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Lesser Knots (Calidris canutus) are high-Arctic breeding waders that migrate to temperate and tropical regions for the non-breeding season. Seasonal mass changes were examined in Lesser Knots in New Zealand at the southern end of their migration. Adults showed a large increase in mass in February before their northward migration in March. They were estimated to depart with a ‘fat’ load of around 45%. Subadult birds, most of which winter in New Zealand over the northern breeding season, also showed a mass increase. Mass increases in winter are well documented for European waders but contrary to the European situation, this increase in subadult birds in New Zealand is unlikely to be an adaptive strategy to insure against periods of negative energy balance. Instead, it may be an endogenously orchestrated byproduct that has not been selected against in the pre-migratory period. Such increases may be more widespread in Arctic waders in the Southern Hemisphere than is realised.