Food density, song rate, and body condition in territory-establishing willow warblers (Phylloscopus trochilus)

Abstract

Early-arriving small male willow warblers (Phylloscopus trochilus) in good body condition were relatively successful at defending their territories. Later arriving neighbouring males tended to be larger but had poorer body condition. When males arrived, food resources were limited but increasing. The song rate varied among males during the period before pair formation and was positively correlated with the male body condition index. Song phrase length and song rate decreased after the performance of other activities, which suggests that singing was energy demanding. The sequence of pairing correlated well with male arrival, song rate, and food abundance. Females apparently take food abundance into account when selecting a mate. A model is presented to explain the adaptation of male and female willow warblers to the availability of food resources in time and space and the role of the male in securing a high-quality breeding territory.