In many bird populations, as mentioned already, some individuals stay behind for the winter while others from the same area leave.1 This situation is found in breeding areas that are able to support some individuals in winter, but not as many as in the breeding season. Such partial migration can be divided into two types: obligate and facultative partial migration.
In obligate partial migration, certain individuals in a population migrate some distance every year regardless of prevailing environmental conditions, while other individuals remain resident every year. Particular individuals behave in the same way throughout their lives. This situation is presumed to occur where migratory behaviour is genetically controlled, and where there is no overwhelming advantage in staying or migrating. In some years, perhaps, the migrants do best and in others the residents, so that in the long term both types persist in the same breeding area (Berthold & Querner 1982b, Biebach 1983). They can maintain their distinctiveness only if they mate assortatively, with most migrants pairing with other migrants and most residents with other residents. This situation could occur, for example, if residents paired up in spring before the migrants arrive (a not uncommon event: for example, see Bearhop et al. 2005). If the two types interbred freely, they would lose their distinctiveness. Such a dual strategy is often viewed as the crucial intermediate stage in the transition from full resident to full migrant, or vice versa (see later).
In this system, the advantage of each type of behaviour depends partly on what other individuals do. If more and more birds became resident, say, the time would come when far more birds stayed than the winter food supply would support. Competition would then increase, leading to greater mortality in the resident sector, eventually tipping the balance in favour of migration. In this way, changes in the relative advantages of the two types of behaviour could ensure that the ratio of residents to migrants in any one population was kept roughly in line with local conditions (Lundberg 1987). The relative advantages of resident versus migrant may, of course, change over the years if environmental conditions change, such as winter temperatures or winter food supplies in breeding areas (see Chapter 21 for examples). They may also differ between areas and, in many partial migrants, a greater proportion of individuals migrates from higher than from lower latitudes. Obligate partial migration could thus be viewed as an 'evo-lutionarily stable strategy' (ESS) because, with the appropriate ratio of migrants and residents in the local breeding population, in the long term the pay-offs for both genotypes, in terms of lifetime reproductive rates, are balanced, and in the conditions prevailing no one type can completely replace the other (Berthold 1984a, Kaitala et al. 1993).
1The term partial migration is used here for the individual variation in behaviour within a population, not for the variation between different populations of the same species.
In facultative partial migration the same individuals might migrate in some years and not in others, depending on conditions at the time. Because birds compete, and vary in dominance or feeding efficiency, some individuals are able to survive in conditions where others would perish unless they moved out (the 'behavioural dominance hypothesis' of partial migration, Kalela 1954, Gauthreaux 1978a, 1982a; Chapter 12). In many partially migrant populations, a higher proportion of juveniles than of adults, and of females than of males, migrate (Chapter 15). This fits the dominance order in such populations, so that when food is scarce, juveniles fare less well than adults, and females less well than males. The subordinate individuals therefore migrate in greater proportion, or for greater distances, than the dominants (Chapter 15). In this situation, facultative partial migration is a 'conditional response' to food supply, dependent partly on social status or on differing priorities related to age or sex. Individuals leave only if it becomes difficult for them to survive locally, and the same individual can vary its behaviour according to circumstance and age (Newton 1979, Smith & Nilsson 1987, Adriaensen & Dhondt 1990, Schwabl & Silverin 1990).
In this facultative system, partial migration need not necessarily involve equal pay-offs for resident and migratory behaviour. As a group, the migrants may survive or reproduce less well than the residents, but better than they themselves would have done if they had stayed behind. The migrants are simply making the best of a bad job. The argument is that each bird responds to conditions at the time, so as to maximise its own survival chances. And as these conditions include the competitive pressure from other individuals, migration becomes to some extent density dependent: the more birds there are relative to resources, the greater the proportion that leaves for the winter. Dominance relationships are held to account for the facts that, in many partial migrants, larger proportions of individuals leave their breeding areas in years of high population or of poor food supplies than in other years, and that sex and age differences are often apparent in the proportions of individuals that leave, the dates they leave and the distances they travel, giving rise to geographical gradients in the sex and age ratios of populations in the non-breeding season (Chapter 15).
The European Robin Erithacus rubecula is a facultative partial migrant in Belgium (Adriaensen & Dhondt 1990). In one study, most males that nested in parks and gardens were resident year-round, whereas most males that nested in woodland were migratory, as were all females from both habitats. Resident males survived, on average, about three times better than migratory individuals (50% versus 17%), and even during the extremely cold winter of 1984-1985, residents still survived best. In addition, resident males were much more likely than migratory ones to obtain a mate (74% versus 44%). On the basis of both survival and mating success, the expected reproductive success of resident males was 2-4 times higher than that of migratory males. So why did migration persist? In ecological jargon, partial migration was a 'conditional strategy with unequal pay-offs': if individuals could find a territory locally in which they had a good chance of surviving the winter, they could stay; if not, they had to migrate, for only then did they stand any chance at all of surviving the winter. On this basis, socially dominant individuals were more likely to become resident, and subordinate ones to migrate. This could explain why females migrated more than males, why migratory and resident males tended to use different breeding habitats, and why young of early broods were less migratory than those of later broods. A similar system seems to operate in other species. For example, Great Crested Grebes Podiceps cristatus are partial migrants at Lough Neagh in Northern Ireland. By the time those that winter on the coast return, resident ones have begun breeding in the best areas, pushing the later arrivals into poorer nesting habitat (K. Perry, in Wernham et al. 2002). The same probably happens in many partial migrants. The two types of partial migration, involving an obligate genetic dimorphism in migratory tendency or a facultative migration dependent on individual circumstances, are not necessarily mutually exclusive. They may represent extremes of a continuum, or both could operate to varying degrees in the same population (Lundberg 1988). In addition, migration might be conceived as a split journey in which the first part was obligatory and under genetic control, and the second part facultative and under environmental control (Terrill 1990). This would account for the fact that, in some migratory species, the whole population departs in autumn, but the distance that most individuals travel varies greatly from year to year depending on the conditions (especially food supplies) encountered en route (Chapter 18).
Attempts have been made to assess the advantages or otherwise of migration by cost-benefit analyses, comparing the survival, reproductive success or energy budgets of birds that behave in different ways. Such comparisons may sometimes be useful, but often they are misleading, as explained in Appendix 20.1. In any case, different outcomes would be expected in obligate and facultative systems, further information on which is given in Chapter 12.
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