Parasites and partridges

Key findings

• Increased parasitism among grey partridges caused by ants in the diet is less likely to cause a decrease in chick survival than a lack of food. 
•  Management should focus more on providing food for chicks than concentrating on effect of parasites.

In the mid and late 1990s a number of estates in East Anglia suffered successive years of poor breeding and their partridge stocks went into decline. Although the precise causes for this reduced breeding success are unknown, it coincided with increased reports of parasitic disease in wild birds. Consequently it was suggested that chick-food availability had become so low that grey partridge chicks were being forced to eat some insects that harboured internal parasites and spread disease. To establish whether diet and disease susceptibility had altered in recent years, we did a three-year study of wild grey partridges in East Anglia.

We caught 85 female grey partridges and fitted them with radio transmitters at three study sites in Norfolk with a history of high unexplained losses. We followed these birds through the breeding season and assessed their breeding success, survival and diet. We also did post-mortem examinations on all partridges found dead or in poor condition.

Among the radio-tagged birds, seven were killed by predators, six died as a result of disease and a further 11 died from a variety of other or unknown causes. Breeding success for the radio-tagged birds was low. Although 54 females laid clutches, only 34 actually hatched young and only 20 broods survived longer than six weeks. Analysis of faeces revealed that chick diet contained a range of insects that was typical of earlier studies (see Figure 1).

However, we found that the chicks consumed more ants than usual. Ants, as well as several other invertebrates eaten by chicks at all sites, are known to be intermediate hosts of internal parasitic worms. The survival of chicks in a brood was, on average, inversely related to the percentage of ants in the diet (see Figure 2). In other words, the more ants eaten by the chicks, the lower the chicks' chances of survival. This may reflect increased disease susceptibility or perhaps a poorer diet.

The post-mortem examinations of 79 birds showed that 33 of these contained nematode or cestode parasitic worms. However, only 14 (17%) of the autopsied birds had levels of parasites that were thought likely to have caused death.

Figure 1. The mean composition of grey partridge brood diet at three sites in Norfolk, 2001-2003

Figure 2. Relationship between the percentage of ants in diet and chick survival for three sites in Norfolk, 2001-2003

Although we found that chick losses were highest where ants formed a high proportion of the diet, and although we cannot dismiss the possible effects of parasitic disease, we think that the apparent decrease in chick survival indicates a continued reduction in food availability for grey partridges over recent years, rather than an increase in parasitism. Accordingly, wild grey partridge management should aim at improving food availability for chicks rather than focusing only on the possible effects of parasites.