Seasonal infection patterns in Willow Grouse Lagopus lagopus L. do not support the presence of parasite-induced winter losses

Abstract

We examined the hypothesis that endoparasites of Willow Grouse (Lagopus lagopus L.) affect host winter losses, by examining two samples of Willow Grouse collected in early autumn and late winter. Body condition of juvenile birds improved from September to February, but parasite-induced host mortalities among hosts in poor condition were probably not the cause. If heavily infected hosts were removed disproportionately from the host population, the number of parasite species per host together with their prevalence and intensities should be lower in winter than in autumn. While overall parasite burden was lower in winter than in autumn, most parasite species showed no seasonal decrease in prevalence or intensities. Only one species, Trichostrongylus tenuis, showed a seasonal decrease in both prevalence and intensity. Mean intensities of Eimeria spp. decreased significantly between seasons, while prevalence did not. If mortality rates were higher among heavily infected hosts, parasite aggregation should be lower in winter than in autumn, but no such pattern was found, except for Eimeria spp. Thus, the results presented in this study yield no clear evidence for parasite-induced mortality in Willow Grouse between autumn and late winter.