The effects of modern agriculture, nest predation and game management on the population ecology of partridges, (Perdix perdix and Alectoris rufa).

Abstract

Studies on the ecology of the grey partridge (Perdix perdix) and red-legged partridge (Alectois rufa) began in the early 1930s and have continued through a period of radical change in their farmland habitat. During the past 10 years this work has shown that chick survival is determined mostIy by the abundance of their insect food, and that the quality of the chick's diet has been progressively reduced by the use of pesticides and by other modern farming techniques. However, the net effects of agricultural changes on a population of partridges are not straightforward since both species are shot to varying extents throughout their range, and in some areas they also benefit from game conservation techniques such as predator control and restocking. Moreover, there is continued controversy about the supposed importance of factors which have been considered to regulate partridge populations (e.g. Manley, 1977).

The aims of the work described here have been to isolate and quantify the causes or the decline or the grey partridge and to explain the associated increase of the red-legged partridge in some areas. In this way it was intended to develop an ecological rationale for the practical conservation and exploitation of these birds, bearing in mind current trends in agriculture. Computer simulation techniques have been extensively used.

Many series of bag records, sometimes extending for as much as 150 years are available for farms throughout Britain showing that there was, almost without doubt, an increase in the grey partridge starting at the end of the eighteenth century. It probably began with the enclosure movement and continued to about 1870, but precise interpretation is impossible because of several improvements to guns and in view of the change from "walking up" to "driving" (see p. 27). From 1870-1914 populations were at an all-time high and by 1908 stocks that had been transported from Eastern Europe also began to flourish in the North American prairies. However, the highest densities were reached in areas such as the Silesian plain (Poland). the Elbe lowlands (in Czechoslovakia) and in East Anglia (UK).

Alfred Newton clearly envisaged the ecological and management value of the detailed records or grey partridges which were being kept on the partridge manors near Cambridge (Newton. 1861) but it was not until 1903 that data were being assembled in a systematic way. This paper is basically an interpretation of all the ensuing studies in the light of my work in Sussex since 1968.

Most attention is directed to the grey partridge, and the future of this as a quarry species on farmland is shown to be very seriously threatened by the inexorable increase in the use of pesticides, and by the removal of hedges and similar nesting cover. Comparative studies on the red-legged partridge have been included because they support much of the argument concerning population processes.

A number of workers, especially on birds, have shown that territorial behaviour can modify the density of breeding populations, but the value of the spacing behaviour, and in particular of variations in territory size, still needs to be thoroughly explored in evolutionary terms. This paper also shows by way of deterministic population simulation models, that density dependent pre-breeding dispersal could have evolved in partridges as a way of reducing nest predation. In any case the spacing is considered to be a means of increasing productivity rather than a means of limiting breeding density.