| Summary: | In many species, na€ive first-time migrants undertakemigration without adults, supposedly on the basis of a‘simple’ vector programme that combines an innatedirectional preference with a temporal programme thatspecifies distance. In strongly dimorphic species in whichthe sexes show distinct ecological requirements, theinnate mechanisms of navigation may be expected todiverge between males and females with respect to theirspecific destinations. Based on captures of juvenileRuff Philomachus pugnax at a migratory stopoverover 21 years, a correlation was found between theyear-specific sex ratio and the global magnetic field disturbanceduring the 2 weeks prior to the peak ofcaptures in that year. This suggests that males andfemales respond differently to geomagnetic disturbancewith changes in either the direction of migration or thelevel of migratory activity, and implies sex-specificity inthe use of their geomagnetic navigational ‘toolbox’
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