Figure 17.2 Natal dispersal patterns of several species ringed as chicks and recovered in a later breeding season. All species show a decline in numbers with increasing distance from the natal site, but the form of the relationship differs between species, and distances are greater in some species than in others. (a) Northern House Martin Delichon urbica (Rheinwald 1975); (b) Eurasian Collared Dove Streptopelia decaocto (Hengeveld 1993); (c) Lesser Kestrel Falco naumanni (Negro et al. 1997); and (d) Blue-footed Booby Sula nebouxii within a colony (Osorio-Beristain & Drummond 1993).

population, which was otherwise stable in composition through the winter, was completely divided in two. Big differences in the natal dispersal differences of Ospreys Pandion haliaetus between eastern North America (median distance about 10 km) and northern Europe (median distance about 100 km) were attributed by Poole (1989) to differences between regions in the availability of nest-sites. In eastern North America, many people put up platforms for nesting Ospreys, giving an unusually high density of nest-sites, enabling birds to settle near their natal sites and take advantage of rich food supplies in coastal areas. This does not happen in Europe, however, where nest-sites are few and far between and where food is less plentiful, so Ospreys must disperse over longer distances. Another indication of the influence of landscape structure on dispersal is that, within species, natal philopatry is much more marked, with higher return rates, in isolated populations (such as those on islands) than in their counterparts occupying similar areas in continuous habitat (Weatherhead & Forbes 1994).

In general, natal dispersal distances tend to be greater in migratory species than in closely related resident ones, and the same is true for migratory and resident populations of the same species. For example, in Blackbirds Turdus merula, natal dispersal distances increased from Denmark through Norway and Sweden to Finland, in line with increasing migratoriness of breeding populations (as measured by proportion migrating and distances travelled) (Main 2002). In partial migrants, such as the Song Sparrow Melospiza melodia, migratory individuals showed greater natal dispersal distances than resident ones (Nice 1933). This was probably because resident ones were resident partly because of their dominance, and were already settled on territories by the time the migrants returned.

However short the usual dispersal distances, they are no handicap to range expansion through continuous habitat. In the House Sparrow Passer domesticus, the median natal dispersal distance is less than 1 km (with 97% of individuals at <20 km), yet in many parts of the world this species has expanded its range into new areas at rates of 15-80 km per year (Summers-Smith, in Wernham et al. 2002).

Seabirds and other colonial species

In colonial species, as expected, natal dispersal is influenced by the distribution of colonies. Nevertheless, as confirmed by ring recoveries, the settlement pattern is essentially the same as in other birds, with most individuals breeding in their natal or neighbouring colonies, and fewer individuals moving to colonies further away. This skewed settling pattern holds for colonial landbirds, such as Sand Martin Riparia riparia (Mead & Harrison 1979) and Lesser Kestrel Falco naumanni (Negro et al. 1997), and for a wide range of colonial seabirds, including gulls and terns, auks, shags and sulids, petrels and others. To give one example, only 36% of surviving Black-legged Kittiwakes Rissa tridactyla (mostly males) bred in their natal colony, a further 43% in other colonies up to 100 km away, and the remainder moved to colonies up to 900 km away (Coulson & de Mevergnies 1992). This pattern was found from ring recoveries of birds old enough to be breeding, and from resightings of individually colour-marked birds.1

In some seabirds, many individuals settle to breed within the same part of a colony where they were raised. This finding is much more frequent than expected if individuals settled at random within their natal colony. It also occurs in a wide range of species, including penguins (Williams 1995), albatrosses (Fisher 1971), shearwaters (Richdale 1963), auks (Gaston et al. 1994, Halley et al. 1995), skuas (Klomp & Furness 1992), shags (Aebischer 1995), and others. Extreme examples are provided by some tropical boobies. In a large colony of Blue-footed Boobies Sula nebouxii, the median distance between natal site and subsequent breeding site was less than 30 m (Figure 17.2; Osorio-Beristain & Drummond 1993), and in a colony of Nazca Boobies Sula granti on Galapagos the median dispersal distances of males and females were 26 m and 105 m respectively. As adults, both sexes retained the same sites year after year (Huyvaert & Anderson 2004).

1For further specific examples for gulls and terns see Mills (1973), Spear et al. (1998), Duncan & Monaghan (1977), Coulson & Mevergnies (1992), Austin (1949), Spendelow et al. (1995); for auks see Harris (1984), Swann & Ramsay (1983), Gaston et al. (1994), Halley et al. (1995); for shags and sulids see Aebischer (1995), Huyvaert & Anderson (2004); for petrels see Richdale (1963), Fisher (1971), Brooke (1978), Thibault (1994).

Such findings on the site-fidelity of seabirds are more remarkable than the bland figures suggest. Take the Short-tailed Shearwater Puffinus tenuirostris, for example, 23 million of which breed annually in burrows and headlands around southeastern Australia, migrating to the northern Pacific for the non-breeding season (Skira 1991). On one tiny island in Bass Straight, a population of a few hundred birds has been monitored for more than 50 years. Over 40% of young hatched on this island later returned there, usually breeding for their first time in their seventh year, and a constant 45% of the breeding population consisted of locally-hatched recruits (Serventy & Curry 1984). Such precision in the selection of a breeding location is extraordinary, considering the wide-ranging migration, the average seven-year period between fledging and first breeding, and the fact that less than a kilometre from the study site was a much larger island holding several hundred thousand nesting shearwaters. Moreover, most Short-tailed Shearwaters returned not just to the island, but to the same small part of the colony where they were hatched. Much the same could be said of most other pelagic seabirds that have been studied.

In any colony, then, breeders typically include some individuals raised within the colony and others that have moved in from elsewhere. The proportion of immigrants recorded in nesting colonies has varied greatly between species and with circumstances at the time. During the establishment and growth phases of a colony, immigration is high, so that most of the occupants have been raised elsewhere, whereas during a decline phase the reverse may be true (for seabirds, see Porter & Coulson 1987, Phillips et al. 1999; for geese, see Larsson et al. 1988, Johnson 1995).

Those gulls and terns that nest on the ground in exposed and often unstable substrates, such as sandbanks, often have to move their breeding places, as sites become washed away or flooded, accessible to mammalian predators, or infested with parasites. Hence, whole colonies can sometimes disband and re-form elsewhere, affecting the dispersal distances of both first-time and established breeders. Among southern African species, entire colony shifts are frequent in King Gulls Larus hartlaubii, Great Crested Terns Sterna bergii, Roseate Terns Sterna dougallii and Cape Cormorants Phalacrocorax capensis (Crawford et al. 1994). These species contrast with others in the same region which occupy more stable substrates and show strong colony persistence, including Jackass Penguins Spheniscus demersus, Cape Gannets Morus capensis, Bank Cormorants Phalacrocorax neglectus, Great Cormorants P. carbo and Great White Pelicans Pelecanus onocrotalus. In some cliff-nesting seabirds, generation after generation has used the same sites for centuries.

In many seabird species, individuals are known to visit colonies for one or more years before they attempt to breed, supposedly acquiring experience and local knowledge on which to assess the relative merits of potential nesting sites. The number of colonies visited by individuals during this pre-breeding phase seems to vary between species. In the Great Skuas Stercorarius skua on Foula, virtually all individuals seemed to visit only their natal colony in their pre-breeding years (Furness 1987, Klomp & Furness 1992), but in European Storm Petrels Hydrobates pelagicus and Atlantic Puffins Fratercula arctica, individuals regularly visited more than one colony, sometimes hundreds of kilometres apart, before settling to breed (Mainwood 1976, Fowler et al. 1982, Harris 1984). The most extreme record was for a Puffin ringed on the Treshnish Isles (Scotland) in late June and caught while visiting the Westman Islands (Iceland) 1087 km away 21 days later (M. P. Harris, in Wernham et al. 2002). By the time they start to visit colonies, some seabird species have spent up to several years of their early life in distant seas, apparently without coming to land, yet they still can find their natal areas.

Sex differences in natal dispersal

In some bird species, both sexes show similar dispersal distances, but in many others one sex moves further than the other, at least as a general tendency. The commonest pattern is for young females to disperse further between hatch site and breeding site than males. This pattern has been found in a wide range of species, including many passerines, owls and raptors, gallinaceous birds, shorebirds and colonial seabirds (Greenwood 1980, Clarke et al. 1997). In all such species, therefore, more males than females in local populations have been raised locally.

Most northern waterfowl show sex-biased dispersal, but with males moving furthest, as documented in swans, geese, shelducks, and in various diving and dabbling ducks (Mihelsons et al. 1986, Rohwer & Anderson 1988, Clarke et al. 1997, Nilsson & Persson 2001). In many such species, in contrast to most other birds, pairing occurs in wintering areas, and the male then accompanies the female to her natal area. Because birds from different breeding areas may share the same wintering places, the males of some species have settled to breed up to several hundreds of kilometres from their natal sites (Salomonsen 1955, Rockwell & Cooke 1977, Cooke et al. 1995). Moreover, the males of some migratory duck species have a different mate each year, so they often change their breeding sites substantially from one year to the next. Unlike dabbling ducks, some sea-ducks, geese and swans normally keep the same mate for several years and change their breeding sites much less often (Savard 1985, Anderson et al. 1992). In some sea-ducks, the partners separate after egg-laying and re-unite on wintering areas, as seen in marked pairs of Common Eiders Somateria mollissima, Barrow's Goldeneyes Bucephala islandica and Harlequin Ducks Histrionicus histrionicus (Robertson & Cooke 1999).

Other species in which males disperse further than females between natal and breeding sites include some shorebirds that show 'sex-role reversal', with the female defending the territory or mate, and the male doing the incubation and chick care. Examples include the Spotted Sandpiper Actitis macularia and various phalaropes Phalaropus spp. (Oring & Lank 1982, Colwell et al. 1988). Like the ducks, they have a social system based on mate defence, but unlike many ducks, they form into pairs in the breeding area. Males also disperse further than females in some lekking species, such as Great Bustard Otis tarda (Alonso & Alonso 1992).

In some group-living birds (cooperative breeders), the young remain with their parents for up to several years before they disperse, mostly over short distances (for Florida Scrub Jay Aphelocoma coerulescens, see Woolfenden & Fitzpatrick 1978; for Acorn Woodpecker Melanerpes formicivorus, see Koenig & Mumme 1987; for Arabian Babbler Turdoides squamiceps, see Zahavi 1989; for Siberian Jay Perisoreus infaustus, see Ekman et al. 1994). Delayed dispersal has developed, it is supposed, in situations where all suitable habitat is occupied by territorial groups, leaving nowhere for unattached birds to live (for experimental evidence in Seychelles Warbler Acrocephalus sechellensis see Komdeur et al. 1995). The adults then gain by allowing their young to remain in the territory, with access to its resources, until an opening becomes available elsewhere. The young pay for their long-term accommodation by helping with territorial defence and (in some but not all species) by feeding subsequent broods. While they forgo reproduction themselves, they may gain some 'inclusive fitness' if they help to raise younger siblings. To become breeders, young males sometimes inherit the territory from their father, or take over another territory nearby, but young females almost always move to another territory, as does any breeding female whose son takes over the home territory. The dispersal of young birds from the natal territory is thus delayed by up to several years, and inbreeding is largely prevented by females moving more often or further than males. In general, however, natal dispersal distances in such species tend to be short.

Competition and natal dispersal

Although a tendency to disperse from the natal site seems to be inherent in birds, several types of evidence point to competition in influencing the distances moved. First, in some studies the young moved further from their natal sites in years of high than low population density (e.g. European Greenfinch Carduelis chloris, Boddy & Sellars 1983; Great Tit Parus major, Greenwood et al. 1979, O'Connor 1981; Marsh Tit P. palustris, Nilsson 1989; Bearded Tit Panurus biarmi-cus, Cramp & Perrins 1993, Spotted Sandpiper Tringa macularia, Oring & Lank 1982). Alternatively, the young moved further in successive years as a local population grew (e.g. Eurasian Sparrowhawk Accipiter nisus, Wyllie & Newton 1991; Lesser Kestrel Falco naumanni, Negro et al. 1997).

Second, the young in other studies moved further from their natal sites in years of low than high food supply (e.g. Coal Tit Parus ater, Sellers 1984; Northern Goshawk Accipiter gentilis, Kenward et al. 1993a, 1993b, Byholm et al. 2003; Great Horned Owl Bubo virginianus, Houston 1978; Common Kestrel Falco tinnuncu-lus, Adriaensen et al. 1998; Barn Owl Tyto alba, Schonfeld 1974, Tengmalm's Owl Aegolius funereus, Lofgren et al. 1986, Sonerud et al. 1988, Saurola 2002; Tawny Owl Strix aluco and Ural Owl S. uralensis, Saurola 2002). The last five species depend on cyclically fluctuating vole populations, and in most the length of their movements fluctuated with the vole cycle in roughly three-year periodicity, with the longest natal dispersal in the low vole years (this affected young hatched in the peak years, which dispersed during the subsequent crashes). In addition, the experimental provision of supplementary food to non-migratory Song Sparrows Melospiza melodia greatly reduced emigration from the study area (Arcese 1989).

Third, in yet other studies, young fledged late in the season dispersed further, on average, than young fledged earlier. In the autumn, when Eurasian Tree Sparrows Passer montanus occupied nest boxes as roost sites, twice as many birds as boxes were present (Pinowski 1965). The available boxes were taken by adults and by young hatched early in the season. By the time late-hatched young attempted to obtain boxes, most were already taken, so that the majority of late-hatched young had to move away, subsequently breeding further from their natal sites. Similar increases in dispersal distances with fledging date were noted in many other species.2 Moreover, in an experimental study of Great Tits Parus

2Examples include Blue Tit Parus caeruleus (Dhondt & Huble 1968), Great Tit P. major (van Balen & Hage 1989), Marsh Tit P. palustris (Nilsson 1989), Northern House Martin Delichon urbica (Rheinwald & Gutscher 1969), European Pied Flycatcher Ficedula hypoleuca (Sokolov 1997), major, when most (90%) of the first brood young had been removed, and competition was thereby reduced, young from late broods settled much nearer to their natal sites than in other years (Kluijver 1971).

These various findings, which derive from both resident and migratory populations, imply that dispersal distances may be density-dependent, and influenced by levels of competition, whether for territories, food or nest (roost) sites. None of this is surprising, but it has taken detailed long-term studies to confirm it. In sedentary populations, competition effects may be manifest mainly in the late summer and autumn, as post-fledging dispersal occurs, but in long-distance migrants that migrate soon after breeding, it may occur mainly after the return migration as birds compete for nesting territories in their home areas. The intensity of competition is dependent on the densities of contenders relative to available nesting sites or territories and, as a rule, adults are dominant over first-year birds, but other factors, such as date of hatching or date of settling on territory, also have an influence on who settles where. It is presumably in part for reasons of dominance that young move further than adults, on average, and late-hatched young further than early-hatched ones.

Other factors known to relate to dispersal behaviour in young birds include individual features, such as: (1) inherent 'personality traits', such as exploratory behaviour (e.g. Great Tit Parus major, Dingemanse et al. 2003); and (2) hatching order, body size, weight or dominance within broods (Greenwood et al. 1979, Nilsson 1989, Part 1990, Strickland 1991, Altwegg et al. 2000, Forero et al. 2002; Byholm et al. 2003). These aspects act in addition to the various external influences, such as landscape structure and habitat patchiness, territory or food availability and population density, discussed above. However, not all studied species have shown such relationships.

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