Classifying edge tolerance in understory birds in Amazonian Forest Islands: a quantitative approach

Species distributions in forest fragments are affected by their tolerance to edge habitats. Here we estimated the extent of edge effects for understory birds to assign them into four categories: (1) edge-associated, (2) edge-tolerant and (3) edge-intolerant and (4) rare to classify. We sampled 38 sites at different distances to the edge (15–2,950m) at five continuous forest sites and 33 forest islands at the Balbina Hydroelectric Reservoir, Brazilian Amazonia. In each site, we used 16 mist-nets from 06:00 to 15:00 over two days in both 2015 and 2016 between July and December, totalling 2,115 captures of 130 species during 21,888 net-hours. Species composition, represented by a multivariate axis (NMDS), along the gradient of increasing distance from forest edges were highly variable up to 100m, converging into a similar pattern from this distance. Species were then classified according to their distribution between edge (100m, n=19) based on a multinomial model. Three species were classified as edge-associated, 19 as edge-tolerant, eight as edge-intolerant, and 100 as rare to classify. Of the 30 species classified, most (n=17) corroborate Parker et al. 1996, apart from 13 intolerant species which were not expected to occur within edge habitats, such as Dixiphia pipra and Xiphorhynchus pardalotus. We show that, although species’ habitat associations assigned based on qualitative data from the literature can be generally accurate, this can be substantially improved using a quantitative approach.