Data in Brief: Twig selection on mountain birch Betula pubescens by winter-feeding willow grouse Lagopus lagopus in a subarctic forest

In a subarctic forest at Kvaløya, northern Norway willow grouse (Lagopus lagopus) fed at snow level by clipping bits of twigs from mountain birch (Betula pubescens) during winter. Birch has two types of twigs ending in a terminal bud: long twigs with a smooth bark, and short twigs with rings of thicker bark. The grouse selected ringed twigs above smooth twigs despite a surplus of smooth twigs in the forest. Ringed twigs had more bark cm^-1 of twig length and a higher relative bark/wood ratio than smooth twigs. Smooth twigs had growth nodes that increased in diameter inwards from the tip. Because of the non-linear relation between the area and the circumference of a circle, the bark/wood ratio decreased for each node. Although being able to clip much thicker twigs, 90 % of smooth twigs clipped by grouse were ≤ 2 mm in diameter. It is concluded that willow grouse fed optimally on birch in winter by selecting twigs to minimize fibrous wood intake.