The terrestrial elements of the St Kilda archipelago are recognised as globally significant for their seabird colonies. The revised nomination of the St Kilda site seeks to extend the World Heritage Site boundaries into the surrounding sea areas in recognition of their importance to these colonies, and with the intention of adding significantly to the outstanding universal value of the site as a whole.

St Kilda is for the most part, deserted in the winter months, but 17 species of seabird come ashore in spring and summer to breed on St Kilda, rendering the archipelago the largest seabird colony in Great Britain and Ireland. Including non-breeding individuals, about one million seabirds populate the sea, land and air at this time. The archipelago is set in a pristine marine environment and is a seabird sanctuary without parallel in the north-east Atlantic.

The internationally accepted criterion for a breeding bird aggregation to merit importance, whether it be in a regional, national, biogeographical or international context is that at least 1% of the relevant population be represented in the aggregation. More than half of the seabird species breeding on St Kilda occur in nationally (in a UK context) important numbers. In the wider global context, however, the St Kilda seabirds assume exceptional biological significance. Populations of seven species of seabird breeding on the archipelago qualify as biogeographically important, in the context of the north-east Atlantic Ocean and the associated coastal fringe. Of these, three are important on a world–wide scale.

  Northern fulmar
(Fulmarus glacialis)
St Kilda hosts the oldest known colony in the eastern Atlantic, and is now the largest northern fulmar colony in Great Britain and Ireland, comprising 66,942 apparently occupied nest sites in 1999 – 3.69% of the north-east Atlantic population.

Northern Fulmar   Great skua
(Catharacta skua)
The great skua breeds only in Iceland, the Faroe Islands, Norway and Great Britain. The breeding population of 169 apparently occupied territories (AOTs) on St Kilda is of biogeographical and global importance, representing 1.38% of the north-east Atlantic population and more than 1% of the world population.

Great skua  
  Manx shearwater Manx shearwater
(Puffinus puffinus)
The mostly nocturnal Manx shearwater breeds on the slopes of St Kilda in important numbers, the population comprising 4,803 apparently occupied burrows (AOBs) –1.26% of the north-east Atlantic population

  Common guillemot Common guillemot
(Uria aalge)
The common guillemot also breeds in biogeographically important numbers on the cliffs of St Kilda comprising 23,378 individuals – 1.17% of the north-east Atlantic population.

  Leach’s storm-petrel
(Oceanodroma leucorhoa)
St Kilda is especially important as the major breeding station of Leach’s storm-petrel in the north-east Atlantic. One of only nine colonies in this region, the islands host 45,433 AOBs –89.29% of the biogeographical breeding population.

Leach's strom-pertrel   Atlantic puffin
(Fratercula arctica)
The Atlantic puffin, widely distributed across the north Atlantic, is the most numerous species of seabird on St Kilda. The 135,752 AOBs on St Kilda represent 4.41% of all Atlantic puffins breeding in the north-east Atlantic. This also represents more than 2% of the world population.

Atlantic puffin  
  Northern Gannet Northern gannet
(Morus bassanus)
Perhaps the most conspicuous of St Kilda’s seabirds, there are 60,428 breeding pairs representing 23.64% of the north-east Atlantic population. In addition, this represents almost 20% of the total world population. Only 44 gannet colonies occur in the world and St Kilda is by far the largest, about half as large again as the second largest colony on Bass Rock, off the east coast of Scotland.

  Razorbill Razorbill
(Alca torda)
The razorbill breeds in nationally important numbers on St Kilda; recent surveys indicate that about 1% of the north-east Atlantic population breeds here.