Daytime roosting and habitat preference of capercaillie Tetrao urogallus males in spring - the importance of forest structure in relation to anti-predator behaviour

Abstract

Radio-equipped adult capercaillie Tetrao urogallus males (N = 10) were tracked during April and May 1996-1997 in a coniferous forest in southeastern Norway, which has been subjected to clear-felling and replanting during the last five decades. With the use of radio-telemetry daytime habitat preference was investigated at two levels of scale: within home range and within forest stands inside home ranges. In addition, roosting habitat preference within stands was investigated by locating heaps of intestinal faeces, henceforth called roosting sites. Within forest stands, forest structure and visibility were measured at roosting sites (N = 94) and at random plots (N = 188). Within home ranges the birds used plantations and old naturally regenerated forest according to availability, and open bogs were avoided. Of the roosting sites, 90% were located underneath the low branches of Norway spruce Picea abies trees. For roosting within old naturally regenerated stands (> 70 years old) males preferred sites with higher tree density, higher vertical and horizontal cover and lower visibility than in the surrounding forest. In young and middle-aged plantations, on the contrary, roosting sites tended to have lower tree density (P = 0.118), but still seemed to have better cover than the surrounding forest (P = 0.117). The forest structure preference of roosting sites was reflected in the use of vegetation types within the old naturally regenerated forest. Because dense cover reduces the probability that the birds are detected by a predator, but at the same time increases the risk of being killed once detected, capercaillie males have to compromise between shelter and outlook. Our data indicate that males prefer good cover at the expense of good overview of the surroundings when selecting roosting sites, and that tree density is usually too dense in younger plantations, probably because outlook is reduced and flying obstructed.