Providing enough of the night sky is visible, nocturnal migrants proved able to use the stars as a guide. When tested in orientation cages, they orientated correctly on clear starry nights, but became inactive or disoriented under overcast skies. They also became confused if star patterns were varied experimentally in a planetarium (Sauer & Sauer 1955, 1960, Hamilton 1962c, Emlen 1967a, 1967b). When Indigo Buntings Passerina cyanea were tested under a natural starry sky during autumn migration, they preferred southerly directions. They maintained this southerly preference under an artificial star pattern imitating the natural sky in a planetarium. But when the artificial star pattern was changed by 180°, the birds changed their directional preference to the north. Under a static night sky, no obvious migratory restlessness occurred. The development of a star compass evidently involved learning, with celestial rotation as a directional reference, and captive Indigo Buntings without early experience of the night sky failed to orientate correctly in a planetarium (Emlen 1967b, 1975). But detecting the rotation of the night sky probably takes considerable time - it could not be determined at a glance. Not surprisingly, therefore, the birds did not depend on the axis of rotation per se; rather, they quickly learnt star patterns that indicated where the axis lay, and thereafter relied on those patterns. As with the sun compass, however, the ability and tendency to acquire this knowledge was apparently innate. Similar results were later obtained with Garden Warblers Sylvia borin (Wiltschko et al. 1987).
Some birds developed this ability under unnatural star patterns, as shown for example by the selective blocking out of constellations in the artificial sky. The key factor was the rotation of the night sky about the Pole Star, and even an extremely simplified and reduced star pattern would suffice, so long as it rotated about a single conspicuous star. Thus, nestling Indigo Buntings raised under an artificial sky with the star Betelgeuse (in the constellation Orion) as the point of rotation treated Betelgeuse as the Pole Star when subsequently tested. The same was found in other migratory species (Emlen 1975).
The use of a star compass has now been demonstrated experimentally in at least six different bird species, but may be widespread in nocturnal migrants. If the birds use the rotating star pattern only to define the position of the poles, then no correction for time of day is necessary. They may, however, gain further information from star patterns. As birds proceed on their journeys, lasting up to several weeks, stars that were once visible disappear below the horizon behind them, while others appear above the horizon in front, another indication that birds are unlikely to rely throughout on particular star patterns. Moreover, many bird species, such as the Lesser Whitethroat Sylvia curruca, change direction during their migration from Europe to Africa, heading first southeast into the Middle East, and then south-southwest into Africa. When shown northerly skies in a planetarium during the autumn migration season, individuals of this species headed southeast, but when shown skies characteristic of more southerly latitudes, they headed southwest (Sauer 1957). This finding indicated that birds could respond appropriately to skies encountered at different points in their journey. Moreover, Pied Flycatchers Ficedula hypoleuca and Redstarts Phoenicurus phoenicurus compensated by changing direction after a simulated east-west displacement in a planetarium, indicating again an appropriate response to sky patterns (Rab0l 1994).
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