AUTUMN TERRITORIES AND POPULATION REGULATION IN THE NUTHATCH SITTA-EUROPAEA - AN EXPERIMENTAL-STUDY

Natural fluctuations in autumn nuthatch population size, which maintains territories year round, were positively correlated with the size of the hazelnut Corylus avellana crop, an imporant food. Two ringed populations of nuthatches in C Sweden were followed for 5 yr. Three large-scale feeding experiments were performed; sunflower seed was liberally supplied form June through November. Autumn population density was not correlated with population density in spring. As supplementary feeding was terminated before onset of winter, the season of food scarcity, and nuthatch populations were censused during autumn, the effects on the populations were not caused by direct competition for food and subsequent starvation, but rather by social behaviour. Population density in autumn increased by c85% when food was added, and the correlation with the hazelnut crop was broken. At the same time, mean territory size decreased by c40%. Under natural conditions juvenile nuthatches did not have smaller territories than adults, but there was a tendency for juveniles to react more strongly to supplementary feeding than adults, suggesting that the forces that determine optimal territory size partly differ between the age groups. -from Author