The Effect of a Parasitic Nematode on the Breeding Production of Red Grouse

(1) The breeding success of female red grouse treated with an anthelmintic to reduce an infection of the nematode Trichostrongylus tenuis was compared over a period of 3 years with that of a control group. Treatment significantly increased the production of young per female in all years. (2) Breeding success showed a significant, negative correlation with parasite numbers in old grouse, both within and between areas (in 4 out of 5 years). (3) The role of parasitic nematodes in the population cycles of red grouse are discussed in relation to current models and previous studies.