Habitat quality drives the spatio-temporal occupation of a migratory songbird, the northern Wheatear Oenanthe oenanthe, in Alpine grasslands

Birds of open alpine grasslands are threatened by the degradation and contraction of their breeding habitat. To assess potential impacts of environmental change in alpine environments, basic ecological knowledge, such as habitat requirements during pre-breeding and breeding periods, is needed. We conducted territory and habitat mapping of a population of the northern Wheatear Oenanthe oenanthe breeding in alpine grasslands and lower elevation open habitats in the Western Italian Alps during two consecutive breeding seasons (2019-2020). For territory mapping, a study area of over 160ha was visited five times, covering an elevation gradient from 1,600 to 2,700m.a.s.l. Low elevation grasslands in the valley were exploited by birds as arrival habitat after spring migration, but were not selected as breeding territories. On territories, birds avoided areas with numerous small trees, selecting patchy habitats with high rock cover. Early arriving males first occupied the territories at lower elevations, and those with taller ground vegetation, but avoided overall higher vegetation cover. Birds were able to initiate the breeding season when snow cover was still high, as long as forbs and grass were present. The expected decrease in the area of open alpine grassland due to shrub encroachment will probably reduce the availability of suitable territory habitat and thus threatens the alpine population. Extensive seasonal grazing could help to maintain alpine grasslands above the treeline, and may provide suitable, but lower quality, habitat even at lower elevations by keeping the area open.