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

<p class="MsoNormal"></p> <p class="MsoNormal">In a subarctic forest at Kvaløya, northern Norway willow grouse (<em>Lagopus lagopus</em>) fed at snow level by clipping bits of twigs from mountain birch (<em>Betula pubescens</em>) 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 <span>≤</span> 2 mm in diameter. It is concluded that willow grouse fed optimally on birch in winter by selecting twigs to minimize fibrous wood intake.</p>