In considering the proximate control of migration, a useful distinction can be drawn between obligate migration (formerly called instinct or calendar migration) and facultative migration (formerly called weather migration). In obligate migration, all main aspects are viewed as under firm internal (genetic) control, mediated by daylength changes, which gives a high degree of annual consistency in the timing, directions and distances of movements (Chapter 20). For the most part, each individual behaves in the same way year after year, migrating at similar dates and for similar distances. Obligate migrants often leave their breeding areas well before food supplies collapse, and while they still have ample opportunity to accumulate body reserves for the journey. They tend to migrate long distances, often to the tropics or beyond.
In contrast, facultative migration is viewed as a direct response to prevailing conditions, especially food supplies, and the same individual may migrate in some years but not in others (Chapter 20). Within a population, the proportions of individuals that leave the breeding range, the dates they leave and the distances they travel, can vary greatly from year to year, as can the rate of progress on migration, all depending on conditions at the time (e.g. Svardson 1957, Terrill 1990, Moore et al. 2003). In consequence, facultative migrants have been seen on migration at almost any date in the non-breeding season (at least into January in the northern hemisphere), and their winter distributions can vary greatly from year to year (Chapters 18 and 19). Although in such facultative migrants, the timing and distance of autumn movements may vary with individual circumstances, other aspects must presumably be under firmer genetic control, notably the directional preferences and the tendency to return at appropriate dates in spring. Compared with obligate migrants, facultative migrants tend to migrate shorter distances, although many exceptions occur. The two types of migrants thus have different distribution patterns in midwinter. Whereas obligate migrants are concentrated in a distinct wintering area, usually at long distance from the breeding area, facultative migrants are typically found over the whole migration route from breeding to wintering areas, usually tailing off with increasing distance from breeding area, but with marked annual variations.
In general, it seems that obligate migration occurs in populations whose food supplies in breeding areas are predictably absent in winter, whereas facultative migration occurs in populations whose food supplies in breeding areas vary greatly from one winter to another, according to weather or other variables. The distinction between obligate and facultative migrants is important because it reflects the degree to which individual behaviour is sensitive to prevailing external conditions, and hence varies from year to year. However, obligate and facultative migrants are best regarded, not as distinct categories, but as opposite ends of a continuum, with predominantly internal control (= rigidity) at one end and predominantly external control (= flexibility) at the other.
Another reason for not drawing a sharp distinction between the two categories is that many birds seem to change from obligate to facultative mode during the course of their journeys, as the endogenous drive to migrate wanes with time and distance, and the stimulus to continue becomes more directly dependent on local conditions (Helms 1963, Terrill & Ohmart 1984, Gwinner et al. 1985a, Terrill 1990). Theoretically, the initial obligate phase of any journey might take the migrant across regions where the probability of overwinter survival is practically zero: where any individuals that attempted to winter there in the past were eliminated by natural selection. As migration continues into more benign areas, and survival probability increases, the bird switches to a facultative mode, in which it benefits by responding to local conditions, stopping where food is abundant. The obligate phase would therefore be expected to be undertaken much more rapidly, on average, than the facultative phase, which involves longer and more variable stops. Such a two-phase migration, with obligate and facultative stages, would also ensure that, in any particular year, the bird migrated no further than necessary. In some species only the tail end of the migration may be facultative, in others the entire journey. Most irruptive migrants are near the latter end of the spectrum.
Arctic-nesting geese provide circumstantial evidence for a two-phase migration, in which the first part is obligatory and the second part facultative. Geese need to leave the arctic every year before survival there becomes impossible, and they tend to depart en masse on about the same dates every year. But once they reach suitable wintering areas, their movements become much more variable in timing and extent, depending on local food availability. They appear to change from a primarily endogenous migratory phase (obligate migration) to a stage when the stimulus for further migration is primarily environmental. In effect, as birds travel south in autumn, the drive to continue becomes increasingly dependent on food and other local conditions (Terrill 1990). The same holds for many other migrants, notably irruptive seed-eaters (Svardson 1957, Newton 1972, Koenig & Knops 2001), but also American Tree Sparrows Spizella arborea (Niles et al. 1969), Chipping Sparrows Spizella passerina (Pulliam & Parker 1979), Snow Buntings Plectrophenax nivalis (Haila et al. 1986), Blackcaps Sylvia atricapilla (Klein et al. 1973), Yellow-rumped Warblers Dendroica coronata (Terrill & Crawford 1988), and many others (Chapters 18 and 19).
Further support for the idea that migration is often two-phase comes from studies of captive birds. In White-throated Sparrows Zonotrichia albicollis, Helms (1963) identified two subdivisions of migratory behaviour in both autumn and spring. The first phase (which he called the motivational subdivision) was characterised by intense and continuous night-time activity, while the second phase (the 'adaptational subdivision') was less intense, with numerous interruptions and greater variability. Helms (1963) aligned these two phases with the behaviour of free-living birds during spring migration, as they switched from an intense, highly directed phase to a more casual 'wandering phase', in which they searched for suitable habitat and took advantage of local opportunities. In addition, observations of the directional preferences of caged migrants revealed an increasing variance in headings towards the end of the migratory period (Wiltschko & Wiltschko 2003).
Experiments on captive birds have also confirmed that individuals can develop migratory restlessness in response to food deprivation in winter, well outside the normal migration period (Biebach 1985, Gwinner et al. 1985a). For example, Garden Warblers Sylvia borin showed spontaneous nocturnal restlessness during the autumn migration period (September-December), but not in January when they would normally have settled in winter quarters. However, they again became active at night in winter if deprived of food (Gwinner et al. 1985a). The fact that deprived birds could put on fat may seem surprising, but they seemed to do so through changes in physiology and behaviour, feeding for much longer each day than normal. After late January, migratory activity could not be reactivated when birds were subjected to restricted food treatment.
Several researchers, working with different species, have been able to initiate or enhance nocturnal restlessness in autumn by restricting food or lowering temperature (reviews Farner 1955, Helms 1963). It is not known whether the birds responded to temperature directly or through their energy needs and body condition.
Was this article helpful?