[A new method of gatheing ecological data on the common buzzard (Buteo buteo) and on the field mouse (Microtus...

In treeless regions the common buzzard uses the towers of electric overhead transmission lines (110-150 kV) for roosting. The streams of excrement bring about flash-overs which can be registered according to date and time of day. These records provide information as to diurnal and annual rhythms, as wells as to the relationships existing between topographical and climatic factors and the population density of the birds.Ninety per cent of the animals leave the roosting place between 42 minutes before and 11 minutes after sunrise. An increase of one unit in cloud density leads to a mean delay of about 2.8 minutes in departure. Excretion during the night exhibits a definite periodical variation. The relationship between population numbers in winter and summer can be measured quantitatively. Regressions for the influence of climatic factors on fluctuations of population density are given. The population dynamics of the field mouse is reflected in the frequency variations of the flash-overs set off by the excrement of the buzzards.