Demography and social organisation of the red-winged fairy-wren, Malurus elegans

The red-winged fairy-wren, Malurus elegans, is endemic to the high-rainfall region of south-western Australia. We studied it in Eucalyptus diversicolor (karri) forest near Manjimup, Western Australia from 1980 to 1995. After a detailed study of breeding biology during 1980-86, we monitored dispersal and survival in known groups during 1987-95. M. elegans bred cooperatively, with 83% of groups (mean size 4.1) including one or more non-breeding males or females that helped to rear young and defend the territory. Survival of breeding adults (78%) and helper males (76%) was high. Territories and groups persisted from year to year, even though one or other of the breeding pair was replaced. Most known dispersals were to a group only 1-2 territories distant. Dispersal was female-biased, mostly in their third or fourth year. A behaviour not recorded in other Malurus spp. was that some birds, chiefly females, joined groups as helpers. The feeding rate of nestlings was not related to group size, but in larger groups the share of work done by the breeding female decreased. Helpers did not enhance the survival of breeding females, and had little overall effect on the production of fledglings. Females produced a mean of 2.4 fledglings, 1.8 independent young and 1.1 yearlings per year; survival of fledglings to the start of the following breeding season was 44.2% (31-61%). We argue that the high levels of adult and juvenile survival influence many aspects of the social system in M. elegans, such as large groups, the presence of female helpers, occurrence of immigrant helpers and delayed dispersal. We suggest that an important benefit of delayed dispersal and group living is in promoting the survival of young birds, and increasing their chance of acquiring a territory.