Four strains identified as West Nile fever virus by inhibited hemagglutination and neutralization tests, enzyme immunoassay, and reverse transcription polymerase chain reaction were isolated during a virological examination of birds and their collected ticks in the natural and synanthropic biocenoses of the Volga delta. The strains were isolated from the great cormorant (Phalacrocorax carbo), the crow (Corvus corone) and its collected Hyalomma marginatum nymphs. The types of interpopulational relations in the ecological system wild-birds-virus-mosquitoes-synanthroic birds-ticks are discussed.