[6] Its closest relative is the black-billed capercaillie, Tetrao parvirostris, which breeds in the larch taiga forests of eastern Russia and parts of northern Mongolia and China.

It is one of the most sexually dimorphic in size of living bird species, only exceeded by the larger types of bustards and a select few members of the pheasant family.

Cocks (males) typically range from 74 to 85 centimetres (29 to 33 inches) in length with wingspan of 90 to 125 cm (35 to 49 in) and an average weight of 4.1 kg (9 lb 1 oz).

Their toe rows of small, elongated horn tacks provide a snowshoe effect that led to the German family name "Rauhfußhühner", literally translated as "rough feet chickens".

The Scottish population became extinct, but has been reintroduced from the Swedish population; in Germany it is on the "Red List" as a species threatened by extinction and is no longer found in the lower mountainous areas of Bavaria; in the Bavarian Forest, the Black Forest and the Harz mountains numbers of surviving western capercaillie decline even under massive efforts to breed them in captivity and release them into the wild.

[1] As reported by the Spanish researcher Félix Rodríguez de la Fuente in his "Fauna" series, the northwestern Spanish subspecies T. u. cantabricus—an Ice Age remnant—was threatened in the 1960s by commercial gathering of holly fruit-bearing branches for sale as Christmas ornaments—a practice imported from Anglo-Saxon or Germanic countries.

In Scotland, the population has declined greatly since the 1960s because of deer fencing, predation and lack of suitable habitat (Caledonian Forest).

A study published in 2022 by NatureScot scientific advisory committee[16] recommended 'renewed intensive measures' to maintain the population, especially steps to assist in the survival of eggs and chicks.

Predators like crows, foxes and pine martens are blamed for the decline, as well as the deer fences, and increased human recreation in the territory which can injure adult birds.It was declared as 'extremely vulnerable' and requiring urgent action.

[17] The western capercaillie is adapted to its original habitats—old coniferous forests with a rich interior structure and dense ground vegetation of Vaccinium species under a light canopy.

They mainly feed on Vaccinium species, especially bilberry, find cover in young tree growth, and use the open spaces when flying.

Western capercaillie, especially the hens with young chicks, require resources that should occur as parts of a small-scaled patchy mosaic: These are food plants, small insects for the chicks, cover in dense young trees or high ground vegetation, old trees with horizontal branches for sleeping.

These criteria are met best in old forest stands with spruce and pine, dense ground vegetation and local tree regrowth on dry slopes in southern to western expositions.

In the highlands and along the ridges of mountain areas in temperate Europe as well as in the taiga region from Fennoscandia to Siberia, the boreal forests show this open structure due to the harsh climate, offering optimal habitats for capercaillie without human influence.

It is highest in sun-flooded open, old mixed forests with spruce, pine, fir and some beech with a rich ground cover of Vaccinium species.

The annual range can be several square kilometres (hundreds of hectares) when storms and heavy snowfall force the birds to winter at lower altitudes.

The western capercaillie lives on a variety of food types, including buds, leaves, berries, insects, grasses and in the winter mostly conifer needles.

Most of the year the droppings are of solid consistency but, with the ripening of blueberries, these dominate the diet and the faeces become formless and bluish black.

The western capercaillie is a highly specialized herbivore, which feeds almost exclusively on blueberry leaves and berries with some grass seeds and fresh shoots of sedges in summertime.

Available insect supply is strongly influenced by weather—dry and warm conditions allow a fast growth of the chicks, cold and rainy weather leads to high mortality.

During winter, when a high snow cover prevents access to ground vegetation, the western capercaillie spends almost all day and night in trees, feeding on coniferous needles of spruce, pine and fir as well as on buds from beech and rowan.

To digest this coarse winter food, the birds need grit: small stones or gastroliths which they actively search for and devour.

During the short winter days the western capercaillie feeds almost constantly and produces a pellet nearly every 10 minutes.

[18] The breeding season of the western capercaillie starts according to spring weather progress, vegetation development and altitude between March and April and lasts until May or June.

The cock postures himself with raised and fanned tail feathers, erect neck, beak pointed skywards, wings held out and drooped and starts his typical aria to impress the females.

Even single human observers may cause the hens to fly off and prevent copulation in this very short time span where they are ready for conception.

In Scandinavia and in Scotland, male western capercaillies are well-known for their combative behavior during mating season, sometimes even challenging and chasing off people who enter their territory.

They seek food independently and prey mainly on insects, like butterfly caterpillars and pupae, ants, myriapodae, ground beetles.

They grow rapidly and most of the energy intake is transformed into the protein of the flight musculature (the white flesh around the breast in chickens).

[31] A traditional gamebird, the capercaillie has been widely hunted with guns and dogs throughout its territory in central and northern Europe.