On sex-specific foraging behaviour in the Willow Warbler, Phylloscopus trochilus

Abstract

The foraging behaviour of Willow Warblers (Phylloscopus trochilus) in relation to the availability of their insect prey was studied at the Tovetorp Field Station and at the Royal Park of Djurgården, Stockholm, Sweden, from 1985 through 1989. The numbers and types of arthropods available to Willow Warblers fluctuated throughout the breeding season. In general, most arthropods were found on the underside of the leaves. Males spent more time gleaning and less time hovering and fly catching when preying chiefly on aphid nymphs then when foraging on other arthropods on birch before budbreak. They performed more fly-catching behaviour and tended to forage in taller trees than did females. Dissimilarities in foraging behaviour between the sexes were attributed to differences in wing length and wing shape. Because females are smaller and have less pointed wings they are better adapted for hovering than males. Consequently, females were better at capturing prey on the underside of birch and willow leaves, which was abundant during the egg-laying and brood-rearing periods. Thus, in birch and willow, females hovered more frequently than males, but there was no difference between males and females foraging in spruce, pine, and oak.