Whatever the pattern of geographical segregation among wintering migrants, if such differences are genetically influenced, through migratory behaviour, it is not hard to envisage how they might have come about (Figure 23.9). Imagine that populations which breed in separate areas come together on common wintering grounds. If they were limited by food in the wintering area, and individuals of one population were better adapted to that area, they would in time be expected to eliminate individuals of the other population completely, or force them by selection to winter elsewhere. As a second scenario, imagine that, under food shortage in their joint wintering area, they competed on equal terms, with the same proportion of each population surviving the winter. If individuals of one population had, on average, a consistently higher reproductive rate than the other, then in time they would be expected to replace individuals of the other population completely in their shared wintering area, either eliminating them altogether or forcing them to winter elsewhere (Chapter 22). This is one way in which a sedentary population
ABC ABC ABC
Figure 23.9 Migration of three neighbouring populations (A, B and C) showing: (A) synheimy (complete sharing of wintering areas), (C) alloheimy (complete separation of wintering areas) and (B) an intermediate situation (partial separation of wintering areas). The upper rectangles indicate breeding areas, the lower rectangles wintering areas, and the arrows the scattering of migrants from each breeding area. A gradual change from pattern 1 to 3 could come from selection resulting from competition between individuals from different breeding populations or from selection to reduce migration distances, eventually giving rise to 'parallel migration' (see text). Pattern 1 could be maintained or developed in species that utilise sporadic habitats or food supplies, whose distribution within the winter range varies markedly from year to year (as for some irruptive migrants, Chapters 18 and 19). To judge from ring recoveries, the intermediate situation (2) is the commonest. From Salomonsen (1955).
could become migratory, under pressure of competition from individuals of another population increasingly wintering in the same area (Sutherland & Dolman 1994, Bell 2000). Likewise, imagine that birds in a given breeding area, where their numbers were limited, divided between two wintering areas. If the birds with a genetic predisposition to go to one wintering area survived consistently better than the birds visiting the other area, the first group would eventually replace the second completely. In these ways, competition could act as a selective force behind a genetic change in migratory habits, leading ultimately to year-round geographical segregation of populations.
Whether the development of alloheimy leads to chain or leapfrog patterns could depend on the competitive relationships between individuals from the different populations, as explained in Chapter 22 (Cox 1968, Holmgren & Lundberg 1993,1 Bell 2000). If individuals in a higher latitude population settling in a wintering area were competitively superior to the individuals of the same species resident there, the immigrants could either eliminate the residents, or force them to become migratory, moving to lower latitudes for the non-breeding season, and thus setting up a system of chain migration (Bell 2000). If, on the other hand, the northern birds were inferior in competition with the more southern residents, the northern ones would benefit from moving to yet lower latitudes, thus becoming leapfrog migrants. Hence, whether one or other system developed could depend on the competitive relationships between individuals from different populations. Such competition might affect the behaviour of all members of the competing inferior population, leading to total segregation in wintering areas, or only some of them, leading to partial segregation in wintering areas.
This raises the question of how synheimy could persist, with populations from different breeding areas wintering together in the same area and taking the same foods. One way would be if they were each limited in numbers on their breeding areas, so that collectively they did not reach the limit of their shared wintering area, and hence did not compete seriously for resources there. Another way would be if they were limited either in breeding or wintering areas by factors other than resources, say by predators or parasites, which held the populations below the level at which serious competition for food occurred. A third way in which synheimy could persist is if the different populations differed in their ecology, reducing any potential competition between them. For example, in many areas resident populations of some species stay near their breeding places year-round, eating whatever foods are available there, while winter visitors of the same species from elsewhere move around from place to place, exploiting temporary abundances of food such as fruit crops or insect outbreaks (for Eurasian species in Africa see Chapter 24). The survival of both residents and migrants in the
1Holmgren & Lundberg (1993) formalised patterns of migration depending on whether dominance at a given wintering area was mediated by prior occupancy or by body size, latitudinal gradients in suitability for breeding and wintering and the cost of migration. In their models, chain immigration would result if both breeding and wintering suitability gradients increase towards lower latitudes and dominance is due to body size, or if breeding suitability increases to higher latitudes and wintering suitability to lower latitudes, and dominance is due to prior occupancy.
same area depends on the mobility of the migrants, and their ability to seek out locally abundant food supplies which are often greater than the local residents can consume themselves in the time available. Resident and migratory populations may also differ in habitat use, as found among Blackcaps Sylvia atricapilla wintering in Spain (Pérez-Tris & Tellería 2002). Fourthly, synheimy of different breeding populations may also persist in species whose habitats and food supplies are sporadically distributed, changing in location from year to year. This situation exists for seed-eating boreal finches, vole-eating raptors and owls, and many freshwater ducks (Chapters 18 and 19). It leads individuals to move around, within and between winters, and prevents their developing strong winter site-fidelity, which underpins the evolution of alloheimy. Finally, it is also possible that some of the examples of synheimy in closely allied taxa that we observe now are recent, arising from human land use, or are transitory, as birds change their behaviour and shift gradually from synheimy to alloheimy.
Habitat segregation between resident and immigrant Robins Erithacus rubec-ula was examined in Spain (Tellería & Pérez-Tris 2004). In September, before the migrants arrived, local Robins were found only in forests, but after migrants had arrived, large numbers were also found in shrub habitats. Some migrants replaced residents in the forest, but in general the majority of residents remained in the optimal forest habitat, while the majority of migrants occupied the scrub (the two types being distinguished on morphological criteria). It seemed that local Robins benefited from prior occupancy of the best habitat, forcing the migrants to occupy apparently less suitable sites. This may be how small southern populations survive despite the annual incursion for the winter of large numbers of con-specifics from higher latitudes.
In consideration of these patterns, body size comparisons between northern hemisphere populations are revealing. In some chain migrants, individuals from northern breeding areas tend to be larger, so it is not hard to imagine that, through aggressive encounters, they could displace smaller individuals from more southern areas, which would then benefit from moving even further south. When both populations move back north in spring, the smaller birds occupy the ground vacated by the large ones. In contrast, in some leapfrog migrants, such as Common Ringed Plover Charadrius hiaticula, Common Redshank Tringa totanus, Common Kestrel Falco tinnunculus and Peregrine Falcon F. peregrinus, it is the individuals from northern populations that are smallest, so they would tend to lose in aggressive interactions with more southern birds and have to move on further south, giving a leapfrog pattern, eventually fixed by natural selection. However, it remains to be seen whether such differences in body size patterns are consistent across all chain and leapfrog migrants. In any case, it is impossible to tell whether the body size differences between populations evolved before or after the development of their migration patterns (for alternative models of the evolution of leapfrog migration see Cox 1968, Bell 1996, 1997).
Competitive superiority of one wintering population over another could arise in a number of different ways: the individuals in one population may be larger, and hence dominant in aggressive interactions or better able to resist cold; they may gain the benefits of prior occupancy if they are already established when individuals of the second population arrive, or they may differ in structure and behaviour in a way that enables them to more efficiently exploit local resources, or better avoid predators and parasites. Alternatively, they may, in their particular breeding range, be able to achieve a higher mean per capita reproductive rate, and simply 'outbreed' the other population, gradually replacing it in their common wintering range where overall numbers are limited (Chapter 22). This latter situation has been explored in a demographic model for a migratory shorebird, the Oystercatcher Haematopus ostralegus, in which different breeding populations shared the same wintering area. Initially, as winter habitat was progressively removed in simulations, all populations decreased in parallel. However, as habitat loss continued, the populations with lower fledgling production began to be disproportionately affected (Goss Custard et al. 1995).
These various considerations suggest that intraspecific competition could be a major driving force behind geographical segregation patterns among migratory bird populations, at least for latitudinal segregation. For longitudinal segregation, minimisation of flight costs could also be involved. Various mathematical models have been devised to explore these patterns further. They indicate that such factors as latitudinal gradients in habitat suitability, migration costs, and annual time budgets, could all add to asymmetric competition as potential selective forces behind different patterns of geographical segregation, including chain, leapfrog and overlap patterns (Cox 1968, Greenberg 1980, Lundberg & Alerstam 1986, Holmgren & Lundberg 1993, Bell 1997, 2000). But on present knowledge, we have no way of judging which selective forces are likely to have applied in particular cases.
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