Species distributions are strongly affected by climate, and climate change is largely impacting on species and populations. Thermal niches have been widely used as proxies for estimating thermal sensitivity of species and have been frequently related to community composition, population trends and latitudinal/elevational shifts in distribution. To our knowledge, no work has yet explored the relationship between thermal niche and change in range size (changes in the number of occupied spatial units over time) in bird species. In this paper, we related a 30 years change in range size to Species Thermal Index (STI: average temperature at occurrence sites) and to other factors (i.e. birds' associated habitats, body mass, hunting status) potentially affecting bird populations/range size. We analysed trends of breeding birds range in Italy for a suite of poorly studied cold-adapted animals potentially sensitive to global warming, and a related group of control species taxonomically similar and with comparable mass but mainly occurring at lower/warmer sites. We found a strong positive correlation between change in range size and STI, confirming that recent climatic warming has favoured species of warmer climates and adversely affected species occupying colder areas. A model including STI and birds' associated habitats was only marginally less supported, with forest species performing better than alpine open habitat and agricultural ones. In line with previous works highlighting effects of recent climate change on community composition, species' population trends and on poleward/upward distributional shifts, we found STI to be the most important predictor of change in range size variation in breeding birds.