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Individual consistency in the arrival dates of territorial male Willow Warblers Phylloscopus trochilus

ABSTRACT The timing of migration is thought to have a genetic basis, and it might be expected that the arrival dates of individuals would be similar from year to year. However, there are many other factors which influence timing, and significant consistency in arrival dates has so far been reported for relatively few species of birds. In this study two methods of analysis – correlation and repeatability – were used to investigate individual consistency in arrival dates of territorial male Willow Warblers Phylloscopus trochilus returning to the breeding site. Both methods indicated significant individual consistency between years, for both adults and first-years. The use of relative arrival dates produced slightly better results than actual arrival dates, possibly due to a partial control for differences in weather conditions from year to year. Heritability of arrival dates was also investigated but the sample size was small and the correlation between known fathers and sons was not significant.