The clockwork of Finnish tetraonid population dynamics

We studied 1964-1983 population dynamics of three species of tetraonids, capercaillie (Tetrao urogallus), black grouse (Tetrao tetrix) and hazel grouse (Bonasa bonasia) in Finland. The data comprise species-specific quantitative estimates of bird numbers in 11 provinces during the 20-yr period. Autocorrelation functions calculated from the trend-corrected data suggest that the population fluctuations are cyclic in all three species, cycles most frequently displaying six-yr periodicity. A model acknowledging long-term trend and incorporating linear combination of sine and cosine transforms of time mimics the fluctuations reasonably well. The coefficients of determination ranged from 25% to 88% depending on species and province. Fitting the same model to 1897-1930 hunting statistics of black grouse from south-western Finland indicates that such a clockwork-like dynamics has been licking at least from the beginning of this century. An analysis of variance of the lengths of increase and decrease phases of the population cycles further confirm the symmetric regularity of the tetraonid dynamics. No differences 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 responsible for the clockwork-like dynamics is explored.