Timing and Speed of Migration in North and West European Populations of Sylvia Warblers

Timing and speed of migration within Europe and North Africa are analysed in five Sylvia warblers using North European and British ringing recoveries. Intraspecific comparisons between populations breeding in Great Britain and in Northern Europe show no difference in the timing of autumn migration but about three weeks earlier spring arrival in Great Britain, indicating that northern populations spend a shorter time on the breeding grounds. Autumn migration speed estimates based on distance and elapsed time between consecutive captures vary between 43 and 93 km d-1 depending on species and origin. High speeds of birds reported up to ten days after ringing suggest that some individuals have long flight-stages when they pass through Europe. Populations breeding in Northern Europe migrate at a higher speed than those in Great Britain. Shorter time on the breeding grounds and higher speed during autumn migration in more northern populations as well as higher speed in species undertaking longer migrations, indicate that migrants adjust their behaviour to a time shortage. Migration behaviour may thus be under selection to economize time. A compensatory increase in migration speed for late migrants, observed in some of the species, is consistent with the existence of such a selection pressure. Calculated spring migration speeds are higher than corresponding autumn speeds, but no obvious differences between northern and more southern populations are evident.