Food Resource Distribution and the Organization of the Parus Guild in a Spruce Forest

Abstract

We studied the abundance of arthropod food within spruce Picea abies in relation to the foraging site selection of tits Parus and the Goldcrest Regulus regulus in winter in Central Sweden. Arthropods, spiders in particular, were most abundant in upper tree parts and in the outer sections of branches. There was a high correlation between food abundance and bird density, based on proportional use of foraging sites over eight sections of spruce suggesting that food distribution is a major factor influencing the segregative distribution of the four bird species. However, the distribution of the bird species cannot be explained solely by food abundance. In outer tree parts there were fewer birds than expected from arthropod biomass. A possible explanation is that the risk of predation by Pygmy Owls Glaucidium passerinum is higher there, reducing the net benefit of foraging in the tree parts with highest food abundance. A further support for the impact of predation comes from the fact that it is the socially least dominant species, the Goldcrest and the Coal Tit P. ater, that use the outer tree parts. Crested Tits P. cristatus are highest in dominance rank, but they mainly forage in the inner tree parts. Willow Tits P. montanus are second in the dominance, but have the lowest food abundance in their foraging sites. Both interspecific competition over food, and the site-dependent risk of predation, seem to govern the ways in which the four bird species select their foraging sites in spruce in winter.