2.3. Density indices of prey animals

Density estimates of grouse were provided by The Finnish Game and Fisheries Institute. Estimates are based on the so-called route (until 1989) and triangle censuses (1989 on). In both methods three men patrols move in chains 20 m apart along a certain route (route censuses) or along compass lines designed in the form of triangles, each side 4 km long. Counters, mainly hunters, record all grouse in the line, determining their species, sex and age. Censuses are carried out in early August. The same triangles are used for snow track counting of mammals in February-March. I used yearly data from 10-12 triangles, which were situated within a circle with a radius of 30 km, the city of Oulu as a centre point. Data from 10-15 triangles outside that area were used to back up the estimates (more about method see Linden et al. 1996). Track indices of mammals were transformed to density estimates by a formula Z = 1.57 x s/md, where Z = animals/1000 ha, s = number of tracks crossing the line, m = length of the census route and d = length of animal’s day track (Formosov & Priklonski, cited in Havas & Sulkava 1987). Density estimates for other land birds were given by Mikola (1986), Rauhala (1994) and Inkeroinen & Mönkkönen (unpubl. data) and for waterfowl by Tynjälä (unpubl. data). All bird estimates except grouse were based on spring cencuses. I calculated summertime estimates taking into account the productivity and mortality rate of different species found in handbooks or research reports (Coombs 1978, Rajala 1979, Valkeajärvi & Ijäs 1994). Density of adult grouse in August censuses included mortality caused by goshawks and was used in their spring density estimates. Productivity of mammals was also obtained from literature (Siivonen 1956, Angerbjörn 1986, Wauters & Dhondt 1990).