The ability of birds to detect sky polarisation patterns, which change with respect to the sun's position (being particularly striking around the time of sunset), has been demonstrated by experiment (Able 1993). At least seven species of normally nocturnal migrants have been shown to respond to manipulations of polarised light from the sky. The birds were tested outdoors in otherwise normal conditions in cages covered by sheet polaroids (Able 1982b, 1989, Moore & Phillips 1988, Phillips & Moore 1992, Helbig & Wiltschko 1989). In each case, the birds changed orientation as predicted by alterations in the alignment of the polaroids. However, the visual stimulus created by this procedure is quite unnatural, and birds sometimes oriented differently under artificial polarised light than under naturally polarised skylight (Helbig & Wiltschko 1989). Nevertheless, the experimental birds were clearly responding to polarised light as an orientation cue, rather than to other sunset features, as discussed in a later section.
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