Modelling at the edge: Habitat types driving the occurrence of common forest bird species at the altitudinal margin of their range

We investigated habitat drivers of occurrence of five common forest bird species (Erithacus rubecula, Phylloscopus collybita, Sylvia atricapilla, Periparus ater, Fringilla coelebs) at the upper margin of their range in the Alps. Our work aimed to i) identify methods to analyse species-habitat relationships at the margin of species distribution, ii) provide recommendations for conservation and management in this changing environment. We recorded species occurrence at 149 points in Trento province (Italy; 1,365-2,200 m a.s.l.), and related occurrence to elevation and cover of habitats in a 100-m radius. We estimated factors affecting occurrence by single-species and multi-response MARS models. We also evaluated factors driving the overall species richness at point counts by means of a glmmPQLapproach. Multi-response models performed well (in general they performed better than single-species models) and suggested important effects of elevation (negative), conifer forest (not significant for P. collybita, positive up to a threshold for other species) and wood pasture (not significant for S. atricapilla, positive for other species). Conifer forest, wood pastures, shrubland and elevation affected the number of species at point counts (positive effect for all variables except for elevation). Multi-response models may help elucidate ecological relationships for 'rare' species, including common species at the margin of their range. Wood pasture promoted the occurrence of common forest species and the number of species, even if scarcely represented, and thus could deserve more consideration in conservation and management planning, given that several species are of high conservation concern.