On the territory of the Kaliningrad region (on the coast of the Curonian Lagoon) investigated the effect of one of Europe's largest breeding colonies of Great Cormorants (Phalacrocorax carbo L.) on the composition and structure of forest ornithocenosis during the nesting period. Cormorants colonization of forest area on the coast of the Curonian Lagoon, led to the replacement of a mature and diverse community of birds on its easy-depleted version with total dominance of a single species, the founder of the colony. In the direction of the forest area outside the colony cormorant to populated part of the colony and then to the abandoned old part of it, the number of breeding species is reduced from 39 to 31. The indices of species richness are also having the minimum values ​​in the main part of the colony of cormorant forest area. The area, densely populated by cormorants, formed a set of extreme conditions for most of birds originally lived on the territory. For the area of ​​the abandoned colony during regenerative successions distribution of the species richness in the community is changed to decrease the degree of dominance and increasing the proportion of a few species.