US citizen Edwin Rist, 22, pleaded guilty to burglary and money-laundering offences in November last year in connection with the theft of 299 bird skins from the Natural History Museum in Tring.
Officers were called to the museum, which belongs to the better-known London museum, after reports of a break-in on June 24, 2009, Hertfordshire Police previously said.
It was later found that 299 brightly-coloured bird skins were missing, believed stolen, from a collections area.
The missing birds formed part of the nation’s natural history collection, assembled over the past 350 years.
A Hertfordshire Police spokeswoman said previously: “The 70 million specimens looked after by the Natural History Museum are a resource of international importance in the development of scientific knowledge.
“The ornithological collections are amongst the most heavily used and are consulted by researchers throughout the world, who either visit Tring or request loans.
“The knowledge gleaned from these collections helps protect endangered species and answer questions about the biodiversity of the world around us.
“The stolen birds are a number of brightly-coloured tropical birds, including cotingas, quetzals and birds of paradise, some of which are endangered species, irreplaceable and, therefore, of especial scientific concern. The stolen specimens are therefore priceless. Police have recovered the majority of the bird skins, although some are still outstanding.”
Rist will be sentenced at St Albans Crown Court on Friday.