Changes in song thrush Turdus Philomelos density and habitat association in apple orchards during the breeding season

Within-season changes in habitat and distribution of breeding birds have been poorly investigated but are likely to be important for several species. With this work, we investigated what factors affect the density of a multi-brooded bird species breeding in apple orchards, the song thrush Turdus philomelos, in three different periods of the breeding season in the Trento province, northern Italy. Birds were counted along transects, and analyses carried out using a Generalised Mixed Linear Model (GLMM) approach. A further analysis assessed the effect of conspecific occurrence and habitat variables on the changes in local density. Thrush density changed during the season, increasing from early to late period, and was mainly affected by tree height (positively), bare soil (negatively in third period), shrubland (negatively), wetland (positively), grassland (negatively), percentage of rows irrigated by droplet system (negatively), percentage of orchard covered by anti-hail nets (negatively), urbanised area (negatively in first period, positively in third period). Intra-seasonal variation in density was affected by tree height (positively), anti-hail nets (negatively), use of herbicide during the late period (negatively), difference in bare ground (negatively), and initial thrush density (negatively), the latter suggesting that best habitats are occupied first. Multi-brooded bird species may show varying density and changing species-habitat relationships during the breeding season, and their distribution could change during the period of reproduction. This pattern could apply to a large number of bird species, requires deeper investigation and could have important implications for conservation and management.