A natural change in the migration of Blackcaps

A new migration route of central European Blackcaps Sylvia atricapilla to winter quarters in Britain and Ireland (as opposed to the western Mediterranean area) was discovered from ring recoveries (Zink 1962, Berthold 1995). Prior to 1960, Blackcaps were rare during winter in Britain and Ireland, but from the late 1980s thousands of individuals wintered regularly (Lack 1986). For nearly 40 years, ringing provided no indication that these birds were raised in Britain or Scandinavia. However, many Blackcaps ringed during the breeding season in Belgium, southern Germany and western Austria have been recovered in a west to northwest direction, some of them in Britain and Ireland. In all these continental areas, some southwest migrants also breed.

It therefore appeared that, within a period of 30 years, a portion of the southwest migrating population in central Europe had shifted their migration toward the west and northwest and that the majority of British-wintering Blackcaps originated from this region. To see whether this new migration had a genetic basis (as opposed to being due to increased wind drift or other prevailing conditions), some wintering Blackcaps were caught in southwest England, bred in captivity in Germany, and then tested for directional preferences in standard conditions. Both the adults and their offspring showed a west-northwest preference, indicating that the new migration route represented an evolutionary change that had apparently arisen within recent decades (Figure 20.4; Berthold et al. 1992a, Helbig 1994, 1996).

Such a change may have started from one or more individuals with an unusual directional tendency that found themselves in a new area where they could survive the winter. These pioneers may have been individuals from the extreme end of a range of directional preferences already existing within the central European source population, or mutant individuals with a new directional preference not previously represented. Alternatively, as suggested by Busse (1992), they may

Figure 20.4 Autumn orientation of Blackcaps Sylvia atricapilla caught in winter in Britain (a), their captive-bred first-generation offspring (b), and a control group from southwest Germany (c). Each symbol shows the direction of the mean vector of one individual during 15-20 tests. The findings indicate that British-wintering Blackcaps have different inherent directional preferences from Blackcaps that breed in the same region of central Europe but winter to the southwest in Iberia. Modified from Helbig (1996).

have been individuals from a normal southeastward migrating population with an inherent fault in their migration control system, which led to their migrating in the reverse direction to normal for that time of year (that is, taking the spring direction in autumn and vice versa, see Chapter 10). But whatever the origin of the pioneers, it would not have been enough merely for them to have been blown off course, because the new route could not have been inherited by their offspring. It must have been genetically influenced. Once started, the selection pressures that may have favoured wintering of continental Blackcaps in Britain rather than in the Mediterranean region could have included: (1) factors acting in the new wintering area, such as progressively milder winters, or improved food supply provided at garden feeders, and at winter fruit bushes planted in recent decades; (2) factors related to the location of the new wintering areas, which involve a shorter migration distance by up to 1500 km and possibly an earlier return to the breeding areas; or (3) factors acting in the former Mediterranean wintering areas such as drought-induced declining food supplies and increasing competition. Other ringed Blackcaps from central Europe have been found in autumn further north in western Europe than the British birds, but have not established wintering populations, possibly because at these higher latitudes the winters are too cold for them.

Captive British-wintering Blackcaps show migratory restlessness at a date that would enable them to arrive in their central European breeding areas by 1 April, whereas Mediterranean wintering birds would not arrive until about 17 April. In addition, the mean testis size at the date of arrival was about 25% greater in the first group than in the second. An early return could enable British-wintering Blackcaps to pair preferentially with one another (assortative mating based on differential arrival times), and perhaps raise an extra brood each year, speeding the evolution of the new habit. A field study revealed that, on average, birds from British wintering areas did indeed arrive in central European breeding areas earlier than those from Spanish wintering areas, and were 2.5 times more likely to mate with one another than at random (Bearhop et al. 2005). The British-wintering birds also settled in the better territories, produced larger clutches and broods. In view of these findings, the British-wintering birds may eventually out-compete and replace the Spanish-wintering ones. It is also surprising that British-breeding Blackcaps have apparently not yet become partially resident, with some individuals remaining all year in their breeding areas. So far, only one recovery of a confirmed British-bred Blackcap has been obtained in Britain in winter, but this may well mark the start of a new habit. Meanwhile, most British Blackcaps continue to winter in Iberia and North Africa.

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