Ecology and conservation of endangered forty-spotted pardalotes

Abstract

The forty-spotted pardalote (Pardalotus quadragintus) is an endangered songbird endemic to eastern Tasmania. These birds specialize on forests containing Eucalyptus viminalis (white gum tree) for foraging, and require tree cavities for nesting. They have experienced both historical range contraction following European settlement and recent population decline within existing habitat. Causes of this decline are uncertain. Ecological theory predicts that habitat specialists will be most vulnerable to habitat loss, rather than factors that disrupt the balance of fecundity and mortality. However, cavity-nesting birds tend to invest heavily in individual nest attempts, and as a result, may be vulnerable to competitors, predators, or parasites which attack the nest. To examine current threats to forty-spotted pardalotes, I conducted three seasons of fieldwork in southeastern Tasmania, monitoring the breeding biology of forty-spotted pardalotes and their generalist competitors, striated pardalotes (Pardalotus striatus). I also collected blood samples from forty-spotted pardalotes to examine their population structure and demographic history. A model presentation experiment and observations of nest takeovers revealed that striated pardalotes were the dominant competitor and usurped 10% of forty-spotted pardalote nests, whereas forty-spotted pardalotes never usurped striated pardalotes. However, the largest source of nest failure was a native ectoparasitic fly (Passeromyia longicornis) which killed 81% of nestlings. Encouragingly, I found that forty-spotted pardalotes occupied nest boxes, resulting in high breeding densities and similar nest success compared to natural cavities. Finally, I developed a dataset of 57,868 SNPs from 159 individual birds sampled throughout major populations in southeastern Tasmania. Despite its larger population size, Maria Island had the least genetic diversity of any population, and there was no evidence of migrants between it and the southern populations. There was little population differentiation and substantial gene flow between Bruny Island and the nearby mainland populations, as well as evidence of ancestry from an extirpated mainland population. This study shows that conservation of habitat specialists needs to include protection of habitat features such as tree cavities and the extent of E. viminalis forest, but also needs to address threats within existing habitat, such as nest parasites. My finding that a native fly parasite is the principal source of mortality forty-spotted pardalote nestlings adds to growing concern about the increasing impacts of avian ectoparasites on their hosts in island systems.