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Only in the least migratory population (Denmark) was an age difference found, with adults moving in greater proportion than juveniles. In the three most migratory populations, in Norway, Sweden and Finland, resident individuals occur only in the most southern areas, all from more northern regions being migratory. From Main (2002).

Only in the least migratory population (Denmark) was an age difference found, with adults moving in greater proportion than juveniles. In the three most migratory populations, in Norway, Sweden and Finland, resident individuals occur only in the most southern areas, all from more northern regions being migratory. From Main (2002).

times (or longevities) and on whether matings were selective or random among genotypes. If matings were assortative, with individuals of similar migration habits pairing together, the change to new migration habits could occur more rapidly than if matings were random between genotypes.

Measures of migratory restlessness, along with breeding experiments, revealed that in Blackcaps, non-migrants, short-distance migrants and long-distance migrants were part of the same continuum of variation in a single trait, as reflected in restlessness. The binary response (migrate or not migrate) is caused by a threshold which divides individuals into those above and below. Thus all birds without measurable migration in the wild have activity levels in captivity at the low end of a continuous distribution, below the limit of expression or detection (Pulido et al. 1996). Similarly, the division between short-distance and long-distance migrants is caused by variation in the numbers of nights on which they show appropriate behaviour. These findings have profound implications for the evolution of migration, because they suggest that ecologically significant transitions between resident or migratory, and between short- and long-distance migration, could come about by selection on a single trait. Of course, other selection would be needed to change the migratory direction and fattening pattern, as discussed later.

Most probably, then, migratory traits are threshold characters, which are determined by multiple genetic loci. This mode of inheritance, based on quantitative genetics, supersedes the idea of a single locus determination of a genetic dimorphism (migrate or not), since the offspring of given pairs are normally intermediate in their behaviour (Berthold et al. 1990b, 1996). Consistency tests on captive birds showed that individuals with a low level of restlessness did not change to a high level of restlessness in subsequent years, although a proportion of individuals that initially showed a high level subsequently showed a lower level (equivalent to wild birds becoming less migratory in later life) (Berthold 1993). In the Blackcap Sylvia atricapilla and Dark-eyed Junco Junco hyemalis, migratory behaviour (= high restlessness) was also expressed in a greater proportion of females than males, raising the possibility of sex-linked inheritance (Berthold 1986, Terrill & Berthold 1989, Holberton 1993). In this situation, any sex difference in migratory tendency would not result entirely from dominance relationships at the time, but also from genetic influence.

These various findings support the view that inborn, temporal patterns of migratory activity serve as time programmes that, together with inherited migration directions, enable inexperienced, first-time migrants to 'automatically' cover the distance between their breeding areas and their specific wintering areas. They embody the so-called vector system of migration, and help to explain why young birds, with no previous experience of a wintering area, can find their own way to an appropriate locality, without help from experienced adults (Chapter 9).

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