Mortality causes and survival rates of hunted and unhunted willow grouse.

Abstract

Few studies have examined the effect of interactions between predation and hunting on the rate of game bird mortality throughout the year. We monitored 134 radiotagged willow grouse (Lagopus lagopus) on 3 contiguous areas, 2 non-hunted and 1 hunted, in central Sweden between 1992 and 1995. We recorded 100 known mortalities: 22 from hunting, 40 from raptors, 19 from mammalian predators, and 19 from other causes. Excluding hunting, cause of death was similar on hunted and non-hunted areas (P > 0.5), with raptors the most important cause of death in all seasons. We did not record any deaths due to mammalian predators in winter or spring, and deaths through collision with power lines only affected winter, but not annual, survival rates. Cox proportional hazard modeling revealed season (autumn, winter-spring, summer) and area (hunted, non-hunted) as the only variables to affect risk of death (P < 0.001). The greatest rates of hunting mortality (0.24) and natural predation (0.32) occurred on the hunted area during autumn. Autumn survival on the hunted area (0.49) was lower (P = 0.029) than on the non-hunted areas (0.71). Survival increased in winter-spring to 0.69 on the hunted area and 0.81 on the non-hunted areas, and remained high (>0.78) through the summer period. We believe hunting mortality was mostly, if not totally, additive to natural mortality in our study. The population density on the hunted area did not decline as expected from the results, and we suggest that immigration from beyond the neighboring non-hunted areas was sustaining the population. We conclude that a harvest model for willow grouse must take account of dispersal and predation rates at a landscape scale.