Capercaillies are essentially folivores, except for the first weeks of their lives when chicks rely also on arthropods.
The habitat of Cantabrian capers differs notably from that of conspecifics in the conifer domain; it is no wonder then that diet is also quite specific. We can tell by the remains of plant tissue in the droppings.
The diet is fairly diverse, and includes canopy species, shrubs, and herbs. More interesting perhaps than bulk diet is which items are more or less frequent in the diet than they are in the environment; the so-called diet selection. Overall, birds show relatively high metabolic demands, thus consuming highly profitable food. In that sense the folivore diet of grouses is an exception, and their digestive tracts show adaptions to the highly fibrous diets: long, cul-de-sac prolongations of the digestive tract (caeca) that basically give more digestive surface and retain longer the digestive products. Bacterial endosymbionts also help digesting the cellulose contained in tissue walls. Highly fibrous food particulates do not enter those caeca, and they are rapidly digested producing dropping like the ones in the picture above. Those are the ones that we normally see in the field. On the other hand, digestion within the caeca takes much longer, and is more thorough; every 24 to 48 hours capers make caecal droppings, which result from a long digestion and look very different: large, brownish and fluid.
The diet of a folivore bird in broad-leaved forest varies markedly with the seasons. Particularly in winter, food availability is very limited. In the oak forest of the south-west, capers seek leaves of evergreen holly tree, bilberry shoots, and even ferns. Holly is the only native tree that keeps green leaves in winter, with the exception of a few relict patches of Scots pine.
Bilberry (Vaccinium myrtillus) and ferns are understory species, which in addition tend to partially stick out of the snow, or grow in gullies and on steep slopes where snow does not accumulate. Compared to northern populations, Cantabrian capers use a higher proportion of understory plants, even in winter; that is a notable ecological differentiation, which likely bears along other important differences in interactions with other species (competition, predation, etc.).
Then in early spring, while courtship and mating begins, beech buds (see above) are important; they are a hardy though abundant resource when very few other plants offer edible tissues. Interestingly, highly abundant species like oaks, birch and heaths are used well below their relative abundance. This pattern may be determined merely by the timing of their relative availability: buds of oak and birch show up later than beech, and later on tender bilberry leaves begin to be available.
In summer, the chick-rearing season, food seems abundant. Cantabrian capers, like in boreal and central-European populations, do consume berries of Vaccinium.
The above is however the diet picture from a given area. The Cantabrian range is rugged and patched, vegetation varies notably, and capercaillies seem to cope with that variability.