The Social Organisation of the White-browed Scrubwren Sericornis frontalis Gould (Acanthizidae) in Arid, Semi-arid and Mesic Environments of Western Australia

Abstract

The social organisation and demography of White-browed Scrubwrens Sericornis frontalis were studied in arid, semi-arid and mesic environments of Western Australia over three years. The highest incidence of co-operative breeding was observed in the semi-arid environment whereas it was least common in the arid environment. Social grouping was also a common occurrence in the mesic environment, but only a small proportion of these groups bred co-operatively. Auxiliary (non-helping) membership in the groups seems to be due to habitat saturation resulting from a high survival rate of birds in the population. Helpers contribute significantly to the feeding of nestlings and the maintenance of nest hygiene and, on average, co- operatively-reared nestlings fledge one to two days earlier than those reared by parents only. This shortened nestling period increases the probability of young fledging before the onset of unfavourable environmental conditions. The low incidence of co-operative breeding in the arid environment seems to be due to the slight competition for breeding space as a consequence of a high mortality rate amongst immature and adult birds. This high mortality rate may be due to the inability of birds to cope with harsh summer conditions after the depletion of body reserves resulting from recent breeding attempts.