The Ecology of the Partridge I. Outline of the population processes with particular reference to chick mortality and nest density.
Although the grey or hungarian partridge (Perdix perdix L.) is valued as a game-bird and is protected over large areas of Britain, nevertheless there is evidence of a general downward trend in partridge numbers during the present century (Middleton 1935; Matheson 1960). There are also very considerable annual fluctuations in the September partridge populations, largely due to variation in breeding success, which is most conveniently measured by the ratio of young birds to old birds (the young: old ratio) before shooting starts. Some evidence exists to show that this young : old ratio has, in fact, decreased during the present century (Middleton 1935; Blank & Ash 1962; Eley Game Advisory Station 1965).
The downward trend in numbers and in breeding success reduce the numbers available for shooting, but ecologically they are quite distinct. The year-to-year changes are a reflection of the action of factors causing fluctuations in the size of the actual population and such factors have been termed by Morris (1959) the 'key factors', because they provide the key to the prediction of future population size. These fluctuations in the population will be dampened by any factors that act in a density dependent manner- that is, that kill an increasing proportion of the population as its density increases - such factors
are often described as regulating.
The level around which the population fluctuates is conditioned by other environmental variables that influence the relationship between the regulating factors and population density.
The present paper, the first in a series describing investigations of various aspects of partridge ecology pertinent to its decline, describes the analysis of census data collected by Blank & Ash (1962); this enables the key factor and regulating factors to be recognized. An account is also given of a brief investigation of the relationship between nesting densities in field boundaries, a measure of population level, and the nature of these boundaries.