In many taxa, the most common form of sex-biased migration timing is protandry-the earlier arrival of males at breeding areas. Here we test this concept across the annual cycle of long-distance migratory birds. Using more than 350 migration tracks of small-bodied trans-Saharan migrants, we quantify differences in male and female migration schedules and test for proximate determinants of sex-specific timing. In autumn, males started migration about 2 days earlier, but this difference did not carry over to arrival at the non-breeding sites. In spring, males on average departed from the African non-breeding sites about 3 days earlier and reached breeding sites ca 4 days ahead of females. A cross-species comparison revealed large variation in the level of protandry and protogyny across the annual cycle. While we found tight links between individual timing of departure and arrival within each migration season, only for males the timing of spring migration was linked to the timing of previous autumn migration. In conclusion, our results demonstrate that protandry is not exclusively a reproductive strategy but rather occurs year-round and the two main proximate determinants for the magnitude of sex-biased arrival times in autumn and spring are sex-specific differences in departure timing and migration duration.

Department of Animal Ecology Ecology and Genetics Uppsala University Uppsala Sweden

Department of Biology and Centre for Environmental and Marine Studies University of Aveiro Aveiro Portugal

Department of Biology and Centre for Environmental and Marine Studies University of Aveiro Aveiro Portugal 5 South Iceland Research Centre University of Iceland Laugarvatn Iceland

Department of Bird Migration Swiss Ornithological Institute Sempach Switzerland

Department of Zoology Palacký University Olomouc Czech Republic 3 Museum of Natural History Olomouc Czech Republic

Institute of Vertebrate Biology The Czech Academy of Sciences Brno Czech Republic

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