Migrants tend to leave particular wintering areas over a limited period of days or weeks, but in roughly the same period each year, some species before others (e.g. King & Mewaldt 1981). This holds in both obligate and facultative migrants. The year-to-year consistency in migratory timing within species is evident in studies on captive passerines. In White-crowned Sparrows Zonotrichia leucophrys gambelii, for example, the standard error was 1.0 day for the mean date of onset of pre-migratory fattening during eight different springs in males kept outdoors in winter quarters (King & Farner 1965, King 1972). Among wild birds, however, subject to differing conditions, the situation is far more variable.
As in autumn, preparation for migration in spring may be delayed in some individuals by poor food supply and body condition (Chapter 27), or by lateness of preceding events in the annual cycle, such as winter moult (Dugger 1997, Saino et al. 2004). Hence, in many populations, departure dates from particular wintering areas are often spread over several weeks (for White Stork Ciconia ciconia see Berthold et al. 2002; for Peregrine Falco peregrinus see McGrady et al. 2002). The same is true for the dates of passage through particular localities en route. Mean migration dates may also differ greatly between years. For example, the mean passage dates (= capture dates) of male Blackcaps Sylvia atricapilla through Israel over seven springs varied between 6 April and 3 May, and in late years the spread in passage dates was also less, as was the difference in mean dates between the sexes (Izhaki & Maitav 1998a). In general, as found in many localities, species that migrate earliest in the spring show bigger variation in mean passage dates between years, and also a bigger range of individual migration dates within years than species that migrate latest in spring (e.g. Francis & Cooke 1986, Hagan et al. 1991).
As in autumn, ringed adults of some species have shown some consistency in migration dates from year to year, as measured either by their departure dates from wintering areas (for Tundra Swans Cygnus columbianus see Rees 1989), or their arrival dates in breeding areas (for Barn Swallows Hirundo rustica see M0ller 2001), or both (for Black-browed Albatrosses Thalassarche melanophrys see Phillips et al. 2005). Individuals that are early relative to other individuals in one year tend also to be early relative to other individuals in other years. As in autumn, such consistency could be due to inherent differences in migration timing between individuals, or to other differences between them, perhaps in their abilities to feed and fatten. Differences in timing between age and sex groups also contribute to the spread of spring migration dates within populations, as explained in Chapters 15 and 27.
In long-distance migrants, a wide spread in departure dates from wintering areas does not necessarily result in a similar wide spread in arrival dates on breeding areas. Birds encounter different conditions on their journeys, and early birds are more likely to be delayed by poor weather or food supply, enabling later ones to catch up with them. In Red Knots Calidris canutus, which migrate in spring from Tierra del Fuego to arctic Canada, departures from successive stopover sites become progressively more concentrated so that from Delaware Bay, the last staging site before the breeding areas, almost the entire population leaves within a 3-day period, usually 28-30 May (Baker et al. 2004). In this species, then, wide variation in departure dates from wintering areas is reduced along the migration route, so that most birds arrive in breeding areas within the same few-day period.
A 'calendar effect' may operate in spring, as well as in autumn, affecting the timing and extent of fattening, and the duration of stopovers. For example, in Semi-palmated Sandpipers Calidris pusilla passing through South Carolina, stopover durations decreased, and fat levels increased, as spring progressed (Lyons & Haig 1995). This would have had the effect of accelerating the migration of 'late' birds within the population, but whether it was an inherent response to daylength (time of year) or a response to increased food supply in late spring is an open question. Similarly, White-throated Sparrows Zonotrichia albicollis departing from their Florida wintering areas late in the migration season accumulated more fat than those that left earlier (Johnston 1966).
In other species, the opposite may hold, as the latest birds to leave wintering areas get even later with respect to earlier ones as they progress along the migration route - the domino effect of Piersma (1987). This situation is likely to hold where food supplies at stopover sites become progressively depleted through the season, so that later birds take longer to fatten (Chapter 27). One consequence would be that the arrival dates of birds in their breeding areas would show greater spread than their preceding departure dates from wintering areas.
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