The behavioural ecology of the thick-billed grasswren

Author: Marina Louter

Louter, Marina, 2016 The behavioural ecology of the thick-billed grasswren, Flinders University, School of Biological Sciences

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This study uses the conservation behaviour framework to understand the ecological context of nesting behaviour and fledging success in a previously unstudied threatened songbird: the thick-billed grasswren (Amytornis modestus raglessi). Data were collected from 2012 to 2015 at Witchelina Nature Reserve, a former pastoral sheep station, located in the arid rangelands of central South Australia. In 2010, the Nature Foundation of South Australia purchased the 124,000 ha reserve and removed all livestock. The re- vegetation and recovery of overgrazed land to ‘natural’ habitat is an opportunity for flora and fauna conservation. This study provides a test of the efficacy of habitat restoration from grazing as a conservation tool for the persistence of a threatened songbird species.

In this thesis I address four main issues in relation to vegetation cover: (1) grazing history and habitat use, (2) predation risk and predation outcome, (3) insect abundance and parental care (including prey size), and (4) genetic relatedness within groups. I radio-tracked adult males to show that the average home range size of thick-billed grasswrens is large (8 ha) and did not differ across the study site. I found that active grasswren territories were more likely located at sites with low prior grazing intensity; vegetation cover and insect abundance were significantly associated with grazing history. I measured predation risk at Witchelina Nature Reserve using artificial nests: rodent predation risk was ~10 %, avian predation risk was ~12 %, and observed predation at natural nests was ~13 %. Exposed artificial nests with little vegetation concealment were depredated by avian predators more often; at natural grasswren nests, active nests had dense vegetation cover and vegetation concealment did not predict predation (suggesting snakes and rodents as main predators). Predation at natural nests was low; of 47 thick-billed grasswren nests, most (74 %) produced fledglings. Vegetation cover predicted parental feeding at nests; parents at sites with dense cover fed nestlings more frequently. Video recordings of feeding at nest showed that grasshoppers and caterpillars were commonly fed to developing nestlings.

Finally, this study shows that thick-billed grasswrens are cooperative breeders, with up to two helpers observed feeding at 50 % of video-monitored nests. Using ddRAD-seq genetic analysis we established that helpers were related to the attending pair. We did not find evidence for extra-pair paternity in offspring of five thick-billed grasswren groups.

In conclusion, vegetation cover was significantly associated with grazing history: areas with high previous grazing history had little vegetation cover and few arthropods. Vegetation cover was related to key parameters of nesting success of the thick-billed grasswren, including presence of active territories, parental feeding activity, and avian nest predation. Collectively, the results of his research can be directly implemented in focused recovery plans for the vulnerable thick-billed grasswren. Conservation of habitat with dense vegetation cover and abundance of preferred chenopod nesting shrubs Maireana pyramidata and Maireana astrotricha throughout the range of the thick-billed grasswren is recommended as a key management approach to conserve this previously unstudied arid zone species under threat.

Keywords: Amytornis, behaviour, conservation, ecology, grasswren

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2016
School: School of Biological Sciences
Supervisor: Sonia Kleindorfer