Two- and three-dimensional territories and territory overlap of Willow Warblers Phylloscopus trochilus in Arctic forests

Space use by animals is usually considered in two horizontal dimensions, less often with a separate analysis of the vertical stratification. Studies on three-dimensional (3D) utilisation of space, with individual movements modelled simultaneously in three dimensions, appeared only recently. In ornithology, such approaches were applied to study home ranges of large birds tagged with radio and GPS transmitters or during visual observations of colour-banded passerine wintering birds and residents in the tropics, but to our knowledge, they were never used for studying 3D territories advertised by singing in passerines during the breeding period in the Palearctic Region. We studied the 3D territoriality of Willow Warblers Phylloscopus trochilus during the breeding period in the Kola Peninsula, Arctic Region, Northwest Russia. Using the method of Cooper et al. (Auk 131:681-693, 2014), we located points at which individually marked males sang using 3D coordinates and analysed 2D and 3D territories and their overlap. We found out that 2D methods, compared to 3D methods, overestimated individual spatial overlap by 8.5% and total spatial overlap by 16.7%. Thus, Willow Warblers in Arctic forests could partition their territorial space in three dimensions. In addition, using the overlap indices adopted for 3D data, we showed that birds could avoid the areas where territory overlap did occur. Furthermore, we demonstrated for the first time that an overestimation of the territory overlaps by 2D versus 3D methods grew higher as the number of neighbour territories increased.

Automated summary

This study investigated the three-dimensional utilization of space and territory advertisement behavior in Arctic forests. The results showed that expanding territory analysis into three dimensions could provide a more accurate estimation of territory overlap compared to two-dimensional methods, and the study also revealed the avoidance behavior of birds in overlapping territories.