Noisy Scrub-bird (Atrichornis clamosus) Recovery Plan
Population data that could be used in a calculation of effective population size Ne, the
Mt Gardner population
Census size: Numbers of singing males (those defending a territory) are given in Table 1. Important points are that the number of singing males dropped to a low of about 40 in the 1960s (and may have dropped as low as 20 to 30 in the 1940s (Smith 1985d) and the number has steadily increased since 1971. Rate of annual increase at Two Peoples Bay from 1970 to 1980 was 8.6% per year (range -2.2% to 18%) and from 1980 to 1991 was 6.4% per year (range -7.3% to 14.6%). At Mt Manypeaks the mean annual rate of increase from 1987 to 1991 was 54.3% (range 26.9% to 81.8%). The carrying capacities of Mt Gardner and Mt Manypeaks are not known, but the carrying capacity of the latter could be estimated from aerial photographs.
The relationship between the census size (number of singing males) and the actual population size (N) is not known. It is known, however, that sub-dominant males and females can exist within defended territories. In one area, after the dominant male was removed three males were successively removed in the same breeding season . However, in other territories where the dominant male was captured he was not replaced in that year. In the past we have assumed a conservative ratio of singing males to N of 2.5. In this Recovery Plan we assume that the minimum viable population size of the Noisy Scrub-bird equates to a census size of 40 singing males, provided that > 90% are in optimal breeding habitat.
Lifetime dispersal: Not known, but males are known to have moved up to 8 km (straight line map distance) from Lakes population to Mt Taylor and also from Lakes population to upper Angove River. Other data are Mt Gardner to Lakes: 3 km; translocated males at Mt Manypeaks up to 5 km from release site, but dispersal from established territories mostly less than 2 km. Evidence at Mt Gardner suggests that only a small proportion of non-territorial males disperse. A female banded as a nestling was recaptured at a nest 2 km away two years later. One female dispersed 500 m to her breeding site one year after fledging and another moved 1200 m.
Generation length: Females can breed at year 1. The oldest known breeding female was age 10 (although this animal spent virtually all of her known life in captivity). Captive data suggest age at first breeding for males is 2-3 years.
Reproductive output: The clutch size is one and females raise a maximum of one chick each year. A second (or even a third) nest is built if the first egg is lost. Lost chicks are not replaced.
Breeding system / Social System: Males are opportunistically polygamous. The dominant male defends a territory and has the opportunity to mate with females who have nesting areas that may overlap or be adjacent to his territory. Sub-dominant males may occur within male territories and may be capable of mating with females even though they are not defending a territory (based on a single sub-dominant male that died during capture which had active gonads).
Sex ratio: Not known to be significantly different to 1:1.
Life table: Parameters not known; the only indication of mortality is based on population increase rates of singing males at Mount Manypeaks (26.9 to 81.8% per annum, mean 54.3% per annum over the four years 1987-1991).