Most of the studies cited in this chapter were concerned with particular species or suites of similar species, so it is hard to tell what proportion of an avifauna has changed in migratory habits in recent decades (apart from migratory timing which has been studied in many species). Over the past 50 years, climate changes have been more marked in some regions than in others, and in particular regions studies reporting changes in migratory behaviour were more likely to be published than those finding no change. However, among the bird species that breed in Britain, 73 provided enough ring recoveries from a sufficiently long period to look for changes in the lengths and directions of migrations. In total, 51 (70%) of these species showed no significant change in these respects during the twentieth century, but in 15 species movements had become shorter, in five they had become longer, while in two the movements had changed in other ways. The 22 species that showed changes were significantly more than the four expected on a significance level of 5%. They included passerines, raptors, waders, waterfowl and seabirds (G. Siriwardena & C. Wernham, in Wernham et al. 2002). Similarly, of 30 species that breed in Germany and provide enough ring recoveries, eight species showed decreasing mean recovery distances with time, while five species showed increasing mean recovery distances (Fiedler et al. 2004). Again the numbers that showed change were significantly greater than the two expected on a significance level of 5%. Such studies confirm that changes in the migration behaviour of birds have been common over the last several decades.

These various observations, along with the breeding experiments discussed in Chapter 20, all serve to confirm that migration is a dynamic phenomenon, subject to continual change in response to prevailing conditions. Some aspects, such as an abrupt change in the direction of migration, imply rapid evolutionary change, but other aspects could represent either genetic or facultative responses to changing conditions. In any case, in any marked long-term change, both are likely to be involved, the birds responding initially by facultative means, and eventually by genetic change, as natural selection comes to bear. Facultative responses are relatively limited (though variable in extent between species) and, if environmental conditions continue to change in the same direction, such responses become inadequate to deal with the new conditions. Only genetic change may then enable the population to respond appropriately to conditions beyond the previous range.

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