- 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.
- Chicks that were fed beetles, snails or ants (known to be potential carriers of parasites) did not contract parasites.
- Growth rates of chicks fed on beetles, snails or ants were higher than those fed just chick crumb.
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, 54 females laid clutches but 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 (Figure 1).
Figure 1: The mean composition of grey partridge brood diet at three sites in Norfolk, 2001-2003
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 (Figure 2).This may reflect increased disease susceptibility or perhaps a poorer diet.
Figure 2: Relationship between the percentage of ants in diet and chick survival for three sites in Norfolk, 2001-2003
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.
The food availability of reared grey partridge chicks was experimentally altered to test whether the consumption of invertebrates identified in the literature as intermediate hosts of parasitic worms caused increased parasitic infection in partridges. We kept 20 batches of 12 one-day-old grey partridge chicks in captivity on a sterile ssubstrate. Each batch was fed with a standard diet of chick crumb but, for the first three weeks, we augmented the diets of 15 batches with ants, beetles or snails, which are all known to be vectors of parasitic disease. Five batches received adult ants, five a variety of small snails and five carabid beetles, with the remaining five batches receiving the standard diet as a control. We allocated the diet randomly to each batch.
We also sealed the joints in the pens containing each batch and employed bio-security measures to prevent cross-contamination. In total, the experiment involved the collection and feeding of over 3,000 beetles, 3,000 snails and over 10,000 ants. We monitored the chicks and examined their faeces microscopically to confirm that the invertebrates had been eaten.
In most cases chicks consumed all invertebrates within seconds of being fed them. We measured food (chick crumb) consumption on a daily basis and body size (wing length, body weight and tarsal length) weekly. After six weeks, we conducted post-mortems on all chicks that had not died previously to establish the number and species of internal parasitic worms.
None of the chicks fed an experimental diet contracted parasites and there was no difference in the amount of non-invertebrate food consumed by the different groups, although the growth rates of chicks eating ants and beetles were higher, on average, than those of chicks eating snails or the control diet.
The lack of parasitic infection suggests that either the infection rate must be very low or that some other factor may be controlling the contraction of disease. Unlike wild grey partridges, the chicks reared under the experimental conditions had all the requirements for good growth, namely a permanent supply of warmth, light, water and a high-protein diet. It is possible that under these ideal conditions grey partridge chicks have a much better immunity to parasitic infection.
Although we found that chick losses were highest in wild partridges where ants formed a high proportion of the diet, and although we cannot dismiss the possible effects of parasitic disease, this was not the case in our experimental study. We believe 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.
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