CECAL THREADWORMS TRICHOSTRONGYLUS-TENUIS IN RED GROUSE LAGOPUS-LAGOPUS-SCOTICUS - EFFECTS OF WEATHER AND HOST DENSITY UPON ESTIMATED WORM BURDENS

Trichostrongylus tenuis eggs were counted in faeces from individually marked wild red grouse for 8 years. Egg counts varied seasonally and a nnually. In some years, a sudden increase in mid-April was consistent with delayed maturation of larvae which had overwintered in the birds in a hypobiotic state. A more gradual increase in summer was probably due to uninterrupted maturation of larvae ingested then. Despite 30-fo ld year-to-year variation in mean egg counts, relative differences in egg counts among known individuals within years tended to persist acro ss years. Rainfall in previous summers explained much of the year-to-y ear variation in egg counts, probably because parasite recruitment was greatest during wet summers. Grouse density was only weakly related t o worm egg counts. The data were not consistent with the hypothesis th at the cyclic-type population fluctuation in red grouse numbers observ ed at the time of this study was caused by the parasites.