Cost-benefit analyses of migration are best conducted on individuals from within the same breeding population, some of which remain year-round while others migrate. For each type, measures are needed of: (1) mortality over winter, which in a genetically controlled system may be expected to be lowest in the migrants wintering in a milder area; (2) mortality over the migration periods, which is expected to be greatest in the migrants because of the risks involved; and (3) reproductive success, which is expected to be similar in both groups or greater in the residents, which might acquire the best territories and start nesting earlier than the migrants. If the long-term net outcome of these three processes favours residents, in a genetically controlled system the population should change towards becoming totally resident, but if the long-term outcome favours migrants, the population should change towards becoming totally migratory. Genetically controlled partial migration would be expected to persist if both types, while perhaps varying in success from year to year, had equal lifetime reproductive success in the long term.
A different situation would be expected in facultative partial migrants, in which the same individuals might migrate in some circumstances and not in others. In these populations, as described above, dominants are most likely to remain in breeding areas, and subordinates to leave. This is because the dominants could prevail in competition for winter territories, food or other resources. Hence, any difference in mortality or reproductive success between the two groups could result from dominance status or 'quality' effects, and not from any genetic differences in migratory behaviour, so would not affect the movement patterns of the population in the longer term. The crucial question here in any one year concerns the optimal strategy for the individual, which would be to stay in some circumstances and to leave in others.
These various considerations indicate that, in cost-benefit analyses of partial migration based on measurements of mortality and breeding success, different outcomes would be expected in obligate and facultative extremes. In a facultative system in which migration was driven by dominance, say, the residents would usually be expected to perform best in all respects, as the migrants are in effect refugees making the best of their misfortune, as in the Robins Erithacus rubecula discussed earlier.
Some studies on the relative merits of migratory versus resident behaviour have involved comparisons of survival and reproductive rates between resident and migratory populations of the same species. In general, such studies are of little value in this respect, mainly because populations (breeding in different areas) differ in survival and reproduction for reasons other than migration (for such comparisons see Harrington et al. 1988, Nichols & Johnson 1990, Hestbeck et al. 1992, Monkkonen 1992). Within species, migratory habits tend to increase with latitude (Chapter 13). In single-brooded species, brood sizes also tend to increase with latitude, so if populations are to remain stable in the long term, average annual mortality must also increase with latitude (for Blue Tit Parus caeruleus, see Snow 1956), in parallel with the migratory habit. In multi-brooded species, in contrast, the number of broods raised per year (and hence the total number of young raised per year) may decline with increasing latitude, in line with decline in the duration of the favourable season. So in these species, if populations are to remain stable, annual mortality must also decline with latitude. The salient point is that latitudinal gradients in reproductive and mortality rates would be expected in many widespread species, regardless of whether they are resident or migratory, or of the length of their migratory journey. In any population that remains numerically stable (with no long-term upward or downward trend), whether migrant or resident, average mortality rate must balance average reproductive rate, whatever these rates happen to be.
Nor can one reliably assess the costs and benefits of migration by comparing the breeding and mortality rates of resident and migratory species nesting in the same area. This is because, over a wide range of latitude, most resident species (because of their diets) can try to breed over a longer period each year than can closely related migrant species in the same area (Chapter 14). Residents would therefore be expected to produce more broods per year and, in stable populations, have correspondingly higher annual mortality rates than closely related migratory species breeding in the same area, an expectation that more or less fits the facts (von Haartman 1968, Greenberg 1980, O'Connor 1986, Monkkonen 1992). Differences in reproduction and mortality between the two groups are not consequences of their migratory or resident behaviour, but of the different seasonal patterns of their particular food supplies, which influence both their migratory behaviour and their reproductive and mortality rates.
Nor can the costs and benefits of migration be assessed from comparisons of the annual energy budgets. Because migrants normally overwinter in warmer climates than occur in winter in their breeding areas, they save maintenance energy by migrating (Greenberg 1986). Even when the energy costs of the return journey are added in, the overall energy budget for the whole non-breeding season could often be lower than it would have been if the birds had overwintered in their breeding areas. Again, however, this fact has not necessarily had any influence on the evolution of migration. For it is not the overall energy budget that matters, but the ease of getting the daily needs in the conditions prevailing. There may be no cost in acquiring a large daily intake, if food is sufficiently plentiful within the area concerned. Whatever the overall requirement, it is an inability to get enough on a day-to-day basis that leads to mortality or reduced reproduction, and exerts the selection pressure for or against migration. In general, therefore, cost-benefit analyses of migration should be treated with caution and, in facultative migrants, individuals that leave breeding areas may reasonably be expected for the same reason to show lower survival and reproduction, on average, than residents.
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