Parasites and Scottish ptarmigan numbers.

Unlike red grouse (Lagopus lagopus scoticus), which have almost 100% prevalence of the parasitic threadworm Trichostrongylus tenuis and frequently high tapeworm numbers, 70% (n=71) of Scottish ptarmigan (Lagopus mutus) had no threadworms and only 4% had tapeworms. Other parasites and pathogenic bacteria were infrequent. Threadworms occurred in 12% (n=25) of birds on granite hills where mean ptarmigan densities were low, and in 43% (n=46) of birds over schists and limestones where mean densities were high. The notion that parasites cause the cyclic-type ptarmigan declines observed on the granite Cairngorms massif is unlikely (ptarmigan fluctuations over the richer rocks are irregular).