Certain peculiarities make Saaremaa's bird life quite unique as compared with Estonian mainland. Many species, which are common breeders in mainland, are qualified as rare (Acrocephalus dumetorum, Alcedo atthis, Strix uralensis), remarkably non-numerous (Ciconia ciconia, Corvus frugilegus, Passer montanus, Sitta europaea) or missing (Aquila pomarina, Aquila chrysaëtos, Bonasa bonasia, Tetrao urogallus, Numenius phaeopus, Gallinago media) in Saaremaa. At the same time, several seabird species, which are rare or non-numerous in mainland, are common here (for example, Anser anser). The only stable population of Anthus petrosus breeds in western Saaremaa. Nucifraga caryocatactes is remarkably numerous here. Several species which belong to the Estonian rarities list, have been met only in Saaremaa (Gavia immer, Aix galericulata, Glareola pratincola, Phalaropus tricolor, Chlidonias hybridus, Alle alle, Monticola saxatilis andPhylloscopus fuscatus).