Temporal and spatial correlation in chick production of willow grouse Lagopus lagopus in Sweden and Norway

We analysed the spatial and temporal dynamics of chick production of willow grouse Lagopus lagopus in the Swedish and Norwegian mountain region using harvest data covering 24––38 years and line-transect counts covering 6––10 years from the period 1960––2003. Juvenile and adult grouse were counted in the bag of harvested willow grouse in late August and throughout September at six sites in Sweden and Norway. In addition, numbers of juvenile and adult willow grouse were obtained from line-transect counts at 21 sites in Sweden just before the hunting season started on 25 August. The juvenile:adult ratio from the harvest data, estimated as number of juveniles per two adults, showed similar long-term averages and distributions between all six sites. The results from the line-transect counts revealed an overall higher average and a greater range of production between sites. We suggest that the difference between estimates based on line-transect counts and harvest data are caused by hunter behaviour. Hunters did not sample the juvenile and adult grouse in proportion to what was present in the population, e.g., productivity estimates derived from harvest samples will underestimate the proportion of juveniles in autumn in most years. We suggest that it is the harvest process that acts to adjust the juvenile to old bird ratio in the harvest data to a similar distribution in different sites. We found little evidence of regular fluctuations/cycles in annual production of juveniles from either harvest data or line-transect data. We conclude that chick production in willow grouse appears to fluctuate more irregularly than was previously believed based on harvest data and can not be used as a potential forecasting tool in willow grouse management as suggested earlier. Spatial correlation was weak and investigating only a limited number of sites to predict the proportion of juveniles in autumn should not be considered as a management tool for large areas such as the state-owned land in Sweden. Line-transect counts are costly, but do provide a more accurate estimate of the proportion of juveniles in the fall population than is revealed by harvest data. A more detailed understanding of hunter behaviour is obviously needed for better interpretation of harvest data.