Demography of the Cooperatively Breeding Splendid Fairy-Wren, Malurus-Splendens (Maluridae)

Abstract

We studied the demography of the splendid fairy-wren, Malurus splendens, a group-territorial, cooperative-breeding passerine, near Perth, Western Australia, from 1973 to 1990. This period included 13 years of below-average rainfall, a number of minor wildfires and one that burnt 95% of the study area in January 1985. Annual survival of breeding males (mean = 70%) and females (mean = 59%) was higher during a period without fire. Survival of breeding females fell to less than 50% from 1987 to 1989, two years after the major fire. Survival of adults did not vary with age, but survival of juveniles was lower (mean=31%) and variable (range 11-59%), particularly among those produced in the first breeding season after the fire in 1985. Splendid fairy-wrens bred during the Austral spring, with minor variation in length of the breeding season depending on rainfall at the start and temperature at the end of the breeding season. Clutch size varied little, but females laid more clutches in years when predation or brood parasitism were high, particularly in the two years after the major fire. Females produced a mean of 2.9 fledglings each season (4.7 in the best year), but productivity was low for several years after the 1985 fire. Helpers attended 60% of all nests but overall had little effect on annual fledgling production. Experienced females were more productive than novices, both with and without helpers. High reproductive effort, indexed by days nesting and caring for fledglings, did not reduce a female's chance of survival to the next breeding season, and annual reproductive effort was not correlated with female survival. Differential dispersal and mortality produced variation in the adult sex ratio. During the fire-free period 1978-84, the numbers of non-breeding males and females increased, because of a lack of available territories rather than a lack of available partners. Variation in natality and survival caused large variations in population density but the density of breeders was more constant (C.V. = 24%) than than of helpers (C.V. = 57%). No decline in the population after the major fire of January 1985 was apparent until the 1988 breeding season; the number of groups declined from 1988 to 1990, some previously occupied territories became vacant and group size decreased. This delayed decline in population is attributed to decreased production of fledglings in the years after the fire and the gradual replacement of experienced breeding females by novices, which were less productive and suffered higher mortality. The major causes of demographic variation (brood parasites and fire) directly affected natality and juvenile survival; indirectly, population density, age structure, sex ratio and group composition were affected. Delayed dispersal, the proximate cause of cooperative breeding in splendid fairy-wrens, is favoured by this demographic environment. The widespread occurrence of similar demographic profiles may in part explain the high frequency of cooperative breeding in Australian birds.