Info

73b

Coulson et al. (1984)

The data are drawn from detailed studies in which attempts were made to identify all individuals present. Records for different years are pooled. N = number marked, % = percentage known to return the next winter in the same area. Sizes of study areas are given in hectares or as length of coastline. Other examples for Neotropical migrants in Rappole et al. (1983), for waterfowl in Robertson & Cooke (1999).

aThis is the percentage of re-sightings (not total birds) that were in the same site, so is not comparable to most of the other data presented in which return rates include mortality as well as movement elsewhere.

bIncludes 43/56 males and 38/55 females, all adults. Other figures of 9/17 (53%) for 2- to 14-year-old birds and 1/12 (9%) for 1-year-old birds.

The data are drawn from detailed studies in which attempts were made to identify all individuals present. Records for different years are pooled. N = number marked, % = percentage known to return the next winter in the same area. Sizes of study areas are given in hectares or as length of coastline. Other examples for Neotropical migrants in Rappole et al. (1983), for waterfowl in Robertson & Cooke (1999).

aThis is the percentage of re-sightings (not total birds) that were in the same site, so is not comparable to most of the other data presented in which return rates include mortality as well as movement elsewhere.

bIncludes 43/56 males and 38/55 females, all adults. Other figures of 9/17 (53%) for 2- to 14-year-old birds and 1/12 (9%) for 1-year-old birds.

them. In one study, shorebird species were ranked in order of decreasing winter site-fidelity (= increasing dispersal) from Purple Sandpiper Calidris maritima (most site-faithful), through Ruddy Turnstone Arenaria interpres, Curlew Numenius arquata, Ringed Plover Charadrius hiaticula, Oystercatcher Haematopus ostralegus, Redshank Tringa totanus, Dunlin Calidris alpina, Bar-tailed Godwit Limosa lapponica to Red Knot Calidris canutus (least site-faithful), the differences being again linked with the degree of year-to-year stability in their habitats and food supplies. Red Knots regularly moved from estuary to estuary during the course of a winter, but many visited the same several estuaries every winter. Except for the last three species, more than 94% of recovered individuals were found in the same section (within 10 km) of a large estuary within and between winters (Rehfisch et al. 2003). In the same study, juvenile Dunlins were found to move longer distances among roosts than did adults.

In some passerines which are territorial in the non-breeding season, site attachment may be so strong that individuals remain throughout each winter in the same 50 m radius, and return there year after year (see later). Other species, while apparently solitary during the non-breeding period, do not appear to defend territories, perhaps because they are more mobile, depending on patchily distributed food supplies (Salewski et al. 2002). Yet others join roaming flocks in the non-breeding season, foraging over wide areas.

The fact that birds have greater freedom to move around in the non-breeding than in the breeding season gives problems in measuring site-fidelity, especially in migrants. In some species that show site-fidelity, such as the Blackcap Sylvia atricapilla, the numbers of birds at particular sites have varied greatly from year to year, and from one part of the winter to another, depending on the size of the local fruit crops that provide their food. This fluctuation was apparently due largely to the behaviour of young birds which settled in their first year mainly in sites where fruit was plentiful, and partly to individuals (young or old) varying their length of stay from year to year, moving on each year when the local crops were eaten (Cuadrado et al. 1995).

Site-fidelity with variable lengths of stay has been noted in some other species that exploit annually variable food supplies, such as Rosy Finch Leucosticte arc-toa (Swenson et al. 1988) and Eurasian Siskin Carduelis spinus (Senar et al. 1992). Typically at any one locality, some individuals seem merely to pass through, others stay for days or weeks and yet others for months, with the proportions varying from year to year in line with food supplies. It fits with the notion that many northern seed-eaters take broadly the same migration route each year, but pass particular latitudes much earlier in some years than in others, and reach the limits of their potential wintering range only in occasional years of widespread food shortage (Chapter 18). Species can therefore vary markedly in distribution from one winter to the next, in association with regional variations in food supplies (for large-scale patterns, see Rey 1995). This is true not only of the finches that depend on sporadic tree-seed crops, but also of some ground-feeding finches whose food may be covered by snow, of some raptors and owls which depend on temporarily and locally high rodent populations, of shorebirds of soft substrates whose prey varies in spatial distribution from year to year, and of some waterfowl whose food supplies are affected by variations in water levels and ice cover. Moreover, in any one year, many species show age-related differences in wintering range (Chapter 15;

Ketterson & Nolan 1976, Gauthreaux 1982a, Dolbeer 1991). The inference that individuals of such species must use widely separated areas in different years has been increasingly supported by ring recoveries (Chapters 18 and 19).

The chance of recording ringed individuals at particular sites increases with their length of stay, and many more individuals could use a site than are present at one time. Because duration of stay is often linked with measured food supplies and social status, greater return rates are recorded in good than in poor food years, in adults than in first-year birds (e.g. 76% vs. 48% in Great Cormorant Phalacrocorax carbo, Yesou 1995), or in territorial than in non-territorial individuals (Cuadrado 1995, Johnson et al. 2001). The potential for observational bias gives uncertainty in how much the recorded variations in return rates between years and localities, or between sex and age groups, are due to differential site-fidelity, to differential survival, or merely to different durations of stay. All three factors could contribute in some degree to recorded return rates, adding to the effect of size of study areas. The question is whether birds move around in winter over distances of metres or kilometres, or over tens, hundreds or thousands of kilometres.

Studies of insectivorous and frugivorous passerines (mainly Syliviidae and Parulidae) in the tropics have reported high winter recurrence, suggesting that at least most surviving individuals have returned to their territories in successive years (Nisbet & Medway 1972, Pearson 1972, Price 1981, Kelsey 1989, Holmes & Sherry 1992, Salewski et al. 2000, Wunderle & Latta 2000, Latta & Faaborg 2001, 2002). On the other hand, studies of wintering passerines (mostly Sylviidae and Turdidae) in the Mediterranean region have generally reported lower return rates, indicating that many birds could have changed winter quarters between years, or within years (Herrera 1978, Herrera & Rodriguez 1979, Finlayson 1980, Cuadrado 1992, 1995, Catry et al. 2003). As the Mediterranean species depend more heavily on fruit in winter, it would not be surprising if they were less territorial and site-faithful than their tropical wintering equivalents which take more insects, and hence have a more consistent food supply. Moreover, some of the studies involved only territorial individuals (e.g. Nisbet & Medway 1972, Kelsey 1989), while others involved a mixture of territorial and non-territorial ones (e.g. Constant & Ebert 1995, Cuadrado 1995). Non-territorial ones would be expected to show less site-fidelity.

Swans, geese and sea-ducks generally show high return rates to wintering sites, while freshwater diving ducks show lower return rates, and dabbling ducks still lower rates (Robertson & Cooke 1999). The differences again relate partly to the stability and permanence of the various habitats that the different species occupy, and to changing water levels and ice cover. Wide dispersal between one winter and the next has been well documented from ring recoveries of several species, including Common Pochard Aythya ferina (see Figure 19.8) and Tufted Duck A. fuligula (M. Kershaw and R. Hearn, in Wernham et al. 2002).

Sex-related differences

Few data are available to compare winter site-fidelity between the sexes. Sex-differences were apparent in the return rates of Harlequin Ducks Histrionicus histrionicus to a wintering site in British Columbia (77% males vs. 62% females, Robertson & Cooke 1999), and of Buffleheads Bucephala albeola to a site in

Maryland (26% males vs. 11% females, Limpert 1980), but at least part of this sex difference may have been due to differential survival rather than differential site-fidelity. In contrast, ring recoveries gave no indication of sex differences in the winter site-fidelity of Black Ducks Anas rubripes, Canvasbacks Aythya valisineria and American Woodcocks Scolopax minor in eastern North America (Nichols & Haramis 1980, Diefenbach et al. 1990), or of Whooper Swans Cygnus cygnus and Bewick's Swans Cygnus columbianus in Britain (Scott 1980, Black & Rees 1984). Such sex differences would not be expected in adult geese and swans because they remain in pairs year-round. So while sex differences in winter site-fidelity may occur in some species, they are apparently absent in others.

Age-related differences

Outside the breeding season, when many birds move away from their nesting areas, the adults often move less far, or stay away for less long, than the young (Chapter 15). This is true whether the species performs a fixed-direction migration or a multi-directional dispersive migration. Some long-lived species, in which individuals do not breed until they are several years old, show a progressive change to shorter distance moves or to shorter periods away from the breeding areas, with increasing age (Chapter 15). Such patterns have been noted in a wide range of bird species, including Grey Heron Ardea cinerea (Olssen 1958), Black Kite Milvus migrans (Schifferli 1967), Eurasian Oystercatcher Haematopus ostralegus (Goss-Custard et al. 1982), Herring Gull Larus argentatus (Coulson & Butterfield 1985), Common Guillemot Uria aalge (Birkhead 1974), Great Cormorant Phalacrocorax carbo (Coulson 1961) and Northern Fulmar Fulmarus gla-cialis (Macdonald 1977). This means that some individual birds occupy different areas in successive winters, as they age, or that they spend progressively less time on their wintering areas as they age.

Studies on other species have indicated increasing winter site-fidelity with increasing age and social status, greater in males than in females (for Great Black-backed Gull Larus marinus see Coulson et al. 1984, for Great Cormorant Phalacrocorax carbo see Yesou 1995, for Mallard Anas platyrhynchos see Nichols & Hines 1987, for swans see Table 17.7), and matching the findings from breeding areas (see above). However, it is usually uncertain how much the higher return

Table 17.7 Percentage return to wintering sites by different age/social classes of swans

Pairs without Pairs with Single adults Yearlings Source young young

Tundra Swan Cygnus 162 78 116 67 43 58 20 55 Scott (1980) columbianus bewickii

Whooper Swan Cygnus 24 88 7 86 63 70 5 60 Black &

cygnus Rees(1984)

Note: Tundra Swan data from Welney Refuge, southeast England; Whooper Swan data from Caerlavorock Refuge (12 km2), southwest Scotland. Some returns occurred after more than one year, and in no group were sex differences detected.

rates of adult birds, compared to young ones, are due to greater site-fidelity (and movements elsewhere) and how much to greater survival from the previous year.

Comparison of breeding and non-breeding site-fidelity

Many species show greater fidelity to breeding than to wintering sites. This reflects not only spatial variation in wintering habitat from year to year, but also the fact that individuals of many short-distance migrants migrate in their first but not in subsequent years, or migrate different distances in different years (Chapters 12, 16 and 18). However, it is also apparent in some long-distance migrants, such as Willow Warbler Phylloscopus trochilus and White Stork Ciconia ciconia (Salewski et al. 2002, Berthold et al. 2002). White Storks show extreme fidelity to their breeding places, normally returning year after year to the same nests, but they are much less faithful to particular wintering places. During studies based on satellite-tracking, four White Storks were tracked from Europe to their African winter quarters several times, one bird on nine successive journeys (Berthold et al. 2002). These birds occupied different areas in Africa from year to year, depending on the food supply. One bird wintered in Tanzania in one year, further north in the Sudan in the second year, but then went as far as South Africa in the third year and Botswana in the fourth. Hence, this one individual wintered in different years in places that extended from the northern tropics to the southern temperate zone.

Other species show the opposite tendency, with less fidelity to their breeding areas than to their wintering areas. In the American Redstart Setophaga ruticilla and Black-throated Blue Warbler Dendroica caerulescens, lower site-fidelity in summer reflected a breeding habitat that varied strongly in suitability from place to place, and from year to year (Holmes & Sherry 1992). The same holds for some arctic-nesting shorebirds, such as Curlew Sandpiper Calidris fer-ruginea and Sanderling Calidris alba whose breeding sites may change from year to year according to snow-melt patterns (Tomkovich & Soloviev 1994), but whose wintering sites are more consistent and are used by the same individuals year on year (Elliott et al. 1977, Evans 1981). To set against these patterns, other species show extreme site-fidelity in both breeding and wintering areas, while others show little or no site-fidelity in either breeding or wintering areas, depending on the degree of predictability in their habitats and food supplies (Chapters 18 and 19).

0 0

Post a comment