Pheasants (Phasianus colchicus) and grey partridges (Perdix perdix) were maintained together on land known to be contaminated with eggs of the caecal nematode Heterakis gallinarum to examine the hypothesis that this shared parasite has a greater impact on grey partridges than on pheasants. Since an inverse relationship between worm intensity and partridge body condition was detected, while no such relationship with pheasant body condition was observed, we were unable to refute this hypothesis. Furthermore, that there was no relationship between worm intensity after the exposure period and partridge body mass prior to the infection trial implies that infection caused the decrease in partridge body condition, and not vice versa. Data consistent with previous observations that H. gallinarum fecundity and survival is greater in pheasants than in partridges suggest that the bulk source of nematode infection to wild grey partridges is reared pheasants, and not the partridges themselves. This, and the differential impact on host body condition, supports the hypothesis that the spread of parasites from increasing numbers of released pheasants has contributed to the decline in wild grey partridge populations in the UK within the past 50 years.
Keywords: Heterakis gallinarum; Key words Apparent competition; Nematode; Perdix perdix; Phasianus colchicus.