There is probably no single aspect of the entire subject of bird migration that challenges our admiration for birds so much as the unerring certainty with which they cover thousands of miles of land and water to come to rest in exactly the same spot where they spent the previous summer or winter. (Frederick C. Lincoln 1935.)
In many bird species, as may be judged from ring recoveries, individuals tend to breed in the same general region where they were raised; and once they have bred, they often use the same territories or even the same nest-sites in successive years. This holds more for one sex than the other, but is true of both territorial and colonial species. Moreover, individuals of some species also winter in the same localities in successive years. Although such site-fidelity is apparent in both resident and migratory populations, in migrants it implies the existence of precise navigational skills and homing behaviour, through which individuals seek out, year after year, the same breeding and wintering localities hundreds or thousands of kilometres apart. Individuals of some migratory species also use the same refuelling sites each year on their journeys, while some waterfowl use the same moulting sites.
The terms site-fidelity and dispersal denote opposite sides of the same coin, as they are both concerned with the distances that separate different places of residence of the same individual. Birds that occur in the same places in successive breeding seasons are said to show site-fidelity, while those that move from one place to another are said to disperse, but because distances vary along a continuum, there is no clear division between the two. Another term in frequent use is philopatry, which signifies continued residence in the place of birth, or return there after a period of absence.
In studies of the site-fidelity and dispersal of individuals, therefore, it is useful to distinguish: (1) natal dispersal, measured by the linear distances between natal and first breeding sites; (2) breeding dispersal, measured by the distances between the breeding sites of successive years; and (3) non-breeding dispersal, measured by the distances between the wintering sites of successive years. In almost all bird species that have been studied, individuals move much greater distances between natal site and breeding site than between the breeding sites of different years (e.g. Newton 1986, 2002, Paradis et al. 1998). Dispersal distances are of interest in their own right, as they relate to the ecology of the species concerned. They also have wider consequences, as they influence gene flow and the genetic structure of populations, the persistence and dynamics of local populations, and the potential for colonisation and range expansion (Newton 2003). This chapter is concerned mainly with the dispersal patterns of different species, and the environmental factors that underlie them.
Site-fidelity and dispersal have been studied primarily with the help of ringing. Most studies have been made by observers working in defined areas, ringing nestlings or adults in one year and noting where they are found in a later year in the same area. Such records are invariably biased in favour of short-distance moves because, being confined to the study area, they are not balanced by the longer moves of other individuals which may have settled outside the area and gone undetected. Moreover, some study areas are so small compared with the natal dispersal distances of the birds themselves that only a tiny proportion of ringed chicks is found breeding there in later years, the majority of survivors having settled elsewhere (see Weatherhead & Forbes 1994 for review of passerine studies). In general, for each species, the larger the study area (up to a point), the greater the proportion of locally raised young later found breeding within its boundaries (see Sokolov 1997 for European Pied Flycatcher Ficedula hypo-leuca). In areas where practically every chick was ringed, other (unringed) individuals breeding within the area are taken as immigrants hatched elsewhere. In most study areas, immigrants greatly outnumber the locally raised birds. Some researchers, aware of the effects of size of study area on the proportions of locally raised young recovered as breeders, have attempted to devise ways of correcting for it (Barrowclough 1978, van Noordwijk 1983, Baker et al. 1995, Koenig et al. 2000), or have used other means of studying dispersal, such as radio-tracking (Walls & Kenward 1998).
Less biased information on dispersal distances comes from recoveries of ringed birds reported by members of the public. Even if all the birds are ringed in a particular locality, the recoveries are not confined to that locality, so can give a much more representative picture of dispersal distances. Although the natal sites are known precisely for birds ringed as chicks, the assumption usually has to be made that individuals of reproductive age recovered in the breeding season were in fact nesting, or had the potential to nest, at the localities where they were reported. Collectively, such records reflect the settling patterns of individuals with respect to the ringing site, regardless of their movements in the period between ringing and recovery, which remain unknown. In some large, long-lived species, care must also be taken to separate the immatures, which may summer in areas partly different from the breeding adults of their population (Chapter 15).
There are obvious benefits to a bird in nesting near where it was raised, and in returning to the same areas each year, providing conditions permit. One is that the individual can benefit from local knowledge. This holds on both breeding and wintering areas, and at any sites the bird might stop on migration. Familiarity and prior ownership might also give a bird an advantage in competitive interactions with other individuals, making it better able to defend its feeding and breeding sites against potential takers. Moreover, through long-term residence over many generations, populations tend to become adapted through natural selection to the conditions prevailing in their particular region. This is evident, for example, in the consistent inherent patterns of size and colour variation found within species across their geographical ranges. So by remaining in (or returning to) the same general breeding area, individuals occupy regions to which both they and their likely breeding partners are best adapted. The benefits of local experience and of local adaptation, acting at the level of the individual, could thus be the main selective forces underlying site-fidelity in birds, where this is feasible.
Another potential advantage of site-fidelity stems from the social cohesion that it facilitates. Many birds pair with the same partner each year, even though they may live separately outside the breeding season. Partners can re-unite if they share a common breeding site (like most birds) or a common breeding and wintering site (like some sea-ducks in which partners separate after egg-laying and re-unite on wintering areas, Robertson & Cooke 1999). In at least some species with long-term pair bonds, breeding success improves as pairs remain together, but declines for a time following a change of mate (Black 1996). For these species then, site-fidelity enables partners that migrate independently to re-mate and gain any resulting reproductive benefits. But whether mate fidelity is a selective force behind the evolution of site-fidelity or an incidental behavioural consequence of site-fidelity is an open question.
Dispersal also has advantages. One is that birds can leave areas where conditions are poor or overcrowded to find somewhere better, in the process exploring around their home area. Some birds occupy successional habitats, which get less suitable over time, while others exploit patchy or ephemeral habitats or food sources, which are available in different places in different years. By moving from one good area to another, as appropriate, individuals may thereby enhance their survival and reproductive prospects. Another advantage of dispersal is that it could reduce inbreeding, which can lower the production and viability of offspring (Greenwood et al. 1978, Keller et al. 1994, Brown & Brown 1998, Daniels
& Walters 2000). Despite the various intensive studies that have been made of colour-ringed bird populations, extremely few brother-sister or parent-offspring matings have been recorded in most wild species. The chances of inbreeding are reduced further if, as in many bird species, one sex tends to settle further from its natal site to breed than the other (Greenwood & Harvey 1982), or if birds are more likely to disperse if close relatives are present in the same group or vicinity (Part 1996, Wheelwright & Mauck 1998, Cockburn et al. 2003). This last pattern is commonly found among species that live in groups of mainly closely related individuals. Conversely, individuals that mate with genetically very different individuals may suffer reduced fitness owing to the breakup of co-adapted gene complexes. Theoretically, an optimal balance between the contrasting risks of inbreeding and outbreeding would allow sufficient genetic mixing without disrupting local adaptations. Dispersal distances influence where this balance is drawn. Again, whether such genetic considerations have influenced the dispersal patterns of birds or are merely consequences of dispersal patterns resulting from other influences remains an open question.
There are clearly both benefits and costs to site-fidelity and dispersal, which are influenced by the ecological needs of the bird and by the environmental conditions prevailing. In some circumstances, it pays to remain in the same general area, in others to move elsewhere. Species vary their behaviour accordingly, and some closely related species which exploit different types of environment or food can differ greatly in their dispersal distances (see later). And whatever factors influence individual movements, they clearly have both ecological and genetic consequences on populations.
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