The English name Yellow Wagtail is commonly used to describe the polytipic species Motacilla flava. More specifically it is also used for the British breeding race M.f.flavissima, and various English names are applied to other races: e.g. Blue-headed Wagtail M.f.flava, Ashy-headed Wagtail M.f.cinereocapilla, Grey-headed Wagtail M.f.thunbergi, Black-headed Wagtail M.f.feldegg, Sykes's Wagtail M.f.beema. As the former name is ambiguous and the latter ones would be unnecessarily ponderous when describing a group of birds of several races. I have tended to use Latin epithets throughout this thesis. However, this does not allow an effective plural noun to be applied, so in addition I have used the term "Wagtails", which, unless otherwise qualified, always refers to the species Motacilla flava. This also allows a degree of variety to be introduced to the text. African savanna regions have a climate which is markedly seasonal, although these seasons do not directly correspond with those experienced in temperate regions. Nevertheless, in the regions in which this work was undertaken, the dry season occurs from about November to March and is a period of relatively severe conditions for many bird species. Consequently I bave tended to refer to this period as winter, which should be taken as synonymous with dry season. As very few studies of the overall biology of Palaearctic migrants wintering in Africa have been previously unuertaken, I have attempted to make this account as comprehensive as possible. Inevitably this makes the text rather long, and I have therefore presented as much data as possible in the form of diagrams, which allow a greater amount of information to be presented in a small space.

<p>This thesis relates modern speciation theory to the effects of sudden changes in the range and size of populations. Special reference is made to the climatic oscillations during the Quaternary ice ages. A meta-analysis of laboratory experiments showed that support for allopatric speciation is weak, especially for the peripatric type of allopatric speciation. Furthermore, factors traditionally believed to increase the likelihood of speciation have had little effect on the generation of reproductive isolation in speciation experiments. However, the method of testing reproductive isolation appeared important, in the sense that experimentally derived sister populations were likely to demonstrate reproductive isolation from each other but not from the unaffected mother population. Raw data from mating tests showed that the poor isolation between mother and daughter populations was an effect of asymmetric mate preferences towards males from the mother population. This suggests that peripatric speciation can be effective in generating reproductive isolation between sister populations. The proposed mechanism is that males become less attractive to females by losing certain secondary sexual traits during population bottlenecks, and that females shift their preferences towards other male traits. Support for this mode of speciation is found in the widespread bird genus <i>Motacilla</i> (wagtails). This genus is characterised by extensive plumage variation and contains a large number of widely distributed taxa in the northern parts of its distribution. This thesis shows that taxonomic diversity of wagtails is inversely related to complexity in song and to diversity in molecular and mitochondrial markers. The northern taxa seem to be descendants of southern populations, which were subjected to bottlenecks during expansions into re-opened habitats after the last ice age. The bottlenecks would not only reduce genetic diversity but also inhibit cultural transmission of song to the leading edge of colonisers, allowing sexual selection on other traits, such as plumage. Rapid plumage differentiation among wagtail taxa appears to be a recurrent process and has lead to convergent evolution, making the currently recognised species <i>Motacilla flava</i> (Yellow Wagtail) polyphyletic.</p>

This thesis relates modern speciation theory to the effects of sudden changes in the range and size of populations. Special reference is made to the climatic oscillations during the Quaternary ice ages. A meta-analysis of laboratory experiments showed that support for allopatric speciation is weak, especially for the peripatric type of allopatric speciation. Furthermore, factors traditionally believed to increase the likelihood of speciation have had little effect on the generation of reproductive isolation in speciation experiments. However, the method of testing reproductive isolation appeared important, in the sense that experimentally derived sister populations were likely to demonstrate reproductive isolation from each other but not from the unaffected mother population. Raw data from mating tests showed that the poor isolation between mother and daughter populations was an effect of asymmetric mate preferences towards males from the mother population. This suggests that peripatric speciation can be effective in generating reproductive isolation between sister populations. The proposed mechanism is that males become less attractive to females by losing certain secondary sexual traits during population bottlenecks, and that females shift their preferences towards other male traits. Support for this mode of speciation is found in the widespread bird genus Motacilla (wagtails). This genus is characterised by extensive plumage variation and contains a large number of widely distributed taxa in the northern parts of its distribution. This thesis shows that taxonomic diversity of wagtails is inversely related to complexity in song and to diversity in molecular and mitochondrial markers. The northern taxa seem to be descendants of southern populations, which were subjected to bottlenecks during expansions into re-opened habitats after the last ice age. The bottlenecks would not only reduce genetic diversity but also inhibit cultural transmission of song to the leading edge of colonisers, allowing sexual selection on other traits, such as plumage. Rapid plumage differentiation among wagtail taxa appears to be a recurrent process and has lead to convergent evolution, making the currently recognised species Motacilla flava (Yellow Wagtail) polyphyletic.