An Ecological Study of the Feeding Habits of the English Titmice

Abstract

Problems of the interaction between predatory animals and the species whereon they feed may be attacked in two ways. When the predator species is sufficiently numerous and the prey is amenable to accurate sampling the advance may be direct. But when the predator is scarce or its foods are diverse; when the prey species are so inaccessibly or unevenly dispersed that they cannot be quantitatively surveyed, then indirect methods of investigation must be used. Early in 1947, at the request of the Agricultural Research Council, the Edward Grey Institute of Field Ornithology began an investigation of the dual problem of the influence of bird predation upon the numbers of the prey species and of the influence of the stocks of prey upon the population level of the carnivores. It was agreed that species of economic importance should not be studied, so that the work should not be done upon birds directly dependent on man's activities for a major part of their food supply. The species selected for study must be resident throughout the year, and must be abundant. Here a difficulty was found, for few bird species, even those usually described as 'common' or 'abundant' are represented in any habitat in such numbers that adequate sampling will not sensibly modify, or even destroy the stock in the course of the investigation. This difficulty in choice of species suitable for a direct approach was especially marked after the decimation of the stocks of so many passerine birds in the great frost of early 1947 (Ticehurst & Hartley 1948). It was therefore decided to make an indirect attack upon the problem by the application of a principle enunciated by Charles Darwin (1859): 'As the species of the same genus usually have, though by no means invariably, much similarity in habits and constitution, and always in structure, the struggle will generally be more severe between them, if they come into competition with each other, than between the species of distinct genera.' A group of closely related species, all reasonably abundant, was needed for work on the implication of this principle. In the English Midlands the Paridae (titmice) were an obvious choice. Four species were to be found regularly feeding in the same habitats: great tit, Parus major L.; blue tit, P. caeruleus L.; coal tit, P. ater L.; marsh tit, P. palustris L. Certain ecological conclusions could be drawn from the results of study of the feeding habits of four congeneric species living side by side. If no distinctions could be made between the foods, feeding places and feeding methods of the four species it might reasonably be assumed that food supply was not the factor limiting their numbers. Conversely, a measure of ecological separation between the species would justify the assumption of some degree of interspecific pressure. Lack