Skylight Polarization Does Not Affect the Migratory Orientation of European Robins

Abstract

Among birds, only the Rock Dove (Columba livia) has been shown so far to be able to perceive the E-vector direction of linearly polarized light (Kreithen and Keeton 1974, Delius et al. 1976). The adaptive significance of this ability remains largely unknown, but may be related to the birds' capacity for orientation (Brines 1980, Brines and Gould 1982). Recently, Able (1982) reported that the orientation of nocturnally migrating White-throated Sparrows (Zonotrichia albicollis), is affected by manipulations of skylight polarization patterns at dusk, and Moore (1986) demonstrated that polarized light influences the early morning (dawn) orientation of night migrants tested during spring on the southwest coast of Louisiana in a similar way. In addition, recent data on the relationship between sunset orientation and polarized light, includes the Yellowrumped Warbler (Dendroica coronata) as a species which responds to polarized light (F. Moore, pers. comm.). Orientation experiments have indicated that there may be significant differences between bird species in their use of sun-related cues, the stars, and the geomagnetic field for the establishment and maintenance of nocturnal orientation (Wiltschko and Wiltschko 1978, Able and Cherry 1985, Moore 1987). In contrast to North American Savannah Sparrows (Passerculus sandwichensis) and White-throated Sparrows, the European Robin (Erithacus rubecula) seems to rely more extensively on the geomagnetic field for their orientation during the night (Wiltschko et al. 1971, Wiltschko and Wiltschko 1975, but cf. Katz and Michelson 1978 and Wallraff and Gelderloos 1978 for some conflicting evidence), as well as during the twilight period after sunset (Sandberg et al., in press a). Katz (1985), however, found that robins displayed uniform orientation during twilight tests under total overcast as well as after sunset under starry skies whereas in twilight tests under clear skies, the robins were able to select an appropriate fall migratory direction. I have undertaken experiments with migrating robins captured at the Falsterbo Bird Observatory (FBO, 55023'N, 12050'E) in south Sweden in order to investigate the possible influence of skylight polarization patterns on the dusk orientation of this species. METHODS