As a breeding population expands from lower to higher latitudes, and birds at the expanding front become increasingly migratory, more and more individuals are likely to concentrate for the winter in the original lower latitude range. Theoretically, the resulting competition for the limited resources there could lead to one of four situations, depending on circumstances:
1. The total numbers wintering in the original range could be limited, preventing any further expansion of breeding range to higher latitudes, so that the amount of wintering habitat limits the extent of the breeding range.
2. If the winter immigrants from high latitudes are competitively superior to the original population, when the two come together on their joint wintering area, the original resident population could itself develop migration to areas beyond its current range, being replaced in its breeding area each winter by the immigrants. This scenario depends on the presence of suitable wintering habitat beyond the original range, and would give rise to a system of 'chain migration' in which breeding populations are seasonally replaced by wintering populations of the same species. In this way, populations from successive latitudes maintain the same south-north sequence in wintering areas as they do in breeding areas, but the whole series is displaced towards the equator in winter (for examples see Chapter 23).
3. If the winter immigrants were inferior in competition with the original breeding population, their numbers could be held at a low level (as in 1 above), or they could develop an even longer migration to winter in previously unoccupied areas beyond the original breeding range. This possibility again depends on suitable wintering habitat being available, and would give rise to a system of 'leapfrog migration' in which the migrant population moves through the area occupied by the resident population to winter beyond it. In this way, populations of a species breeding at different latitudes reverse their sequence in winter, with those breeding furthest north wintering furthest south (or vice versa in the southern hemisphere) (for examples see Chapter 23).
4. Alternatively, if the immigrants were competitively superior, they could eliminate the original population completely. The species would then persist as a single population breeding at high latitude and wintering at low latitude, and the original breeding range would be vacant during that part of the year when the wintering population was away on its higher latitude breeding range. One way in which the migrant population could achieve competitive superiority is through the greater fecundity expected at higher latitudes. Even if the two populations then competed on equal terms on their shared wintering range, with no difference between them in average survival rates, the more fecund population (with greater average reproductive rate) would in time outbreed the other, either holding it at much lower level or eliminating it altogether in their shared range.
Given these possibilities, the transition from resident to long-distance migrant emerges as a three-stage process, beginning with the origin (maintenance or re-expression) of the migratory habit, followed by the establishment of a fully migratory population, and in some cases ending with the disappearance of ancestral resident population. On the latter process, the general pattern that emerges involves a 'rolling forward' of the breeding range of a species as new breeding populations are established at higher latitudes and old ones are lost at the trailing edge. This process could have been greatly facilitated by the development of productive high-latitude habitats in the wake of glacial retreat, but may also continue into periods of climatic stability (Bell 2000). On this mechanism, improvements for breeding in one part of the range can, through winter competition, lead to retreat or extinction in another part. The implication is that, were it not for the seasonal influx of migrants from higher latitudes, the same species might be resident over a wider range of latitudes. The species can winter at lower latitudes, but year-round residency is reduced or prevented there by competition for winter food supplies with conspecifics that breed at higher latitude.
This consideration may help to explain why many migratory birds do not breed in their wintering areas even though conditions seem suitable there year-round. Take the Osprey Pandion haliaetus, for example, which does not breed in the tropics and subtropics of South America and Africa, even though the bulk of the northern continental populations winter there, and the immatures from these populations remain there year-round. Osprey s can obviously survive through the year at low latitudes, and the most likely reason they do not remain to nest there is that they can achieve higher individual reproductive output by migrating to nest at higher latitudes, returning for the intervening winters. These various considerations imply that intraspecific competition for resources in low-latitude wintering areas may be a major driving force behind patterns of latitudinal distribution in migratory birds.
Shorebirds contain many high-latitude migrants. Global populations of such species may well be limited by the availability of coastal food supplies in the non-breeding season, and as a consequence are entirely accommodated in the breeding season in the high-latitude habitats where they achieve the greatest breeding output, taking migration losses into account. Many of these high-latitude species could probably breed at lower latitudes and be more widespread at that season if highly productive tundra habitat were not available (Myers 1980). On this view, intra-specific competition on the wintering grounds could have a major role in influencing the overall population sizes, distributions and migrations of birds (for further discussion see Chapter 23).
Many seabird species, once restricted to arctic-nesting areas, spread southward during the twentieth century, greatly increasing their numbers (Newton 2003). These range expansions were in a direction opposite to that expected under global warming, but could be explained by birds being originally confined to areas where they could achieve the highest fecundity and, as these areas became filled, spreading southward during a period of overall population growth. Examples of such species include Northern Fulmar Fulmarus glacialis, Gannet Morus bassanus, various large gull species, and Common Eider Somateria mollisima. The increase and spread of most of these species has been linked to greatly increased food supplies resulting largely from the development of the fishing industry, and the resulting waste produced from the processing of fish at sea. Populations breeding over a wide span of latitude share a common wintering area, in which competition would have been reduced as food supplies increased. These various events may illustrate how the breeding range of a migrant may be limited by competition for restricted food supplies in winter quarters. If this food supply increases and the overall population grows, there may be competition for nesting sites in the existing range, which may promote an expansion in breeding range, if necessary into lower latitude areas where breeding success may be lower than in the high-latitude range.
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