Experiments in which individuals showing high levels of migratory restlessness were paired together showed that the trait persisted or increased in the offspring. Similarly, when individuals showing low levels of restlessness were paired together, the trait persisted or decreased in the offspring. These findings confirmed, for both Blackcaps Sylvia atricapilla and European Robins Erithacus rubecula, that the amount of migratory restlessness was to a large extent genetically pre-programmed (Berthold & Querner 1981, Biebach 1983). Moreover, cross-breeding of migrant with non-migrant strains (using Blackcaps from south Germany and the Cape Verde Islands respectively) resulted in the partial transmission of migratory activity to 33% of the first-generation offspring, again indicating that the urge to migrate was inherited (Berthold et al. 1990b). Starting with a partially migratory population, high or low levels of migratory restlessness (assumed to reflect migratori-ness or sedentariness) could be selected to phenotypic uniformity within as few as 3-6 generations. More precisely, after three generations of selection, sedentary behaviour was fixed in the migratory line, whereas selection over 5-6 generations was needed to produce an almost exclusively migratory stock. These experiments thus revealed not only high heritability of migratory behaviour, but also how rapidly migratory behaviour could change in response to strong selection pressure. In nature, selection is never likely to be that strong, so substantial changes would be expected to take longer, perhaps decades or more depending on generation times.
In another study, Blackcaps Sylvia atricapilla from widely separated populations (southern Finland, southern Germany, southern France and the Canary Islands off North Africa) were hand-raised and tested under identical conditions. The proportion of migrants (showing a given level of migratory restlessness) among birds from the four areas differed markedly: from 100% of birds from Finland and Germany, to 80% of those from southern France, and to only 23% of those from the Canary Islands. Furthermore, the number of nights of migratory restlessness shown by migratory individuals differed between populations, declining north to south (Figure 20.2; Berthold & Querner 1981). Appropriate differences in physiology (moult and fat accumulation) and morphology (wing shape) were also found between the different populations (Berthold & Querner 1982a). Cross-breeding of individuals from two of these populations (southern Germany and Canary Islands) gave offspring of intermediate characteristics, again implying that migratory features were under genetic control (Figure 20.2). Intermediate migratory behaviour was also found in hybrids between migratory
European Quail Coturnix c. coturnix and non-migratory Japanese Quail C. c. japonica (Deregnaucourt et al. 2005).
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