Most dietary studies on predator species available in the literature are based on single populations, with no meta-analysis across populations in distinct areas of their range. Here, we performed a systematic review of the available data on the food habits of barn owl (Tyto alba) across Italy, collected during the last 40 years in relation to the modeled patterns of richness of small mammal communities. The overall dataset came from 212 sites, with multiple surveys for some sites, and a total of 279 samples. There was a significant effect of sample size on niche breadth in each site. There was a significant difference in terms of dietary breadth among six distinct areas of Italy, with diet breadth being lower in Sardinia and higher in Latium-Abruzzi, Tuscany, and North-eastern Italy. Potential small mammal prey diversity was significantly different across distinct study sites, with Apulia and Sardinia having lower diversity than the rest of the sites. Potential small mammal prey diversity affected the niche breadth per site. Overall, the plasticity of the food niche breadth of the barn owl reflected not only local conditions but also more general distribution patterns of small mammal communities.