Natural variability

Another interesting finding confirmed in captive birds concerns the level of individual variation in migratory restlessness and directions apparent within some populations. Among Blackcaps Sylvia atricapilla that breed in the Mediterranean region, some are resident, while others travel mainly short distances up to 1300 km. Correspondingly, migratory restlessness among captive individuals from this region varied by a factor of 100. Migratory directions in the Mediterranean population range from west through south to east, spanning an angle of about 180°, as shown by ring recoveries. In contrast, Blackcaps that breed in Germany are wholly migratory, travelling distances from about 700 to 4500 km to reach various winter quarters which extend from southern France to the Ivory Coast. Their migration directions span about 90° to judge from ring recoveries (Berthold 1996). The amount of migratory restlessness displayed by captive individuals from this region varies by a factor of about six (between about 150 and 900 hours), and occurs over generally longer periods than in the Mediterranean birds. Individuals from resident populations of other species have also shown low levels of migratory restlessness in spring and autumn, including tropical Stonechats Saxicola torquata (Helm & Gwinner 2005). The important point is that not all individuals in the same population are equal in their migratory behaviour, and that substantial variation exists on which natural selection can act if necessary.

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