rDNA repeat units (rDNA RU) are an extremely important part of any genome. rDNA RU arrays form nucleolus organizing regions (NOR) in chromosomes, and the functional status of NOR is an indicator of cell and tissue physiological status. Each rDNA RU consists of pre-rRNA gene transcribing in a single pre-rRNA molecule and intergenic spacer (IGS) which plays a key role in pre-rRNA transcription regulation. Despite recent animal genome-wide studies structural organization of rDNA RUs and in particular IGS in the majority of vertebrate is still underinvestigated. The lack of data on avian IGS sequence makes difficult studying NOR regulation mechanisms and rDNA evolution in birds.