A comparison of the ecology of an assemblage of ground-dwelling birds in an arid environment

One assemblage of species which appears particularly sensitive to environmental changes in Australia since European settlement is the ground-dwelling birds. In order to diagnose the reasons why species decline, it is necessary to gain an understanding of the species' ecology and context. This study examines the ecological similarities and differences between five ground-dwelling bird species, which have differed in their responses to disturbance in Western Australia. Of the species studied, the Whitebrowed Babbler Pomatostomus superciliosus and Variegated Fairy-wren Malurus lamberti remain common throughout pastoral and wheatbelt areas. The Southern Scrub-robin Drymodes brunneopygia and White-browed Scrubwren Sericornis frontalis have declined in some areas of the wheatbelt but not in pastoral areas, whereas the Thick-billed Grasswren Amytomis textilis has disappeared from the wheatbelt and has shown marked contraction in its range in pastoral areas. From 1994 to 1996 inclusive, birds were studied within a 55 hectare area on Peron Peninsula, Western Australia. Colour-banding of individuals confirmed that all species were sedentary, with considerable interspecific overlap of territories or home ranges. Scrub-robin pairs showed a high degree of site fidelity and required relatively large areas of suitable habitat to establish territories. Scrub-robins and grasswrens gleaned most of their prey from litter and sand. The babbler probed within litter and sand, as well as in crevices or under the bark of dead branches on and above the ground. In contrast, the scrubwren and fairy-wren gleaned prey from branches and foliage. All species consumed a wide variety of invertebrates and some berries, and showed broad similarities in the types of food taken. Relative to the other bird species studied, the scrub-robin's diet had a large ant component, while the grasswren's diet had a substantial seed component. The grasswren was the most specialised in its use of nest substrates, with nests placed in the centre of dense low shrubs or climbers. The scrubwren and scrub-robin predominantly used ground substrates, namely litter and dead branches. Fairy-wrens used a wide variety of nest substrates low to the ground, including dead and live plant material, while babblers nested in a selection of tall shrubs and acacias. Each bird species showed similarities with at least one other species in foraging or nesting substrate use. Despite these similarities, the responses of these five species to post-settlement habitat changes in Western Australia have differed. Three species which have declined in the wheatbelt, namely the scrub-robin, scrubwren and grasswren, showed a preference for litter substrates in which to nest or forage within. In the wheatbelt region, the degradation of the litter layer is often greater than that found in pastoral areas. The loss of suitable nesting or foraging litter substrates may explain the decline of the scrub-robin and scrubwren in some wheatbelt areas. As well as foraging on the ground, the grasswren required specific nesting substrates in the shrub layer and therefore may be susceptible to grazing-induced changes in the shrub community in both wheatbelt and pastoral regions. In contrast, the babbler and fairywren foraged in a wider range of substrates above the ground and used a wide range of nest substrates, including tall shrubs and dead substrates, which remain abundant in disturbed areas. In Acacia Sandplain habitat on Peron Peninsula and the adjacent pastoral property of Nanga Station, grasswren presence was correlated with the abundance and structure of potential nesting substrates. It is possible that grazing-induced changes to the foliage density and plant species composition of the shrub layer may increase the risk of grasswren nest predation. Alternatively, these changes in the shrub community may decrease an important seed component of the grasswren’s diet