This paper presents an analysis of the semantics of bird denominations in Mari: an attempt is made to define the factors, or features, motivating bird denominations.Analysis is based on a set of words of inner origin that are part of the corpus of bird names compiled by the author. The results show that the ornithonomy of the Mari language, created over centuries, constitutes a well-shaped system. It reflects a variety of features associated with the appearance, way of life of the birds, sounds they produce, etc. Many bird terms reflect features of appearance. It is interesting to note that the names of birds not seen for some reason may relate to the characteristics of the birds’ voices. In some cases, terms are based on a combination of features. In dialects, different names for same birds may occur, as observed in the sources.