Winter breeding of a subtropical rainforest passerine: spatio-temporal distribution of prey and the sex-related foraging behaviour of the Southern Logrunner, Orthonyx temminckii

Abstract

Most birds breed when food availability is at a maximum. The Logrunner Orthonyx temminckii, a ground-dwelling subtropical insectivore, breeds during the winter-dry season, when its invertebrate prey is believed to be least abundant. We investigated the spatio-temporal distribution of leaf-litter invertebrates and linked this to the foraging ecology and breeding of Logrunners. Fortnightly leaf-litter invertebrate samples revealed that the biomass of Logrunner prey did not vary significantly between the late-wet, early-dry and late-dry seasons, or between three microhabitats—forest interior, treefall gaps and gap edges. Further, the biomass of insect larvae, a nutritious and easily handled food source for young birds, remained constant through egg laying, hatching and fledging. During the dry winter Logrunner breeding season, the rainforest leaf litter may retain moisture and provide thermal insulation, thereby maintaining suitable habitat for invertebrate populations. Whereas invertebrate biomass remained stable during the breeding season, Logrunner foraging rates increased, presumably to increase calorific intake. Although not feeding nestlings directly, males provisioned females away from the nest. Therefore, we suggest that an interplay of several drivers has led to the evolution of winter breeding in these ground-foraging rainforest passerines. Although longer term investigations are required to determine whether their food source is more consistent in winter than summer, our data show that winter breeding coincided with the ready availability of nutritious, high calorie, leaf-litter invertebrates. This would enable Logrunners to rear chicks at a time that is usually considered inimical to these purposes.