Migratory Systems as Adaptive Responses to Spatial and Temporal Variability in Orientation Stimuli

Abstract

The relationship among the numerous environmental stimuli nocturnally migrating birds can use to guide migration continues to challenge researchers 30 years after it was demonstrated that migrants could orient not only by a variety of celestial cues (Kramer 1953; Sauer 1957; Emlen 1967; Moore 1980; Able 1982), but by the Earth’s magnetic field as well (Wiltschko and Wiltschko 1972). In particular, in the field of avian orientation there is a long tradition of attempts to determine the hierarchical relationship among available orientation stimuli. For example, do nocturnal migrants preferentially rely on one orientation stimulus over another? Of even more interest, can directional information from one stimulus serve as a reference to calibrate an orientation response to another? Indeed, a vast body of literature has addressed this last question, and for a while a consensus was reached among researchers. In both North American (Savannah sparrow, Passerculus sandwichensis) and European (pied flycatcher, Ficedula hypoleuca) breeding species, young birds can use celestial information (plane of skylight polarization, sunset position or stars) to calibrate a migratory orientation response to the Earth’s magnetic field (Bingman 1983; Bingman et al. 1985; Able and Bingman 1987; Able and Able 1990, 1993; Prinz and Wiltschko 1992). This seems to be a universal feature of avian migratory orientation systems, and also makes adaptive sense. While rotation of the Earth relative to the sky provides a spatially and temporally stable reference for defining geographic north and south, changing geomagnetic declination in time and space renders the Earth’s magnetic field relatively unreliable as a geographic reference.