Between birch and berries: seasonal dietary niche partitioning among sympatric alpine herbivorous birds

Niche partitioning is important for the coexistence of closely related species, allowing species to reduce overlap in resource use despite shared ecological requirements. In alpine environments, harsh climatic conditions and low habitat complexity constrain opportunities for ecological segregation, making seasonal resource fluctuation especially important for maintaining coexistence. Under such conditions, both historical niche divergence and flexible responses to changing resources are likely to shape the existing patterns of dietary overlap. Here, we examined seasonal dietary niche partitioning between two sympatric herbivorous birds, willow ptarmigan Lagopus lagopus and rock ptarmigan L. muta during the winter–spring transition over six years in central Norway. Using DNA metabarcoding (ITS) of 400 fecal samples, we characterized plant diets and examined interspecific differences in composition, seasonal shift, niche breadth and overlap. There was strong evidence for dietary differentiation between species and across seasons, with species‐specific seasonal shifts in plant occurrence. During winter, partitioning was mostly quantitative with both species relying 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 appeared to maintain a relatively stable and specialized niche, while increasing the use of alpine heaths. However, analyses at wider spatial scales may reveal different patterns. 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.