New Routes And Range Expansion

The fact that regular migrants are much more likely to turn up in unexpected places than are non-migrants emphasises the connection between migration and vagrancy, as does the concentration of vagrant records in the normal migration seasons. Although migrants travel longer distances than non-migrants, and are often blown off course, they are not necessarily more likely to stay and establish themselves in new areas. When migrants arrive in a new area, they are usually in a migratory state, programmed to re-orientate and continue their journeys. Ring recoveries confirm that many soon move on again (Table 10.3).

Not all vagrants return to their usual range, however, and some have established territories, and occasionally paired up with local species and bred. Examples from Britain include: a male Pied-billed Grebe Podilymbus podiceps which paired with a Little Grebe Tachybaptus ruficollis and produced three hybrid young; a female Blue-winged Teal Anas discors which paired with a Northern Shoveler Anas clypeata and raised three hybrid young; a male and female American Black Duck Anas rubripes each of which paired with a Mallard Anas platyrhynchos and produced young; and a female Lesser Crested Tern Sterna ben-galensis which paired with a Sandwich Tern Sterna sandwichensis and produced young in at least three years. In addition, individual Black-browed Albatrosses Diomedea melanophris from the Southern Ocean set up territories and displayed (without success) in Northern Gannet Morus bassanus colonies over periods of 3, 23 and 35 years respectively (at Bass Rock, southeast Scotland, in 1967-1969, Hermaness, Shetland, in 1972-1995; and Faeroes in 1860-1894; the first two cases may have involved the same bird in different colonies, and may also have been the bird seen in Iceland in 1966).

In addition, some vagrants, having once wintered in an area, return there year after year, as in normal homing behaviour, with examples from Britain involving a King Eider Somateria spectabilis (1986-1991), Whistling Swan Cygnus columbi-anus (1985-1986) and Wallcreeper Tichodroma muraria (1976-1977), and in springsummer even an Ancient Murrelet Synthliboramphus antiquus which normally occurs in the North Pacific, but was seen off Lundy Island in three successive summers (Cottridge & Vinicombe 1996). The record holder, however, was a Ring-billed Gull Larus delawarensis, which turned up in 13 successive winters in Stromness, Orkney, Scotland (Fraser & Rogers 2002). These birds were not ringed, but their individual identification was assumed because they occupied the same places in successive years. Other records of winter site fidelity by vagrants have come from California, involving Tufted Duck Aythya fuligula (with one drake returning to the same locality for five years), Black-headed Gull Larus ridibundus, Little Gull Larus minutus and Brown Thrasher Toxostoma rufum (Robertson 1980). On the other hand, the two records of a North American Belted Kingfisher Ceryle alcyon in Britain were 71 years apart (1908 and 1979), yet still came from the same inland locality (Cottridge & Vinicombe 1996).

Other recoveries imply fidelity to off-route stopover sites by vagrants. An adult male Bluethroat Luscinia svecica was ringed on 17 May 1958 in Devon, southwest England, more than 300 km west of the usual migration route, and retrapped at the same site on 5 May 1963. Another first-winter male Bluethroat ringed at the same site on 21 September 1966 was retrapped there on 14 September 1968. This suggests that, once having taken a particular migration route, at least some individuals repeat it in successive years, even though it involves a detour (see Chapter 8 for other instances in birds that were radio-tracked in successive years). Such recoveries confirm that at least some out-of-range individuals survive and return at later dates.

Some of the Siberian vagrants that appear in western Europe have managed to survive into winter. Records are frequent for Richard's Pipits Anthus richardi,

Pallas's Leaf Warblers Phylloscopus proregulus and Yellow-browed (Inornate) Warblers Phylloscopus inornatus, but even rarer species have occasionally been seen then, including Olive-backed Pipit Anthus hodgsoni, Dusky Warbler P. fuscatus, Little Bunting Emberiza pusilla, Rustic Bunting E. rustica and others. Moreover, Richard's Pipits now winter regularly in Spain (Cramp 1988), and Yellow-browed (Inornate) Warblers and Pallas's Reed Buntings Emberiza pallasi are seen there occasionally, as are Richard's Pipits and Common Rosefinches Carpodacus erythrinus in Morocco, and Red-breasted Flycatchers Ficedula parva in the Canary Islands. It is, of course, almost impossible to distinguish a true vagrant (a bird well off the normal migration route) from a regular migrant on a route used by only a small number of individuals.

Genetically controlled deviations from the normal migration direction occasionally lead to the establishment of new migration routes and wintering areas, as illustrated by Blackcaps Sylvia atricapilla that now migrate from central Europe to winter in Britain (Chapter 20). Is it possible, then, that many of the vagrants which turn up in particular localities in increasing numbers are in the process of establishing new wintering populations, and that birds with appropriate deviant directions are gradually increasing in the population? As in the Blackcap, this is likely to occur only if: (1) the new wintering area permits good overwinter survival, and (2) the deviant individuals more often pair with one another than with individuals with 'normal' directional tendencies. This could happen, for example, if birds from the new wintering area returned to the breeding area at a somewhat different date than those from the regular wintering area (for example, see Bearhop et al. 2005). Although such assortative mating is not essential for any genetically controlled new behaviour to be passed on, it would greatly facilitate its establishment and spread through the population.

Not long ago, bird-watchers considered the Pallas's Leaf Warbler Phylloscopus proregulus to be one of the rarest vagrants reaching northwest Europe. Breeding in Siberia, it provided only three records in Britain prior to 1958, yet after 1980, the annual total in several years exceeded 100 individuals (Figure 10.1; Cottridge & Vinicombe 1996). The Yellow-browed (Inornate) Warbler Phylloscopus inornatus from Siberia increased even more, culminating in a record number of 615 in 1985. It is now the commonest Siberian vagrant to western Europe. Possibly both species have established new wintering areas, for which Britain lies on the route. They seem to arrive slightly earlier in the north of Britain than in the southwest, suggesting some onward movement, perhaps to Iberia or western Africa. Occasional spring records may reflect a return passage.

In North America, many parulid warblers have extended their breeding ranges far to the northwest since the end of the last glacial period. Most mirror-image vagrants from such populations probably end up far offshore over the Pacific Ocean and die. There are, however, several records of banded birds being recovered at the same west coast location in 2-3 consecutive winters, and the 'Myrtle' race of the Yellow-rumped Warbler Dendroica coronata now has a regular west coast winter range separated from the main winter range to the south and east, at locations consistent with mirror-image migration (Diamond 1982). Hence, this process, if under genetic control, might also be producing new migration routes, wintering ranges, and migratory divides (Chapter 20).

Such developments may also be occurring in Lesser Black-backed Gulls Larus fuscus which are wintering in increasing numbers on the east coast of North

Table 10.5 Transatlantic ring recoveries of various bird species some of which breed on both sides, so would not otherwise have been recognised as vagrants. The list gives vagrants only, and regular transatlantic migrants are listed in the footnote
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