Little is known about the cestode parasites of red grouse Lagopus lagopus scoticus. The carcasses of 71 free-living red grouse collected from northeastern Scotland (UK) between June 1991 and September 1992 were examined for the cestode Paroniella urogalli. Over 70% of young and old birds were infected in summer and winter. Cestode abundance ranged from 0 to 29 (X̄ ± SE = 5.54 ± 0.81) worms per host and did not differ significantly between young and old grouse, or between seasons. Cestode intensities did vary significantly between the sexes, but in opposite directions at two sites. A significantly greater total biomass (dry weight) of cestodes was found in grouse in the summer. Cestode biomass also was greater in young than in old grouse, although there was no significant difference after controlling for the effects of site, year and season. However, in the summer total cestode biomass in young grouse was significantly greater per unit bodyweight than in old grouse. No evidence was found for an effect of cestodiasis on grouse condition but a significant positive correlation existed between grouse comb condition and cestode abundance. Estimates of mean per capita worm mass suggested an intensity dependent reduction in the weight of individual cestodes in birds in their first summer. Per capita cestode mass was significantly lower in winter than summer, which may suggest that the production of mature proglottids varies with season. There was also a significant negative correlation between numbers of the nematode Trichostrongylus tenuis and the total biomass of P. urogalli in grouse.

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