Directional preferences

The timing and duration of migratory restlessness, and patterns of fattening, are not the only features under endogenous control, as the same applies to directions. Birds taking different directions in the wild show the same directional preferences when tested in captivity. Some migrants in spring retrace their path from the previous autumn, but others take different routes at the two seasons (so-called loop migrants, Chapter 22). When tested for directional preferences in orientation cages, hand-reared Garden Warblers Sylvia borin kept in constant (12L:12D) conditions changed their mean heading from southwest to southeast part way through their autumn migration period. This corresponded with a change they would normally make part way through their journey between central Europe and Africa (Gwinner & Wiltschko 1978, 1980). They made no such change in spring, when they return by a more direct northerly route, requiring no change in direction during the journey. The whole pattern was in line with the loop migration routes between Europe and Africa revealed by ring recoveries. Other spontaneous shifts in directional preferences were also recorded during the migration seasons of captive Blackcaps Sylvia atricapilla (Helbig et al. 1989), Pied Flycatchers Ficedula hypoleuca (Beck & Wiltschko 1988) and Yellow-faced Honeyeaters Lichenostomus chrysops (Munro & Wiltschko 1993), and may well be widespread among birds.

The most obvious difference between seasons, namely the direction of travel, is apparently controlled by daylength changes and their effects on the physiological state of the bird. By appropriate manipulation of photoperiod, Emlen (1969) brought two groups of captive Indigo Buntings Passerina cyanea into spring and autumn migratory condition at the same time as one another. He then tested the directional preferences of both groups under identical planetarium skies. Birds in autumn condition oriented southward, those in spring condition northward. In some earlier experiments, Dark-eyed Juncos Junco hyemalis and American Crows Corvus brachyrhychos, which had been exposed to long photoperiods in midwinter, moved northward when released. However, castrates of these species migrated southeast after release, as did non-photostimulated control birds (Rowan 1925, 1932). This finding suggested that the effects of daylength could be mediated by the differing levels of gonadal hormones present in autumn and spring. A later study showed that the orientation of captive White-throated Sparrows Zonotrichia albicollis could be reversed by altering the temporal pattern of administration of the hormones prolactin and corticosterone. Birds injected with prolactin 4 hours after they had been injected with corticosterone oriented southward, whereas birds given prolactin 12 hours after corticosterone oriented northward (Martin & Meier 1973).

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