Multiple strategies and cues define baleen whale migration.

Migration is a do-or-die life-history strategy, yet factors shaping it remain poorly studied in many marine taxa, as direct observations are difficult. Building on animal migration theory, we used satellite tracking data from 21 southern right whales tagged in Aotearoa New Zealand to investigate migratory cues, timing and routes. Whales followed either a direct westward route or a longer, indirect route via higher latitudes when migrating from their wintering grounds to summer feeding grounds. Departure timing from wintering grounds was the only significant predictor of route choice, independent of sex, age class, body length or condition. Early-departing whales were more likely to follow the longer route, taking more days to migrate, yet they arrived at shared feeding grounds at similar times to whales using the direct route. These contrasting behaviours are hypothesized to reflect trade-offs between time- and energy-minimization strategies. During migration, whales may use multiple external cues to navigate, including gradients associated with ocean currents, bathymetry and magnetic inclination. The availability of two distinct migratory routes, combined with individual flexibility in route choice, may enhance population resilience in a changing ocean.