Spatial association between forest heterogeneity and breeding territory boundaries of two forest songbirds

Human activities and natural disturbances create spatial heterogeneity within forested landscapes, leading to both sharp and gradual boundaries in vegetation and abiotic attributes, such as rocks. Those boundaries may affect the detailed delineation of avian territories (independently of their general location), but their role is largely unknown. We tested, using a spatial analysis approach, whether spatial heterogeneity of vegetation and abiotic attributes were associated with territory boundaries of ten black-throated blue warblers (Dendroica caerulescens) and 14 ovenbirds (Seiurus aurocapillus). The study was conducted during summer 1999 in a mature deciduous forest near Québec City, Canada. Singing males were mapped from repeated surveys at 756 points, 25 m apart, on a 49 ha grid. Spatial heterogeneity was obtained from 27 attributes measured at each point. Boundaries of bird territories, vegetation, and abiotic attributes were delineated using the lattice-wombling boundary detection algorithm. The spatial association between territory and microhabitat boundaries was computed using the spatial overlap statistics. There was significant spatial overlap between territory boundaries and those of 15 and 17 attributes for black-throated blue warbler and ovenbird, respectively. The attributes most strongly associated with territory boundaries were conifer seedling cover, grass and total vegetation cover between 0-2 m high for black-throated blue warbler and fern cover, vegetation-covered rocks and shrub diversity for ovenbird. Complementary to this, a redundancy analysis (RDA) was used to compare attributes associated with the general occurrence of males to those whose boundaries were associated specifically with territory boundaries. Most attributes whose boundaries were associated with territory boundaries did not correspond to resource attributes, i.e., those where birds were detected most frequently. We conclude that soft boundaries associated with spatial heterogeneity may help shape forest bird territories by providing landmarks not necessarily related to resources used within territories.