Avifaunas of three large cities (Hamburg, Berlin and Warsaw) situated in the northern lowlands of Central Europe along an 850-km-long west-east line were compared. Estimates of several species' breeding populations in these cities were used to calculate their densities in the total area of breeding habitat in each city used by the species in question. The relationship of the densities of several common species was compared. Of 39 species analysed, 18 showed higher densities in Hamburg, 4 in Warsaw and 2 in Berlin. A gradient of increasing density from Warsaw to Berlin and Hamburg (Western gradient) was found for 16 species: Columba palumbus, Troglodytes troglodytes, Prunella modularis, Turdus merula, T. philomelos, T. viscivorus, Sylvia atricapilla, Phylloscopus collybita, Ph. trochilus, Regulus regulus, R. ignicapillus, Garrulus glandarius, Parus cristatus, Certhia brachydactyla, Fringilla coelebs, Pyrrhula pyrrhula. An opposite (Eastern) gradient was found for only 3 species: Columba livia f. domestica, Oriolus oriolus and Passer domesticus. Suggested causes for most of the larger differences among the three cities in the densities of particular species are the variation in the richness of vegetation (higher in both German cities) and differences in the stage of synurbization (i.e. adaptation to the urban environment) of specific avian species. In a few cases, factors such as the geographic range of a species, the attitude of humans (in the case of the Feral Pigeon), and specific, slight variations in habitat are put forward to explain the differences found.
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