A spatially explicit assessment of within-season changes in environmental suitability for farmland birds along an altitudinal gradient

The intra-seasonal adjustment in site selection, and hence in distribution of multi-brooded avian species, is a poorly investigated topic of potentially high relevance for conservation. This phenomenon may be especially important among multi-brooded species occurring in habitats undergoing large modifications. We used correlative species distribution models to carry out a spatially explicit assessment of variation in habitat suitability between early and late periods of the breeding season for four European farmland passerine species (woodlark Lullula arborea, skylark Alauda arvensis, cirl Emberiza cirlus and corn bunting E. calandra). We counted birds at transects (surveyed in early and late season) in Northern Apennines (Lombardy, Italy), and modelled their spatial distribution with MaxEnt, separately for early and late season periods. A comparison of predicted environmental suitability (ES) at occurrence sites showed significant differences between early and late period ES at both early and late occurrence sites. Three species showed differences in elevation between early and late occurrence sites. The proportion of 1-ha cells predicted as potentially suitable during the entire breeding season ranged from 29.3% (corn bunting) to 52.8% (woodlark) of the at least once suitable cells. In multi-brooded birds, a dynamic pattern of species occurrence may be likely and particularly important in farmland habitats along altitudinal gradients. The resulting within-season switches in habitat and distribution could be widespread among farmland birds and could have critical implications for their conservation. Moreover, such habitat switches provide an additional reason for preserving habitat heterogeneity. The intra-seasonal variation in habitat association/distribution should become a focal topic for researchers.