Winter habitat selection and food choice of the capercaillie (Tetrao urogallus) in the French Pyrenees.

Abstract

In winter, the capercaillie, Tetrao urogallus, occurs in a variety of upland forest types. The species seems, therefore, to be adaptable in terms of both the habitat composition of its winter range and of the food that it will eat. This study seeks to establish whether the birds distribute themselves at random in relation to forest habitat types and to food resources, and if they select certain types of food in preference to others. The study took place in the central French Pyrenees (Luchon, Haute Garonne), where the species is widespread, at an altitude of 1,000-2,100 m in a 690-ha mixed forest of beech, Fagus sylvatica, and silver fir, Abies alba, with mountain pine, Pinus uncinata, Scots pine, P. sylvestris, and aralia pine, P. cembra, near the upper tree limit. There are also afforested areas of Norway spruce, Picea abies, and larch, Larix deciduas. We described the habitat by the number of different tree species and by the composition of vegetation types recorded at the intersection points of a 200-m squared grid. Habitats were described in the same way whenever capercaillie were encountered along transects during the 1979-1980 and 1980-1981 winters. Droppings (n = 261) were systematically collected along the transects and their contents described by visual identification of the main items. Compositional analysis was used to compare habitat availability with the locations of the birds and with their diet. Three out of four encounters (76%, n = 118) were in the forest and 10% along its edge; 70% of the birds were observed at altitudes between 1,700 and 1,900 m a.s.l. As regards habitat use, Sorbus spp. (percentage cover: 22.9% at bird locations versus only 4.4% at grid points), Scots pine and larch were most used relative to availability. As regards winter diet, the species eaten most in relation to availability was mountain pine, followed by Norway spruce, Arolla pine, larch, silver fir and Scots pine. The choice probably reflects a greater nutritional value of the needles of these trees. Deciduous trees featured little in the diet, although buds of beech (2.6% of total number of items, n = 353) and willow, Salix spp. or birch, Betula alba (11.3%) were noted. Birds seem to choose tree species on the wintering grounds on the basis not only of their nutritional value, but also of their protective value against environmental hazards. Contrary to what results may suggest, the capercaillie, which appears to cope well with disturbance, is declining across its whole range. We attribute this to the fact that at the individual level, only juveniles can choose their habitat and their diet. Once this choice has been made, the individual remains territorial for the rest of its life, becoming wholly dependent on its wintering site and on the food available there.