Species coexistence in harsh alpine environments is often facilitated by niche partitioning, yet the stability of these mechanisms is being challenged by climate change and associated environmental shifts. Understanding how closely related species coexist in such a resource-limited environment requires quantifying the extent of niche differentiation and how overlap varies across seasons. Here, we examined seasonal dietary niche partitioning between two sympatric herbivore birds, willow ptarmigan (Lagopus lagopus) and rock ptarmigan (L. muta) during the winter-spring transition across six years in central Norway. Using fecal DNA metabarcoding (ITS2) from 400 genetically identified samples of ptarmigan, we characterized plant diets and examined interspecific differences in diet composition, seasonal shifts, niche breadth and dietary overlap. Model-based multivariate analysis showed strong evidence for dietary differentiation between species and across seasons, with species-specific seasonal shifts in plant occurrence. During winter, partitioning was mostly quantitative where both species relied on birch (Betula sp.), but rock ptarmigan also heavily utilized crowberry (Empetrum nigrum). In spring, dietary richness and divergence increased, driven by an expansion of niche breadth in willow ptarmigan towards emerging bilberry (Vaccinium myrtillus). In contrast, rock ptarmigan maintained a relatively stable and specialized niche, while increasing use of alpine heaths such as alpine bearberry (Arctous alpina) and alpine azalea (Kalmia procumbens). Based on our results, we demonstrate that fine-scale partitioning of shared resources can structure dietary niches in sympatric alpine herbivores and that seasonal divergence is mainly driven by shifts in the relative use of shared taxa rather than complete turnover in diet composition. These findings suggest that coexistence in a low-productivity alpine system may depend on persistent specialization and seasonal flexibility of both ptarmigan species. We suggest that ongoing changes in alpine vegetation, such as shrub encroachment, may alter resource overlap and reshape niche boundaries, particularly for habitat specialists restricted to high-alpine areas.