Co

Calidris melanotos) that have colonised eastern Siberia from North America continue to winter in the New World (Table 22.1). It is as though the migration routes of all these various species retrace their ancestral routes of spread. As a species expands its breeding range, it simply adds step after step onto its already existing migration route. On this view, the route we see now could be interpreted largely as a consequence of post-glacial colonisation history, as has long been appreciated (Thomson 1926, Cox 1968).

On the basis of such findings, Bohning-Gaese et al. (1998) suggested that existing long-distance migration patterns prevent species from spreading from Eurasia to North America, or vice versa, thus explaining why many long-distance migrants occur on one land mass or the other but not on both. However, several species breed right across both North America and Eurasia, and winter in both the New and Old Worlds respectively. In this group, birds from North America winter in central and South America, while their conspecifics from Eurasia winter mainly in Africa and Southeast Asia. Examples include many shorebirds and waterfowl, Peregrine Falcon Falco peregrinus and Merlin F. columbarius, and several passerines, such as Barn Swallow Hirundo rustica and Sand (Bank) Martin

Table 22.1 Species that breed in both Eurasia and North America, but winter entirely in the Old World or entirely in the New World

Recent colonists from Siberia to Alaska that winter in the Old World

Recent colonists from Alaska to Siberia that winter in the New World

Recent colonists from Europe to Greenland and northeast Canada that winter in the Old World

Northern Wheatear Oenanthe oenanthe Bluethroat Luscinia svecica Arctic Warbler Phylloscopus borealis

Yellow Wagtail Motacilla flava

White Wagtail Motacilla alba

Red-throated Pipit Anthus cervinus Siberian Tit Parus cinctus

Rustic Bunting Emberiza rustica

Rufous-necked Stint Calidris ruficollis

Bar-tailed Godwit Limosa lapponica

Gray-cheeked Thrush Catharus minimus Pectoral Sandpiper Calidris melanotos

Baird's Sandpiper Calidris bairdii

Buff-breasted Sandpiper Tryngites subruficollis

Long-billed Dowitcher Limnodromus scolopaceus

Snow Goose Chen caerulescens Sandhill Crane Grus canadensis

Northern Wheatear Oenanthe oenanthe Common Ringed Plover Charadrius hiaticula

Red Knot Calidris canutus islandica Ruddy Turnstone Arenaria interpres

Brent Goose Branta bernicla hrota

Riparia riparia. At some time in the past, all these species presumably colonised one northern land mass from the other. They show that not all species have been constrained in range expansion by the difficulty of evolving new migration routes. It may be largely a matter of time, with the longest-established species having evolved new routes to new wintering areas, and the most recent colonists still retaining ancestral routes to the old wintering areas.

Similar patterns are apparent within the northern land masses. Some bird species breed across the whole of northern Eurasia, for example, yet winter entirely in Africa or entirely in Southeast Asia (Table 22.2). Birds breeding at the most distant end of Eurasia thus travel across the entire west-east span of this land mass, as well as to lower latitudes, to reach their wintering areas. The most likely explanation is that these species have spread to breed across Eurasia in post-glacial times from one end to the other, yet retained their ancestral wintering grounds. Some species, such as Greenish Warbler Phylloscopus trochiloides and Common Rosefinch Carpodacus erythrinus, are known to have spread into Europe since the nineteenth century from breeding areas further east, and all have retained their migration to Southeast Asia. Yet again, their long migration routes can be regarded as a retracement of ancestral routes of spread, another legacy of biogeo-graphical history (Landsborough Thompson 1926, Cox 1968). Such routes would presumably not persist unless they served their purpose in present conditions, but a difficulty in evolving (by gradual change) new routes to totally new wintering areas may be the main reason for their continuing existence.

Most of the species that breed across Eurasia, from one end to the other, are resident species, while relatively few are long-distance migrants. The latter tend to have more restricted distributions, breeding in one half or the other. This finding is consistent with the view that expansion of breeding range is constrained in long-distance migrants by the difficulty of evolving appropriate new migration routes (Bensch 1999), although this situation may change through time. An analysis of the migrations of arctic-nesting shorebirds indicated that migration routes were constrained not by distance as such, but by distance across seas and other unfavourable areas, possibly because of the complex adaptations required for barrier-crossing and extensive detour migrations (Henningsson & Alerstam 2005).

Some other migratory species that breed across Eurasia have split wintering grounds, with western populations moving southwest into Africa and eastern populations into Southeast Asia. These species may have survived the glaciations in more than one refuge, one lying near or within Africa and the other near or within Southeast Asia. For example, the Great Reed Warbler Acrocephalus arundi-naceus winters in both regions, so may have had two refugia (or groups of refugia), from which it subsequently spread across the Palaearctic. But it is also possible that such species survived the last glaciation at one end of the land mass, spread across the whole land mass following ice melt and vegetation growth, and then developed a secondary wintering area at the other end of the land mass. Other examples of similar patterns are given in the later section on migratory divides.

Such patterns are also evident on a smaller scale, for example within Europe. Thus, all Red-backed Shrikes Lanius collurio migrating from Europe to Africa cross at the eastern side of the Mediterranean, including those from Spain that start their autumn journey by flying northeast then east. Some other summer visitors that

Table 22.2 Different migration patterns of passerine species that breed across much of Eurasia and winter in the tropics

(1) Western and eastern (2) Western and eastern (3) Western breeding populations winter in Africa and eastern populations winter entirely breeding populations winter populations in Southeast Asia, with a migratory divide in Africa entirely in Southeast Asia a

Table 22.2 Different migration patterns of passerine species that breed across much of Eurasia and winter in the tropics

Greater Whitethroat

Scarlet Rosefinch

Grasshopper Warbler

Tawny Pipit

Sylvia communis

Carpodacus erythrinus

Locustella naevia

Anthus campestris

Garden Warbler

Lanceolated Warbler

Great Reed Warbler

Tree Pipit

Sylvia borin

Locustella lanceolata

Acrocephalus arundinaceus

Anthus trivialis

Willow Warbler

Arctic Warbler

Lesser Whitethroat

Red-throated Pipit

Phylloscopus trochilus

Phylloscopus borealis

Sylvia curruca

Anthus cervinus

Spotted Flycatcher

Greenish Warbler

Common Chiffchaff

Yellow Wagtail

Muscicapa striata

Phylloscopus trochiloides

Phylloscopus collybita

Motacilla flava

Common Redstart

Red-breasted Flycatcher

Eurasian Golden Oriole

Bluethroat

Phoenicurus phoenicurus

Ficedula parva

Oriolus oriolus

Luscinia svecica

Northern Wheatear

Taiga Flycatcher

Sand Martin

Black Redstart

Oenanthe oenanthe

Ficedula albicilla

Riparia riparia

Phoenicurus ochruros

Rock Thrush

Pechora Pipit

Barn Swallow

Desert Wheatear

Monticola saxatilis

Anthus gustavi

Hirundo rustica

Oenanthe deserti

Ortolan Bunting

Siberian Accentor

Red-rumped Swallow

Blue Rock Thrush

Emberiza hortulana

Prunella montanella

Hirundo daurica

Monticola solitarius

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