The effects of needle loss in coniferous forests in South-west Sweden on the winter foraging behaviour of willow tits Parus montanus

Abstract

The foraging behaviour of the willow tit Parus montanus was studied in two coniferous forest areas in SW Sweden, showing high (H) and low (L) percentages of needle loss, respectively. The needle loss is presumed to be accelerated mainly by air pollution and leads to (1) thinning of the tree conopies which could make birds easier for predators to detect and (2) reduction in the abundance of arthropods, which are important food of wintering passerines. Willow tits foraging in pine Pinus sylvestris in the H-area spent proportionally more time scanning for predators and less time handling prey than individuals in the L-area. In the H-area the tits also joined larger mixed-species flocks which included more species. These responses are interpreted as increased time stress in damaged forest but nevertheless the birds showed no tendency to lower their body mass. The results are discussed in relation to winter survival and the consequences of needle loss for bird populations in coniferous forests.