Censusing tetranoids by the Finnish wildlife triangle method: principles and some applications

Abstract

Tetraonid birds are censused in August and in January-March in the Finnish Game triangle scheme, which is based on about 1200 permanent triangle-shaped routes, each 12 km in length . We compared the results of late-summer and winter censuses in order to develop transformations for further use. The summer and winter densities were significantly correlated at both the triangle and game district level. When the numbers of observations are compared, the winter figures are on average 20 % of the latesummer figures in the Capercaillie ; the corresponding figures are 71, 16 and 125 %for the Black Grouse, Hazel Hen and Willow Grouse, respectively . It should be noted that these are not absolute figures and not fully comparable, since the breadth of the census belt is 60 m in August whereas the belt breadth in the winter census is not known. Using transformations obtained in this study, we transformed the old late-summer brood data (1964-85) from southern Finland to relative winter figures, in order to have data comparable to those for southern Soviet Karelia, where actual winter censuses have been made . All the four species have decreased considerably (-65 %) in Finland, whilst the reverse trend is evident for Karelia (relative change during the period +42 %); the figures of the mid 1980s suggest a similar density in Finland and Karelia. The changes in grouse densities partly accord with the predictions based on changes in habitat distribution and (other) human activities in Finland and Karelia. The paper discusses the possibility of deriving estimates of population density from winter census data obtained from a census belt ofunknown breadth, central issues being the basal and lateral detectabilities.