Stable isotopes reveal regional movement patterns in an endangered bustard

Stable isotope analysis is a valuable technique to infer animal movement between isotopically distinct landscapes. For birds in terrestrial systems, it is usually only applied at continental scales, often relying on global isotopic patterns. In contrast, we used this technique to investigate movement patterns of Ludwig's bustard (Neotis ludwigii) at a regional scale, where such information is needed to improve the conservation status of this species. We analysed carbon and nitrogen isotopic compositions of feathers from bustards across two biomes of the semi-arid rangelands of the Karoo, South Africa, to investigate movement and explore sex and age movement strategy differences. We used a linear discriminant function analysis based on growing feathers to classify fully grown feathers to a Succulent or Nama Karoo biome origin. Six of 12 birds for which all primary feathers were analysed had at least one feather classified as having grown in the Succulent Karoo, supporting the theory that these birds are partial migrants. Feathers from two satellite-tracked bustards broadly supported the conclusions of the analysis, although food base differences resulting from local rainfall variation probably obscured geographic signals at finer scales. There was no apparent difference in movement strategies between the sexes, but juvenile feathers were almost exclusively assigned to the Nama Karoo, suggesting that most breeding occurs in this biome. Adult and juvenile feathers also had significantly different isotope ratios, which could relate to diet or to differing metabolic processes. This study demonstrates that with a good understanding of the system, carbon and nitrogen stable isotopes can be useful to infer general movement patterns of birds at a regional level.