In ornithological studies migration is generally considered to be a seasonal, bidirectional movement between geographical areas. In a partial migratory population only some of the individuals in the population migrate. Generally, partial migration is considered to be an ‘evolutionarily stable state’ only if the pay-offs (life-time reproductive success) of both morphs in the population are balanced. In contrast to this hypothesis, most case studies show that resident species, populations and individuals do better than migrants. The European robin is a partial migrant showing a decline in migratory behaviour, the proportion of migrants declining from north to south. In the British robin, almost all settled males as well as 30-50% of the females are resident. Mean local survival of resident males (50%) is higher than local survival of migrant males (17%), and during cold winters survival of residents decreases by about 50%. The probability of breeding however is two to four times higher in residents than in migrants and resident and migratory robins are habitat separated both in the breeding season and in winter, with 70% of breeding males being migratory in the woodland, but in the park and gardens most males are resident. Almost all females robins are migratory (Adriaensen and Dhondt 1990).
Birds often show some form of social segregation during winter, both at large geographical scales (a consequence of differential migration) and at the regional or local level, when comparing different habitats or micro-habitat. Female European robins (Erithacus rubecula) greatly outnumber males in southern Iberia, which confirms that this species is a differential migrant with a strong latitudinal segregation of the sexes. Furthermore, sex, age and body size influence the habitat distribution of robins in winter. Subordinate birds (females, juveniles and small individuals) are generally more common in habitats with a greater shrub development and comparatively scarce in woodlands with relatively little undergrowth. Birds wintering in woodlands are in better condition than birds wintering in shrub-land. These results are consistent with the hypothesis that proposes that social dominance, mediated by differences in size and experience, is important in determining the habitat segregation of sex and age classes (Catry et al. 2004).
Female European robins (Erithacus rubecula) have adopted three different pairing strategies. These are, in order of frequency: moving on to a male’s territory; fusing her territory with that of a neighbouring male; and being joined by a male on her territory. Although females are not free to choose between strategies during a particular season, individuals frequently changed strategy between different years. Their behaviour suggested that they are exercising mate choice, although the scope for such choice is limited by a strong tendency for a female to pair in areas where she had lived previously. Males with large territories are more likely to pair and tend to pair earlier than those defending small territories (Harper 1985).
- Adriaensen.F., & Dhondt.A.A., (1990) Population Dynamics and Partial Migration of the European Robin in Different Habitats. Journal of Animal Ecology, 59, 1077-1090.
- Catry.P., Campos.A., Almada.V., & Cresswell.W., (2004) Winter segregation of migrant European robins (Erithacus rubecula) in relation to sex, age and size. Journal of Avian Biology, 35, 204-209.
- Harper.C.G.D, (1985) Pairing strategies and mate choice in female robins Erithacus rubecula. Animal Behaviour, 33, 862-875.