Links between breeding and wintering sites
To what extent do birds from particular breeding areas migrate to the same wintering areas as one another, as opposed to different wintering areas? Their 'migratory connectivity' could be considered strong or weak, depending on the extent to which individuals from a given breeding area distribute themselves among different wintering areas, mixing with other individuals from different breeding areas. The answer to this question has implications for the ecology and genetic structure of populations, as well as for their conservation.
Among geese and swans, birds from separate (often isolated) breeding areas migrate along traditional routes to separate (often isolated) wintering areas. For example, Barnacle Geese Branta leucopsis from Greenland, Spitzbergen and Novya Zemlya migrate each autumn to separate parts of western Europe, with very little exchange of ringed individuals between the three populations (see Figure 23.3). Hence, even though pair formation occurs in winter in this species, breeding populations remain almost genetically isolated. Some populations of other goose species are subspecifically distinct, implying that this isolation is of long standing, as in the Greenland and European races of the Greater White-fronted Goose Anser albifrons, both of which winter in Europe but in different parts. Because juvenile geese migrate with their parents, they winter in the same areas, and find their breeding partners there. In most species of geese, swans and cranes, which use narrow migration routes and a limited number of staging and wintering sites, many individuals clearly remain in close proximity to one another for much of the year. In a sense, they migrate as a community, but do not necessarily stay together all the time. In a much less extreme situation, populations of many bird species remain partially segregated year-round, through parallel, chain and leapfrog migration patterns (Chapter 23), so that birds from different breeding areas clearly do not mix at random in winter.
In other species, however, ringing and satellite-based radio-tracking have shown that different birds from the same breeding locality can migrate to widely separated wintering places, and conversely that birds from a single wintering locality can migrate to widely separated breeding places. Individuals may return year after year to their own breeding and wintering sites, but have different sets of neighbours at the two seasons. For example, Peregrine Falcons Falco peregrinus caught wintering on a 50-km stretch of coast in eastern Mexico were tracked to breeding areas that lay across much of North America and western Greenland, with a west-east spread of more than 5000 km (see Figure 23.5; McGrady et al. 2002). Similarly, Peregrines from any one breeding area wintered at sites scattered over a wide range of latitudes from southern North America to northern South America, mixing with Peregrines from other breeding areas (Chapter 8). Again, individuals tracked in more than one year showed fidelity to their own breeding and wintering sites. In some species, even breeding partners tracked by satellite wintered in areas separated by more than 1000 km, as shown in Ospreys Pandion haliaetus, Greater Spotted Eagles Aquila clanga and others (Chapter 8; Kjellen et al. 1997, Meyburg et al. 1998). In contrast to the geese discussed above, such species could be said to show weak migratory connectivity. Species that breed or winter in different areas in different years, according to conditions, are also likely to show weak connectivity.
Dispersal patterns depend largely on the ecology of the species themselves, particularly on the level of stability in their habitats and food supplies. But such patterns in turn have a major influence on the genetic structure of populations and their propensity to subspeciate: when there is little movement of individuals among populations, considerable genetic substructuring can arise. Several studies have drawn attention to the link between dispersal and subspeciation (Rensch 1933, Belliure et al. 2000, Newton 2003). This subject is outside the scope of this book, except to point out that species with large geographical ranges in which individuals disperse over long distances between different breeding areas typically have few, if any, subspecies, whereas species in which individuals disperse over short distances have many subspecies (Newton 2003). This is evident even within families in which species vary in their dispersive behaviour, including finches, owls, raptors and waterfowl. More recent studies of dispersal patterns of Great Tits Parus major have shown the potential for genetic substructuring at a more local scale, over distances of kilometres or tens of kilometres, related to spatial variation in habitat quality (Garant et al. 2005, Postma & van Noordwijk 2005). How long this local substructuring lasts remains to be seen.
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