Winter predation and the role of spacing behaviour in low density red grouse populations.

Abstract

The role of breeding production and spacing behaviour in causing the year to year changes in the breeding population of Lagopus species is not clear and has led to debate within the scientific literature. Bergerud, Mossop and Myrberget (1985) provide correlative evidence that changes in numbers are associated with changes in breeding production, that mortality is not associated with periods of territorial behaviour and conclude that breeding production is sufficient to cause changes in population size. Watson & Moss (1987) criticized this analysis pointing out that correlations were not evidence of cause and effect and that previous removal experiments demonstrate the presence of non-territorial birds. Bergerud (1987) answered these points and felt the debate was lost in semantics and what was ultimately needed is a clear unequivocal hypothesis with a valid disproof.

To date a clear statement of a hypothesis and disproof have not been produced. Watson & Moss (1970) considered 4 main pre-requisites are required to demonstrate the role of spacing behaviour but Hudson & Dobson (in press) reviewed these for Red Grouse and showed that such conditions, including the role of removal experiments, have yet to be met. Workers generally agree that territorial behaviour can limit density but much of the debate lies on whether changes in spacing behaviour cause density to fall and can maintain populations at low density.

It is apparent that more than 30 years of intensive research in north-east Scotland have failed to clarify the causal mechanisms involved in changes in grouse numbers and fall under the criticism Chitty (1967) levelled at Lack's study on Great Tits 'Something must be wrong if the present 19 years of data have failed to eliminate even one of two hypotheses with which the study began' and felt 'short-term experiments are not obviously inferior ways of deciding between rival points of view'. With hindsight the failure of these studies has arisen from indirect measures of population loss. For example, Watson, Moss, Rothery & Parr (1984) conducted detailed population data. Which have shown that direct causes of mortality on their study area were insufficient to account for changes and so losses due to emigration were invoked and assumed to be caused by intrinsic changes in spacing behaviour. This approach was denigrated by Darwin who said 'never to trust in science to the principle of exclusion'.

In this respect sound population data coupled with carefully designed field experiments at low density may provide a fresh and clear view of the situation. We have initiated a series of experiments in both Scotland and England and this short paper summarises both population data and initial experimental results. A more detailed analysis will be published elsewhere.