THE CLOCKWORK OF FINNISH TETRAONID POPULATION-DYNAMICS

We studied 1964-1983 population dynamics of three species of tetraonid s, capercaillie (Tetrao urogallus), black grouse (Tetrao tetrix) and h azel grouse (Bonasa bonasia) in Finland. The data comprise species-spe cific quantitative estimates of bird numbers in 11 provinces during th e 20-yr period. Autocorrelation functions calculated from the trend-co rrected data suggest that the population fluctuations are cyclic in al l three species, cycles most frequently displaying six-yr periodicity. A model acknowledging long-term trend and incorporating linear combin ation of sine and cosine transforms of time mimics the fluctuations re asonably well. The coefficients of determination ranged from 25% to 88 % depending on species and province. Fitting the same model to 1897-19 30 hunting statistics of black grouse from south-western Finland indic ates that such a clockwork-like dynamics has been licking at least fro m the beginning of this century. An analysis of variance of the length s of increase and decrease phases of the population cycles further con firm the symmetric regularity of the tetraonid dynamics. No difference s were found among species or provinces. A theoretical analysis, based on projection matrix techniques, shows that it is unlikely that such a regular cyclicity could be driven solely by intrinsic components of the population structure. The domain of other possible factors respons ible for the clockwork-like dynamics is explored.