Autumn Territories and Population Regulation in the Nuthatch Sitta europaea: An Experimental Study

Publisher Website
Google Scholar
Abstract
(1) Natural fluctuations in autumn population size of the nuthatch (Sitta europaea), a passerine bird that maintains territoires the year round, were positively correlated with the size of the hazelnut (Corylus avellana) crop, an important food. (2) To study the influence of the autumn food supply in more detail, two ringed populations of nuthatches in central Sweden were followed closely for 5 years. (3) Three large-scale feeding experiments were performed. During each experiment sunflower (Helianthus annuus) seed was liberally supplied from June through November. (4) Autumn population density was not correlated with population density in spring. (5) As supplementary feeding was terminated before the onset of winter, which is 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. (6) Population density in autumn increased by about 85% when food was added, and the correlation with the hazelnut crop was broken. At the same time, mean territory size decreased by about 40%. (7) Under natural conditions juvenile nuthatches did not have smaller territories than adults. However, there was a tendency for juveniles to react more strongly to the supplementary feeding than adults, suggesting that the forces that determine optimal territory size partly differ between the two age groups.