Control of population size in birds: the grey partridge as a case study.
The ideas most ornithologists have on the control of population size are in large part formed by studies of insects. This is not surprising considering that case studies on the control of population size in birds have accounted for less than a tenth of those on animals generally. For instance, out of thirty studies included by Stubbs (1977) and of forty-five reviewed by Connell & Sousa (1983), the majority concerned insects and only six referred to birds. A recent review restricted to insects alone cited sixty-three case studies (Stiling 1988).
The numerical dominance of insects - 22,500 species in Britain, compared to 1,700 for vascular plants, 210 for birds and 58 for mammals - is obvious, but, in many respects, the study of birds is ahead of studies on other organisms. After all, birds have clear advantages: they are for the most part diurnal, are attractive to look at, and have family lives that we can relate to. Moreover, they are easy to count or even to census, especially when nesting, and they can be individually marked or radio-tracked with relative ease. Estimates of bird population density which reach back over more than 20 years have been available for some time; for example, Tanner (1966) mentions thirteen species. We take the view that ornithologists could contribute far more towards studies of the control of population size than, in general, they do. Rather than survey the whole order, even if that were possible, we give the example of the grey partridge Perdix perdix. It is the one we know best, and we believe principles arise which are of general application.
In 1913, when the British Ecological Society was founded, partridges were more than ten times as numerous in Britain as they are now. There must have been over seven million partridges in the country at the end of the breeding season, because they provided a sustainable yield of about 2.5 million shot per year (Potts 1986). None of this was achieved with the benefit of ecological science as we know it. There had not even been an official 'inquiry' on the partridge, as there had been on the red grouse (Lovat 1911). Nor was it the outcome of laissez-faire or ignorance. Partridges were managed for their yield over about half the area of the countryside of Britain, and there was a great store of management expertise, accrued from a tradition of practical experimentation by estate owners.
A Rip van Winkle amongst these owners, having slept for the past 75 years, would, on waking, surely marvel at our Land-Rovers, D-vacs and computers. We feel, though, that he would easily understand our current recommendations for partridge management. In passing we note that he would understand them better now than if he had woken in the 1960s when predation was considered of little significance. Maynard-Smith (1978) wrote, 'ecology is still a branch of science in which it is usually better to rely on the judgement of an experienced practitioner rather than on the predictions of a theorist'. We like to think that we have at last caught up!
The purpose of this paper is twofold. First, we summarize some of our research into the control of partridge population size, with particular attention to annual variation, to population trends and to the question of what determines the density of a population at equilibrium; the partridge offers a good subject here because in a number of detailed studies its equilibrium densities have varied from less than I to more than 50 pairs per km2. Second, we consider how the study of bird populations has advanced since the review of Lack (1966), and we propose a way forward for the future.