The study of migratory behavior is, at a fundamental level, the study of geographic variation in behavior. This is necessarily the case when residency grounds at either end of the migratory route differ in spatial extent, because different directional movement patterns will be required of individuals from different parts of the two ranges. As many migratory animals are widespread, substantial differences in migratory routes exist among populations of single species (examples in Dadswell et al. 1987, Baker 1991, Dingle 1991, Groot and Margolis 1991, Berthold 1993). Because the individuals that migrate often have not done so before and often do not have older migration-experienced individuals to follow (e.g., Baker 1991, Berthold 1996), the study of navigational mechanisms and their genetic underpinnings is essential to understanding migratory behavior. Naturalists have long been captivated by the problem of control mechanisms in migratory behavior. As early as 1702, von Pernau suggested that birds were “driven at the proper time by a hidden drive.” In modern terms, this amounted to suggesting that migratory behavior was triggered by innate or genetically programmed stimuli, rather than by environmental factors alone. This speculation was supported by the discovery of endogenous annual cycles more than 260 years later (see Gwinner 1986 for a review). Our understanding of the control mechanisms of migratory behavior has expanded rapidly during the last 90 years. Research in the field has elucidated, for example, genetic and endocrine control mechanisms and their interface with environmental cues such as photoperiod, relationships between environmental conditions, such as weather and food availability and the timing of migration, and unique physiological and morphological correlates of migratory behavior (for reviews, see Berthold and Terrill 1991, Berthold 1996). Birds, fishes, and insects have proven especially valuable subjects for the study of migratory behavior, and all show substantial population differentiation in migratory patterns within species (Dadswell et al. 1987, Dingle 1991, Groot and Margolis 1991, Berthold 1993). In this chapter I focus on migratory behavior in birds, especially that in the blackcap, Sylvia atricapilla, an Old World warbler that has been the subject of extensive research in my laboratory since the early 1970s.
Geographic variation in call structure, likelihood, and call-song associations across subspecies boundaries, migratory patterns, and habitat types in the Marsh Wren (Cistothorus palustris)