A trans-African migrant shows repeatable route choice in males and repeatable timing in females

Migrant bird populations often show substantial variation in route choice and timing. Determining whether this population-level variation is driven by between-individual differences and/or flexibility within individuals is key to identifying drivers of migration patterns. 'Repeatability' (R, the proportion of population-level variation attributable to between-individual variation) has become a central metric for the relative consistency of individual behaviour. Individual repeatability in migratory route choice and timing is often reported to vary between seasonal and regional contexts and may also differ between demographic groups (e.g. sexes), but interpreting repeatability requires careful consideration of the underlying changes in between- and within-individual variation. We GPS-tracked repeat migrations for eight male and five female Eleonora's falcons Falco eleonorae and quantified the magnitude of within- and between-individual variation and the individual repeatability of their seasonal routes and timing at 100 km intervals all across Africa. We did this across both sexes, and then separately for males and females. We found greater between-individual variation in spring routes, albeit with substantial regional fluctuations in both seasons. The greatest between-individual variation in routes occurred during the spring desert-crossing, but this coincided with high within-individual variation, and thus only low repeatability of route choice. Route repeatability instead peaked (R = 0.6-0.8) through the Horn of Africa in spring, and during the rainforest-crossing in autumn. Variation and repeatability of timing was stable across regions, with generally higher between-individual variation and repeatability in spring. Sex-specific analyses suggest males exhibit higher route repeatability, while females exhibit stronger seasonal contrasts in timing repeatability. Such sex differences were unexpected, but overall, between-individual variation and repeatability in routes and timings appear greater where environmental and annual cycle constraints are weaker. Route repeatability is especially high where falcons show fidelity to stop-over sites, or individual barrier-crossing preferences. Individual routines may be acquired through early-life exploration-refinement.