1. Substantial plastic variation in phenology in response to environmental heterogeneity through time in the same population has been uncovered in many species. However, our understanding of differences in reaction norms of phenology among populations from a given species remains limited. 2. Since the plasticity of phenological traits is often influenced by local thermal conditions, we expect local temperature to generate variation in the reaction norms between populations. 3. Here, we explored temporal variation in parturition date across 11 populations of the common lizard (Zootoca vivipara) from four mountain chains as a function of air temperatures during mid-gestation. We characterized among-population variation to assess how local weather conditions (mean and variance of ambient temperatures during mid-gestation) and habitat openness (an index of anthropogenic disturbance) influence the thermal reaction norms of the parturition date. 4. Our results provide evidence of interactive effects of anthropogenic disturbance and thermal conditions, with earlier parturition dates in warmer years on average especially in closed habitats. 5. Variation in the reaction norms for parturition date was correlated with mean local thermal conditions at a broad geographical scale. However populations exposed to variable thermal conditions had flatter thermal reaction norms. 6. Assessing whether environmental heterogeneity drives differentiation among reaction norms is crucial to estimate the capacity of different populations to contend with projected climatic and anthropogenic challenges.