Apparent primary cue

Source

Magnetic versus stellar cues

Indigo Bunting Passerina cyanea

European Robin Erithacus rubecula

Sylvia species

Bobolink Dolichonyx oryzivorus

Magnetic versus sunset cues

Savannah Sparrow Passerculus sandwichensis

Savannah Sparrow Passerculus sandwichensis

White-throated Sparrow Zonotrichia albicollis Dunnock Prunella modularis

European Robin Erithacus rubecula

Magnetic field, planetarium stars

Magnetic field, stars

Magnetic field, stars

Magnetic field, planetarium stars

Sunset, magnetic field

Sunset, magnetic field, stars

Sunset, magnetic field

Sunset, magnetic field Sunset, magnetic field

Stars

Magnetic field

Magnetic field Stars

Stars

Magnetic field

Magnetic field Magnetic field

Sunset by mirrors; Sunset sunset, magnetic field

Sunset by mirrors Sunset

Sunset by clock Sunset shift

Magnetic field Magnetic field

Sunset by mirrors Sunset

Emlen (1967a, 1967b)

Wiltschko & Wiltschko (1975b), Bingman (1987) Wiltschko & Wiltschko (1975a) Beason (1987)

Moore (1982, 1985)

Moore (1985)

Able & Cherry (1986)

Bingman & Wiltschko (1988) Sandberg (1991)

Northern Wheatear Oenanthe oenanthe Yellow-faced Honeyeater Lichenostomus chrysops

Polarised skylight versus sun

White-throated Sparrow Zonotrichia albicollis

White-throated Sparrow Zonotrichia albicollis

Yellow-rumped Warbler Dendroica coronata Northern Waterthrush Seiurus noveboracensis Kentucky Warbler Oporornis formosus Blackcap Sylvia atricapilla

Sunset, magnetic field Sun, magnetic field and magnetic cues

Sunset, magnetic field

Polarised light, magnetic field Sunset, magnetic field

Sunrise, magnetic field

Sunset, magnetic field

Blackcap Sylvia atricapilla Sunset

European Robin Erithacus rubecula From Able (1993).

Magnetic field Magnetic field

Polarised light by polaroids

Polarised light by polaroids Polarised light by polaroids

Polarised light by polaroids

Polarised light by polaroids Depolarised skylight

Sunset

Magnetic field

Polarised light Polarised light Polarised light Polarised light

Polarised light Polarised light

Sandberg et al. (1991) Munro & Wiltschko (1993)

Able (1982b, 1989) Able (1989)

Phillips & Moore (1992), Moore & Phillips (1988)

Moore (1986)

Helbig &

Wiltschko (1989) Helbig (1990, 1991a)

shifted magnetic field (Able & Able 1995). Subsequent experiments revealed that Savannah Sparrows used polarised light cues from the region of sky near the horizon to calibrate the magnetic compass at both sunrise and sunset (Muheim et al. 2006). Once a bird has established geographic north and south with respect to the local geomagnetic field, magnetic cues could come to assume a greater role in navigation. In another experiment, Catharus thrushes caught on migration were exposed to a deflected magnetic field during twilight, and then released and radio-tracked on their subsequent night flights (Cochran et al. 2004, Cochran & Wikelski 2007). Their tracks indicated that the thrushes recalibrated their magnetic compass in relation to twilight cues, and then relied on their (miscalibrated) magnetic compass for their nocturnal flight, apparently ignoring stellar cues. The experimental birds changed to normal orientation again on succeeding nights, apparently having recalibrated their magnetic compass (correctly) back to north. Daily recalibration of the magnetic compass could explain how birds cope with changes in magnetic declination during the route, as well as various local magnetic anomalies; it could also explain how birds operating with a magnetic inclination compass can cross the equator without becoming disoriented. The birds' view of the twilight sky near the horizon may also be decisive for the calibration rank between magnetic and celestial cues (Muheim et al. 2006).

In practice, birds probably use information from several compass mechanisms, with emphasis on whatever cues are most reliable in the conditions prevailing, switching from one type of cue to another during the course of a journey, depending on location, weather and light values (Wiltschko et al. 1998, Muheim et al. 2003). The present variety of orientation mechanisms found among birds may reflect some traits that birds have evolved from reptilian ancestors (for example, magnetic and sun-based orientation), whereas other traits (such as stellar orientation) are as yet known only from birds, and may be specific to them.

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