2D and 3D 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. Only recently, studies on three-dimensional (3D) space use appeared with modelling of individual movements simultaneously in three dimensions. In ornithology, such works were done when studying home ranges of large birds tagged with radio transmitters and GPS locators or during visual observations of colour-banded passerine wintering birds and residents in tropics. To our knowledge, such methods have not been 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, NW Russia. Using the method of Cooper et al. (2014), we located points with 3D coordinates at which individually marked males sang and analysed 2D and 3D territories and their overlap in R. We found 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 can partition their territorial space in 3 dimensions. With the overlap indices adopted for use with 3D data, we also proved that they could avoid areas of overlap. Furthermore, it was demonstrated for the first time that the overestimation of the territory overlaps by 2D versus 3D methods grew higher as the number of neighbour territories increased. In dense populations, 3D territories are arranged like layers of roof tiles, avoiding substantial overlaps.