Song structure and singing behaviour in two nesting populations of Willow Warbler Phylloscopus trochilus acredula of different density were studied in Moscow and Kostroma Regions, Russia, from 1994-1997. In total 520 songs of 9 males from the low-density population and 566 songs of 12 males from the high density population were analysed. Singing periods in the high density Willow Warbler population (11 males, 758 min.) were longer (58% of the time) than in the low density population (13 males, 866 min.) (58%). In the high density population, 51% of singing time consisted of “singing duels” (singing in gaps between the songs of a neighbouring male, and singing during the singing of a neighbouring male), while in the low density population only 7% did. The initial phrases of Willow Warbler songs were significantly longer in the high density population, with up to 9-13 elements, while there were no occurrences of this length in the low density population. Songs as a whole were longer and more variable in the high density population. Thus changes in structure, singing activity and behaviour are linked to male-male interaction and density of the nesting community.
song structure, population density, Phylloscopus trochilus, singing behaviour