Resource-Partitioning between Three Synoptic Thornbills (Acanthizidae : 'Acanthiza' Vigors and Horsfield)

Abstract

Resource-partitioning among three syntopic species of thornbill ('Acanthiza pusilla', 'A. reguloides' and 'A. lineata') were studied at Wollomombi on the Northern Tablelands of New South Wales. All three species are mainly insectivorous and weigh about 7 grams. Abundance of arthropods, on foliage and grass, was measured monthly. There were great differences between summer peaks and winter troughs (> 10:1) and also between the 'normal' year of 1978/79 and the two drought years that followed (ca. 4:1). ... The three species were ecologically separated as they fed on different substrates and on different species of plants and at different heights. 'A. pusilla' foraged mainly on dense foliage of understorey shrubs, 'A. reguloides' foraged mainly on the ground or on the bark of trees and shrubs, and 'A. lineata' foraged mainly on the foliage of eucalypts. Foraging behaviour changed seasonally and with the size and composition of the group. For example 'A. reguloides' fed high in trees when in small groups but on the ground when in large groups. All three species foraged differently when in mixed-species flocks, even in areas where they were the nuclear species. ... The monthly overlap in foraging, between species, ranged from 3 to 43%. Overlaps were lowest when food was scarce but during prolonged periods of scarcity rose to comparatively high levels well before food increased. I believe that at such times either that a species is forced by intra-specific competition, in its specialised foraging role, to broaden its niche; or that overall scarcity is so severe that all species have to abandon specialisation and search for whatever prey is available, with a consequent increase in overlap.