Where nothing stands still: quantifying nomadism in Australian arid-zone birds

Context: Nomadism is a movement strategy in response to non-seasonal environmental variability. Knowledge of nomadic species movements is poor but is necessary to understand life histories and develop appropriate conservation strategies. Objectives: We provide a first quantification of nomadism among Australia’s arid bird community, which is presumed to be highly nomadic, by measuring variation in species’ occurrence and abundance among years to determine whether there are clear nomadic and non-nomadic strategists. Methods: We surveyed birds annually from 2012 to 2016. We measured how many years each species was present at a site and estimated inter-annual variability in species abundance, using both measures to infer species movement patterns. We used results to inform existing movement classifications. Results: Most arid species showed low site persistence, with species detected at the same site, on average, 1.8 out of the five survey years. Movement varied along a continuum rather than grouping into distinct nomadic and non-nomadic groups. Species classified as nomadic showed higher variation in abundance and lower site persistence than species classified as resident. Our method of quantifying nomadism closely replicated existing expert-derived movement classifications of arid zone bird species. Conclusions: Rather than a fixed attribute, movements of many species in our study can be heavily environment-dependent, and individuals of a single species can display a continuum of movements in different times and places. This complicates the conservation of species, but the growing recognition of the complexity of species movements offers opportunities for a more nuanced understanding of the relationship between species and environment