Ecological niche change as a strategy for survival: local adjustments shape distinct population trends in the Eurasian Skylark

The Eurasian skylark (Alauda arvensis) is an open-habitat passerine bird that is declining throughout Europe, mainly due to agricultural intensification. However, population responses to environmental changes are often geographic- and environment-dependent, preventing generalisation and transferability of ecological relationships. For three sub-regions of Lombardy (northern Italy) characterised by both different environmental conditions and human pressures (the Alps, Plain, Oltrepò), we decomposed population trends (1992–2021) of the species and assessed changes of the realised Grinnellian niches over three decades. In each sub-region, although the niche did not show divergence, we found an overall reduction over time. In the Alps, contraction of niche occupancy in meadows and pastures was not associated with a population decline. It is likely due to extensive availability of natural grasslands that counteracted the niche reduction and promoted a positive population trend (+164%). Conversely, in the Plain and the Oltrepò sub-regions, the observed population declines (−99% and −36%, respectively) could be linked to a general decrease of niche occupancy in arable lands. In the Oltrepò, differently from the Plain, a gradual expansion of the niche in meadows and pastures at higher altitudes could have counterbalanced the former reduction, playing a significant role to mitigate the negative population trend in the area. The joint application of population trend analysis and niche modelling, as well as the decomposition of population responses in relation to different environmental contexts, can contribute to a better understanding of ecological processes affecting population dynamics and to support policy makers for planning targeted conservation strategies.