A total of 121 areas in Iceland have been defined as internationally important for the 81 bird species that breed in Iceland or are regular visitors here. Criteria used to define important bird areas (IBAs) in Iceland come from BirdLife International. The IBA criteria are mainly based on quantitative population estimates, i.e., how many birds use a specific site in Iceland, and how large a percentage of the total population this represents. For some bird species, detailed data exist on both the numbers of birds at the site and the overall population size. For many other species, however, assessment is based on limited and often decades-old data.
The 121 areas in Iceland designated as internationally important sites for birdlife are classified into three categories: (a) seabird colonies, (b) rocky coast and shallow marine waters, and (c) wetlands and other inland areas. Several IBAs fall into two or even all three categories. Such sites are discussed in each relevant category, in order to provide a better overview of comparable sites. An example of this is the bay of Breiðafjörður as a seabird colony and Breiðafjörður under the category of rocky coast and shallow marine waters.
Most IBAs in Iceland are important as breeding sites. Fewer have international importance as staging sites, wintering sites, or moulting sites. Several Icelandic IBAs serve all four functions, including Lake Mývatn and Breiðafjörður.
The number of IBAs designated for each bird species varies greatly. Among breeding birds, the northern fulmar (Fulmarus glacialis) tops the list, with 38 colonies designated as IBAs. The red knot (Calidris canutus) has 8 important staging areas in Iceland and is the passage migrant with the largest number of IBAs in Iceland. No areas in Iceland have been defined as IBAs for 40 bird species. No international IBA criteria have been established for 21 of these species. The remaining 19 are so uncommon in Iceland compared to other geographic locations that no Icelandic sites are considered to be of international importance for them.
The boundaries of many areas are rather loosely delineated. For species with a wide breeding range, such as heathland birds and birds of prey, protected areas must inevitably be large if they are to encompass a significant part of the population in question. Seabird colonies, on the other hand, tend to be easily delimited.
The IINH has published a research report on IBAs in Iceland: Mikilvæg fuglasvæði á Íslandi (Fjölrit Náttúrufræðistofnunar No. 55, in Icelandic with English summary, PDF format). This report contains a more detailed breakdown of the data and methodology used in the selection process, discussion of all relevant bird species and groups, and an assessment of prior knowledge. It also contains an overview of all areas in Iceland designated as internationally important for one or more bird species.