Relatively little is known about the hormonal control of bird migration, but the endocrine system presumably mediates all physiological aspects, including: (1) fuel deposition and use, coupled with associated changes in body composition; (2) increased haematocrit production for enhanced oxygen transport during long flights; and (3) modification of different aspects of behaviour, notably the change in diurnal rhythm (for example to promote night flights in otherwise daytime species) (Wingfield et al. 1990). Both autumn and spring migrations have the common requirements of fuel deposition and rapid mobilisation, and other associated changes in body composition (such as breast muscle enlargement, Chapter 5). However, spring migration usually coincides with gonadal growth and other preparations for breeding, whereas autumn migration follows gonadal regression or the ending of moult. Gonadal hormones are therefore present in the bloodstream at much higher levels in spring than in autumn (Wingfield et al. 1990, Wingfield & Silverin 2002).
In spring, gonadal hormones seem to influence hyperphagia, fat deposition and the haematocrit content of the blood. Intact and castrated males of various finches showed similar amounts of fattening and migratory restlessness, but castrated birds began about a week later (Lofts & Marshall 1961, Morton & Mewaldt 1962, King & Farner 1965). This held for birds castrated in late winter, after daylength had begun to increase. Birds castrated before the winter solstice, still experiencing shortening days, showed no pre-nuptial moult in spring, no fattening and much reduced migratory restlessness (for White-throated Sparrow Zonotricia albicollis see Weise 1967, for White-crowned Sparrow Z. leucophrys see Stetson & Erickson 1971). However, implants of testosterone in gonadectomised White-crowned Sparrows (no testosterone previously detectable) stimulated fattening and restlessness to varying degrees in spring, at the same time as in control birds (Mattocks 1976, Schwabl et al. 1988). It thus seemed that the hormone testosterone was involved in the preparation for spring migration.
In contrast, gonadal hormones cannot be involved in triggering autumn migration because the gonads have regressed by then. In fact, free-living male Pied Flycatchers Ficedula hypoleuca dosed with testosterone did not start migration after breeding, but remained in the breeding areas as long as testosterone levels remained high (Silverin 2003). The same was true of female Song Sparrows Melospiza melodia treated with oestradiol (Runfeldt & Wingfield 1985). In the latter species, untreated males that were paired with oestradiol-implanted females continued to show territorial behaviour and elevated testosterone levels well into autumn. Behavioural interactions between partners ensured that both remained in breeding condition for up to three months longer than control pairs in which neither partner was hormonally treated. While gonad removal in either sex suppresses hyperphagia and fattening in spring, it has no such effect in autumn (see also Rowan 1932, Lofts & Marshall 1960).
During migration, thyroid hormones seem to play no more than their normal role in regulating metabolism (changes in which may be substantial during migration). They seem less important in migration than in moult, but may interact with growth hormone to influence fat deposition and muscle growth. The hormone prolactin (from the anterior lobe of the hypophysis) is often considered fundamental to the migratory condition of birds, especially migratory fattening (Dobrynina 1990). The injection of prolactin can induce or accelerate fat deposition and, when combined with adrenal hormones, it can also stimulate migratory restlessness, but its actions are not fully understood. As reported earlier, it may also be involved in influencing the directional switch between autumn and spring.
The role of the adrenal hormone corticosterone in migratory restlessness was shown in a study of Garden Warblers Sylvia borin (Schwabl et al. 1991a). These nocturnal migrants had high levels of circulating corticosterone during the night, and lower levels during the day. When corticosterone secretion was artificially interrupted, nocturnal restlessness ceased. Other links between high corticoster-one, fattening and migration have been shown in recent field studies (Holberton 1999, Piersma et al. 2000, Lohmus et al. 2003, Long & Holberton 2004). In Bar-tailed Godwits Limosa lapponica at a stopover site, corticosterone levels were higher in birds that had just arrived than in refuelling birds, but increased again as birds fattened (Landys-Ciannelli et al. 2002). Adrenocorticosteroids may therefore help to regulate hyperphagia, fattening and restlessness at appropriate times. Corti-costerone has also been found at high levels in birds sampled during sudden weather movements (Wingfield 2003).
Almost certainly, other hormones are involved in migration but their role is even less clear, partly because of the difficulty of separating cause and effect in correlations (Ramenofsky 1990, Wingfield et al. 1990). In the response to daylength, the pineal-melatonin system seems to be involved (Brandstätter 2003), and in nocturnal migrants melatonin profiles change during the migration season (Fusani & Gwinner 2005). Melatonin is a hormone that modulates day-night rhythms. In Blackcaps Sylvia atricapilla from migratory populations, night levels of melatonin were lower during the migration season, when birds showed nocturnal activity, than at other times of year when they were active only by day. In contrast, Blackcaps from non-migratory populations showed no seasonal reduction in melatonin levels. In other experiments on migratory Blackcaps, long migratory flights and long refuelling stopovers were simulated by depriving birds of food for two days, and subsequently re-providing food. In both autumn and spring, nocturnal activity was suppressed and melatonin increased in the night following food reintroduction, the response depending on the amount of body fat. These studies revealed a relationship between melatonin and migratory restlessness, influenced by fat levels and food availability.
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