The occurrence of extra-pair paternity (EPP) in populations of socially monogamous animals has become a significant focus in many ethological research. Among avian species, one of the earliest predominantly socially monogamous species where EPP was documented is the Willow Warbler Phylloscopus trochilus (Linnaeus 1758). Nevertheless, comprehensive data on EPP across the species’ range are limited. Studies of Willow Warblers using species-specific microsatellite loci (Phtr1, Phtr2, and Phtr3), conducted in 2015 on the southern Kola Peninsula, revealed a high EPP frequency of 37.5%, with extra-pair young (EPY) present in 83.3% of nests. Studies from other regions of European Russia (Karelia, Pskov region, and Mordovia) reported lower EPP frequencies (12–18%) and EPY occurrences (67–75% of nests). These findings suggested a hypothesis that EPP rates increase toward the peripheral regions of the species’ range. However, subsequent studies in 2023, using the same methodology on the same Kola Peninsula population, unexpectedly documented a low EPP rate of only 2.9%, with EPY found in only three of the 17 families surveyed. Thus, the 2023 EPP rate on the Kola Peninsula was the lowest reported for Willow Warbler populations in European Russia. These results highlight the potential for considerable interannual and regional variation in EPP rates within a single species, underscoring the need for caution when drawing conclusions based on short-term data and the importance of long-term studies across diverse regions.