds.
The cabinets were labelled with Latin species names organised in evolutionary order. Only a small number among the vast collection were disturbed.
Mr Wylie said it is possible the birds were taken over a longer period of time before the break-in was discovered.
He said 299 birds would fill up to six bin bags and some specimens had one metre-long tail feathers.
The senior officer said: "Clearly there are collectors out there who may want these species.
"There may also be a need within the fishing market because of the nature of the features and colour of feathers these birds display.
"There is also a theory they could be used in dresses and jewellery. There are markets out there for them and we keep an open mind."
The missing birds were all preserved "skins" from which internal organs and eyes have been removed.
Researchers study feather patterns and other features of the specimens to learn more about how they have evolved.
Prof Lane said DNA samples can also be recovered for genetic analysis.
The Natural History Museum holds 70 million specimens brought together over 350 years. The majority are held at its South Kensington headquarters.
The ornithological collection in Tring is one of the world's largest and holds 750,000 birds representing 95% of known species.
* Anyone with information should contact Mr Wylie on 0845 3300222 or Crimestoppers, anonymously, on 0800 555 111.Reuse content