Although much information is available on the occurrence of helminth parasites, little is known concerning the composition, size and variability of parasite populations in a particular host species.

Comprehensive data have been compiled from the examination of 135 juvenile and 223 adult starlings from Northern England over a period of nineteen months. Only four of the fifteen species recorded are found in more than 25 per cent of juveniles or adults.

During the first quarter of the year the incidence of infection is low and the numbers of parasites are small, but in the second quarter the incidence and intensity of infection increase rapidly to their maximum levels. Newly fledged juveniles, when they first appear at the end of May, are also universally and heavily infected. The incidence and level of infection in both groups gradually decrease during the remaining six months of the year.

The numbers of parasite species and individuals appear to be related to the incidence of infection. When the incidence is low infected birds always harbour small numbers of parasites; when it is high a large proportion of birds carry heavy or very heavy infections.