A US citizen has been arrested in connection with the theft of 299 rare bird skins from the Natural History Museum at Tring in Hertfordshire.
Police were called to a break-in at the museum in Akeman Street, which belongs to the better-known London museum, on 24 June 2009.
Police said 299 brightly-coloured stuffed birds were missing, believed stolen, from a collections area.
A Hertfordshire Police spokesman said a man, aged 22, had now been arrested.
"Police have recovered the majority of the bird skins," he added.
Speaking at the time, Professor Richard Lane, director of science at the museum, said the birds formed part of the collection assembled over the past 350 years.
He said the items were of scientific interest, and many were irreplaceable and "literally priceless".
There are some 750,000 bird skins, representing 95% of known living species, held at the museum.
Some of them appear on the Red List of endangered species.
Professor Lane said the ornithological collections were used by researchers throughout the world, who either visit Tring or request loans.
He said: "The knowledge gleaned from these collections can help protect endangered species and answer questions about the biodiversity of the world around us.
"It is very distressing that we should have been deliberately targeted in this manner."