• average planetary magnetic index;
  • Finland;
  • sex differences;
  • sex ratio

In many species, naïve first-time migrants undertake migration without adults, supposedly on the basis of a ‘simple’ vector programme that combines an innate directional preference with a temporal programme that specifies distance. In strongly dimorphic species in which the sexes show distinct ecological requirements, the innate mechanisms of navigation may be expected to diverge between males and females with respect to their specific destinations. Based on captures of juvenile Ruff Philomachus pugnax at a migratory stopover over 21 years, a correlation was found between the year-specific sex ratio and the global magnetic field disturbance during the 2 weeks prior to the peak of captures in that year. This suggests that males and females respond differently to geomagnetic disturbance with changes in either the direction of migration or the level of migratory activity, and implies sex-specificity in the use of their geomagnetic navigational ‘toolbox’.