The Influence of Habitat on the Population Ecology of Partridges.

Abstract

This study examines the role of habitat in the population dynamics of Grey and Red-legged Partridges at the time of pre-breeding dispersal and nesting. The primary objective was to establish the major aspects of habitat influencing Partridges and suggest possible ways in which farmland may be managed to increase Partridge populations.
The thesis begins with an outline of previous studies of Partridges in Britain relevant to the aims of this work and a discussion of the wider implications of these aims with respect to the study of hedgerows and bird populations and the importance of emigration and habitat quality in vertebrate population dynamics. The sources and details of long term Partridge population data are given and methods of quantifying habitat variations on farmland are developed and described. A recruitment model is developed which uses the long term population data to measure the attractiveness of a set of estates to Grey and Red-legged Partridges. The model shows that recruitment of yearling Partridges into the breeding population of each estate decreases with increasing adult density and that the depressing 'effect' of adults on recruitment differs significantly between estates. These differences, termed recruitment efficiencies, are used as indices of the estate's attractiveness to Partridges. The model also shows that recruitment increased with the density of wild yearlings available to enter the breeding population and that Red-legged Partridge recruitment was increased by releasing reared birds, while Grey Partridge recruitment was not. Grey Partridge recruitment efficiency and breeding density were positively related to the amount of nesting habitat available, the amount of dead grass within the nesting habitat and, in some areas, the height of the earth bank at the base of the nesting habitat. Red-legged Partridge recruitment efficiency and breeding density increased with the amount of nesting habitat available and the amount of nettle in the nesting habitat. Grey Partridges nested where amounts of residual ground vegetation, excluding nettle, were greatest, especially the amount of dead grass. The greater the amount of dead grass around a nest, the higher its chances of success. Red-legged Partridges nested where amounts of residual ground vegetation were greatest (including nettle), but nest success was determined by the height of the new growth of vegetation. The effects of hedgerow management on nesting habitat quality are examined and it is shown that regular trimming every other year produces the most suitable nesting habitat for both species of Partridge. The implications of these results for habitat management and Partridge population regulation are discussed and further intensive and experimental research is recommended.