Movement ecology of Australian arid-zone birds

Abstract

The movements of nomadic species are poorly understood, being highly dynamic over time and space. There is an urgent need to better understand this group as current conservation approaches appear not to be providing adequate protection. In this thesis, I evaluate a survey method to monitor this dynamic group, assess environmental variables driving their movements over time, and explore how rainfall structures the overall avian community.
To date, no standardized, large-scale monitoring has been carried out for arid zone or nomadic species. In Chapter 2, I describe a protocol for surveying this group over five years using two survey methods. Detection probabilities and robust density estimates were generated for 64 species and showed the majority fluctuated markedly over time. Line transect surveys were more effective for species richness and abundance measures. This survey method provides the first standardized density estimates for this assemblage and can be easily repeated in future for this and other remote, arid biological communities.
The movement characteristics of species with unpredictable, aseasonal movements are very poorly understood and difficult to measure. In Chapter 3, I develop a method for quantifying extent of bird movement by analysing changes in species’ site persistence and variability in inter-annual densities over time, and compare results with existing movement classifications. Continuous variation in extent of species movement indicated that a binary grouping of resident versus nomadic species is inappropriate. Existing movement classifications likely underestimate species movements within arid regions of their distribution, suggesting that caution is needed when using sweeping species-level classifications, especially for Australian birds whose movements can be heavily environment-dependent.
Unlike regular migration, which is triggered by seasonal cues such as day length and temperature, movement in nomads is thought to be triggered by less predictable environmental conditions. In Chapter 4, I explore the relative influence of dynamic and static environmental variables on species abundance. Dynamic variables were more important for nomadic versus resident species but static variables were equally important for nomadic and resident species. These findings suggest that habitat structure is important to consider in addition to dynamic environmental features for understanding nomadic species movements.
Pulse events are thought to be positively correlated with nomadic species movements and to play an important role in structuring arid biological communities. In Chapter 5, I investigate how structural changes observed in Australia’s arid bird community relate to rainfall and vegetation greenness. The importance of rainfall was mediated through vegetation growth and the community is dynamic at a local level but exhibits stability at a landscape level, underscoring the importance of connectivity between suitable habitats within this landscape to enable species to reshuffle among sites.
In sum, my thesis highlights the importance of standardized data for enabling an empirical approach to understanding nomadic and arid-zone bird species. Findings will advance our understanding of these species’ dynamics and lay groundwork for improving protection of this group by identifying further research priorities.