Data from: Sex-specific arrival times: hybridizing skuas provide empirical support for the role of sex-ratios

In migratory animals, protandry - earlier arrival of males on the breeding grounds - prevails over protogyny - females preceding males. In theory, sex-differences in timing of arrival are driven by the operational sex ratio, shifting towards protogyny in female biased populations. However, empirical support for this hypothesis is so far lacking. To test this hypothesis, we analysed arrival data from three populations of the long distance migratory South Polar Skuas (Catharacta maccormicki). These population differed in their operational sex ratio caused by the unidirectional hybridisation of male South Polar Skuas with female Brown Skuas (C. antarctica lonnbergi). We found that arrival times were protandrous in allopatry, shifting towards protogyny in female biased populations when breeding in sympatry. This unique finding is consistent with the theoretical predictions that sex-specific arrival times should be influenced by sex ratio and that protogyny should be observed in populations with female biased operational sex ratio.