Birds normally leave their wintering areas so as to reach their nesting areas in time to breed at the most favourable season. Many migrants winter so far from their breeding areas that they could not judge conditions there from their position in wintering areas. They can only leave their wintering areas at a time that natural selection has decreed is appropriate, using an internal timer combined with local conditions as a cue. Among species wintering in the temperate zone, the main known environmental stimulus for spring migration (superimposed on an endogenous rhythm) is increasing daylength which promotes extra feeding, fattening and migratory restlessness at appropriate dates for the population concerned (Rowan 1925, Wolfson 1952, Lofts et al. 1963, King 1972). In species that undergo a spring moult, this is also initiated by increasing daylengths, as is gonad growth, each of these processes occurring in appropriate overlapping sequence through the season. The role of daylength has been shown repeatedly in experiments on captive birds, in which longer-than-natural photoperiods advance all spring-occurring processes, whether gonad growth, pre-nuptial moult or migration.
Migratory birds can also respond to daylengths encountered on the journey. Helm & Gwinner (2006) exposed captive Stonechats Saxicola torquata at spring migration time to two different photoperiodic regimes, one typically experienced during migration through the temperate region (fast change) and the other typical of lower latitudes (slow change). These small short-term differences in daylengths had longer-term effects on the birds. Slow-change migrants continued migratory activity longer than fast-change migrants, delayed the growth and subsequent regression of their testes, moulted, and developed autumn migratory activity later in the year. These changes were appropriate to those needed if the birds had started migration at widely different latitudes.
In many wild species, then, gonad growth and sperm formation begin before the birds leave their wintering areas, and continue during migration (for Eurasian migrants see Rowan & Batrawi 1939, Lofts 1962, Marshall 1952b, for North American migrants see Blanchard 1941, Wolfson 1942). Some species have been seen to copulate while on migration, and females have been found with sperm in their reproductive tracts (Quay 1989, Moore & McDonald 1993). This seems to be usual in wild geese, for example, in some populations of which copulation and egg-formation begin even before birds have left their winter quarters (McLandress & Raveling 1983). Egg formation in these geese takes around 12 days, and laying can occur within a few days after arrival in nesting areas. In yet other species, most gonad growth occurs after the birds have arrived in breeding areas, and copulation is seen only after establishment of a territory. Much depends on the interval between arrival and egg-laying, which can span days or weeks, depending on the species and the ecological circumstances in which it lives. In general, it is in late-arriving species (relative to the latitude) that gonad development is most advanced on arrival, and in which the interval between arrival and egg-laying is shortest (Berthold 1996).
In species that undergo a spring 'pre-nuptial' body moult into breeding plumage, the process also starts in winter quarters and proceeds through migration. However, its timing varies greatly between individuals in the same population. Among shorebirds seen at spring stopover sites, some individuals are in predominantly winter plumage, while others at the same sites at the same time are in predominantly breeding plumage. Individual nutritional status may have a big influence on the timing and duration of this moult (Chapter 20).
Daylength is not the only factor to which birds respond in preparing for spring migration. The effects of daylength may be modified by both temperatures and food supplies. For example, captive White-crowned Sparrows Zonotrichia leucophrys exposed to air temperatures increasing from 5°C to 26°C advanced the time of migratory restlessness compared with control birds (Eyster 1954, Lewis & Farner 1973), but it is hard to tell whether temperature acted directly on the birds, or through its effect on their energy needs and the food available for fuelling. In Catharus thrushes studied during spring migration using doubly-labelled water, energy expenditure increased by nearly 20%, reflecting greater thermoregulatory costs, as ambient temperature dropped from 20°C to 10°C (Wikelski et al. 2003). On nights below 21°C, passage birds stayed at their stopover sites rather than migrating (Cochran & Wikelski 2005). Research on both wild and captive birds of many species has shown the importance of food supplies in affecting fuelling rates (Chapter 27).
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