Factors affecting the survival of
Birds of Conservation Concern

by David Leech

5.1.2 Parasitism and Disease

• Parasitism by the nematode Heterakis gallinarum significantly reduces the body condition of grey partridges (Tompkins et al., 2001) and has recently been proposed as a factor involved in the population decline of this species (Tompkins et al., 2000).

Parasitism may be responsible for the rapid decline of the grey partridge.

• Mortality due to parasitism is unlikely to be the sole cause of population extinction as it is a density-dependent process. As the size of the host population decreases, individual survival rates increase because the probability of parasite transmission to previously uninfected individuals is reduced. However, H. gallinarum also parasitises the ring-necked pheasant (Phasianus colchicus) causing a less severe reduction in body condition (Tompkins et al., 1999, 2000). A decrease in grey partridge abundance may therefore have a reduced affect on the transmission rate of the parasite, and the population may continue to decline.

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