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Ecological patterns in the formation of the avifauna and bird populations of the Iturup Island were analyzed. Route counts were used along transects of unlimited width. As many as 116 bird species were registered, including 36% revealed to be ubiquitous, another 34% local and further 26% highly local. 71 species were shown to nest. The taxonomic structure of the avifauna, represented by species from 15 orders, corresponded to the zonal and landscape features of the island’s territories located at the eastern margins of northern Eurasia. Passeriformes (37%), Charadriiformes (22%), Anseriformes (9%), Procellariiformes (8%) prevailed. The zoogeographic peculiarity of the local avifauna was a combination of elements of the Far Eastern island, Pacific, Siberian and Chinese faunistic complexes, some Siberian-American and widespread species, as well as Japanese island endemics. The avifauna around the Iturup Island, formed by a system of general zonal-landscape and altitudinal-belt patterns, united ecological groups of marine and land species, including montane ones (n = 9). Local avifauna in coastal marine habitats included 41 species, vs 55 in forests, 37 in light forests, and 46 in tall grasses, the population density being 607, 785, 968, 518 individuals/km², respectively. The similarity coefficients for the populations of forest habitats were 19–48%, vs 22% for light forests, 20–37% for tall grasses, while in the sea coast and adjacent water areas they amounted to 21–52%. The populations of terrestrial habitats were dominated by the Oriental Turtle-dove, the Pacific Swift, the Siberian Rubythroat, the Japanese Bush-warbler, the Marsh Tit, the Coal Tit, the Eurasian Nutcracker, the Oriental Greenfinch, the Eurasian Siskin, the Long-tailed Rosefinch, the Masked Bunting, and the Grey Bunting. The populations of the coast and adjacent sea area were dominated by the Japanese Cormorant, the Rufous-necked Stint, the Slaty-backed Gull, the Black-tailed Gull, and the Black-backed Wagtail. In open sea areas, the most common were the White-winged Scoter, the Black-footed and Laysan albatrosses, the Northern Fulmar, and the Short-tailed Shearwater.