Availability of nest hollows and breeding population size of eclectus parrots, Eclectus roratus on Cape York Peninsula, Australia

Abstract

The distribution of the Australian mainland endemic subspecies of the eclectus parrot, Eclectus roratus macgillivrayi, is currently confined to the lowland rainforests of the Iron-McIlwraith Ranges of eastern Cape York Peninsula. Females breed in large hollows in emergent rainforest trees that are readily visible from above. Aerial surveys were used to sample 58% of the rainforest (454 km2) of the Iron Range region to estimate the density of these nest trees. Corrections for overcounting bias (not all observed emergent trees were active nest trees) and undercounting bias (not all active nest trees were visible from the air) were made by ground-truthing over 70 trees. The tree count data were treated in two different ways, producing estimates of 417 (s.e. = 25) and 462 (s.e. = 31) nest trees for the Iron Range region. Long-term observational data on the number of eclectus parrots associated with each nest tree were used to estimate the population size of eclectus parrots at Iron Range: 538-596 breeding females, and 1059-1173 males. These results have three implications. First, this relatively low population estimate suggests that the Australian subspecies of eclectus parrots should be considered vulnerable to habitat loss or perturbation, especially in light of their complex social system, male-biased adult sex ratio, low breeding success and high variance in reproductive success among females. Second, the low density of nest trees suggests that eclectus parrots are absent from the rainforests of Lockerbie Scrub and the Jardine dunefields because these areas are too small. Finally, if eclectus parrots persisted in the Iron-McIlwraith region during the rainforest contractions of Pleistocene glacial maxima (e.g. 14000-17000 years ago), the refugium in this region must have been fairly substantial in order to support a viable population - probably larger than previously assumed.