Bell, Christopher Paul; (1995) An enquiry into evolutionary aspects of geographic variation in avian biology, with special reference to the yellow wagtail and meadow pipit. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The Yellow Wagtail is a species with a range of characteristics which are of special interest to evolutionary biologists, including marked racial variation and leapfrog migration. Field observations of the behaviour of breeding Yellow Wagtails at three sites in Europe are presented, together with an analysis of geographic variation in the morphometries and clutch size of the species, and of field and clutch size data on its close relative the Meadow Pipit. Proposed explanations of leapfrog migration and other anomalous migration patterns are discussed. Theories which assume the role of competition, relying on evolutionary explanations of differences in competitive ability between populations are criticised, and an alternative explanation of leapfrog migration is proposed, based on the relationship between seasonality in breeding and wintering areas. Some aspects of clutch size variation in the two study species are shown to be explicable in terms of variation in the distance to wintering sites and in the wintering space available to different populations, and their effect on breeding population density. Behavioral patterns in Yellow Wagtails in the three study sites are compared, including delivery frequency, quantities of food delivered, central place foraging, brooding of nestlings and nest vigilance, as are growth rates in Yellow Wagtails and Meadow Pipits. The significance of these patterns for geographic variation in life history is discussed. Male Yellow Wagtails mainly supplement food deliveries to nestlings by females when demand is high, such as in large broods or at times of high demand per nestling. Nest guarding roles vary between sites, with more emphasis on nest guarding by males in Spain. Wind is shown to profoundly affect the behaviour of adult Yellow Wagtails brooding nestlings, and to depress growth rates in Yellow Wagtail and Meadow Pipit nestlings. Geographic size variation in Yellow Wagtails corresponds with variation in temperature, altitude, and humidity of breeding sites, and shape variation is discussed with reference to differences in diet and habitat between the three field sites. Some aspects of morphometric variation correspond to subspecies differences, while others vary clinally across subspecies boundaries.

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