Individual variation in migration patterns of Iberian little bustards.

Background: Migration is a critical behavioural strategy across many taxa, allowing individuals to respond to seasonal shifts in environmental conditions and resource availability. Understanding animal movements, their plasticity, and their variation within-and among-individuals, is essential for assessing species resilience to environmental changes and developing effective conservation strategies. This study investigates the individual variation in migration patterns of the little bustard (Tetrax tetrax), one of Europe's most threatened birds, using tracking data from 66 individuals from Southwest Iberia.

Methods: We applied an innovative, yet straightforward approach, combining cluster analysis on net displacement values with boosted regression trees, to classify migration patterns of 105 year-round tracks and explore among-and within-individual variation in migratory behaviour. This approached was then complemented with traditional linear mixed models on the daily ND values throughout the year.

Results: Our analysis identified four main migration patterns: residents (63.8%), short-distance summer migrants (22.9%), medium-distance summer-winter migrants (6.7%), and long-distance summer migrants (5.7%). Different strategies were found in the same years and breeding areas, suggesting a high among-individual variation in migratory behaviour. Most individuals tracked for multiple years maintained consistent strategies (76%), suggesting a low intra-individual variation in migratory behaviour. This high among-and low within-individual variation was supported when using linear mixed models. While migratory polymorphism (resident vs. migrant) can provide populations with adaptability to environmental changes, the predominance of resident little bustards in Southwest Iberia may become maladaptive due to accelerating environmental changes in the region, particularly as a result of agriculture intensification and rising temperatures. Nevertheless, almost a quarter of all tracked individuals (24%) changed strategies between years, indicating some capacity to adapt to shifting conditions.

Conclusion: The framework presented here for little bustards can be easily applied to other tracking data, guiding conservation strategies and offering a practical tool for classifying movement patterns and exploring individual variation or plasticity.