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aFrom 21 December (winter solstice), only one possible date which birds might use as an anchor point. From King & Mewalt (1981).

aFrom 21 December (winter solstice), only one possible date which birds might use as an anchor point. From King & Mewalt (1981).

The response to daylength could enable individuals from the same population (or the same individual in different years) to leave different wintering areas in order of southernmost to northernmost, in accordance with the distances they have to travel. Experimental support has been provided in the Pied Flycatcher Ficedula hypoleuca, in which captive males exposed to winter daylengths typical of southern Europe began pre-nuptial moult, migratory activity and gonadal maturation about one month earlier than control birds held under the normal African photoperiods (Coppack & Both 2002, Coppack et al. 2003). Because spring daylengths are longer in southern Europe than in tropical Africa, these flycatchers presumably perceived the date as being later than it actually was, and prepared earlier for migration. In addition, captive Garden Warblers Sylvia borin kept in winter in photoperiodic conditions found at latitude 20°S (that is, longer but decreasing daylengths) initiated gonad growth and migratory activity significantly earlier than conspecifics held under a shorter and more constant equatorial photoperiod (Gwinner 1987). Such a built-in response to photoperiod not only regulates the normal date of migration, but could enable birds to react appropriately to location changes between winters.

In some long-distance migrants, part of the population winters north of the equator and another part south of the equator. Even the same individuals may winter north or south of the equator in different years. This has been shown, for example, in radio-tagged White Storks Ciconia ciconia, in which one individual wintered at localities between 10°N and 30°S over four different years, each time returning successfully at an appropriate date to its nesting site in central Europe (Berthold et al. 2002). One female, tracked on return migration in six different years, set off, on average, around 24 February in four years when she was at 29-34°S, but about 18 days later, on average on 14 March, when she was at 3°S-4°N. She took an average of 75 days over the 11 000 km longer journey, arriving on 24 April-28 May in different years, and an average of 57 days over the 7000 km journey, arriving on 25 April-25 May in different years (Table 8.4). There was no significant difference in arrival dates, according to length of journey. She bred at the same site in all six years, and the spread in her autumn departure dates from the breeding locality was only 14 days (15-29 August) (Berthold et al. 2004).

While daylength response may reduce the delay in dates of arrival in breeding areas among birds wintering furthest away, it may not eliminate it altogether. Among eight radio-tagged Willow Ptarmigan Lagopus lagopus, autumn migration distances and spring arrival dates were correlated, the furthest migrating individuals arriving back latest in spring (Gruys 1993). More strikingly, Pied Avocets Recurvirostra avosetta breeding in the same colonies in Germany are known to winter either in mid-European latitudes or about 1000 km to the southwest in Mediterranean latitudes (Hotker 2002). Individuals wintering in mid-latitudes arrived significantly earlier at their breeding sites than those wintering further south. Their arrival dates also varied more from year to year in relation to local spring temperatures than did those from further south. As in many other birds, earlier arrival was associated with better breeding success, which implied a cost in more distant wintering. This raises the question of why any avocets winter so far south of their breeding areas when they do not survive any better there (Hotker 2002). This could be related to competition and dominance relationships on wintering areas, which lead some individuals to migrate further than others (Pienkowski & Evans 1984). A similar but slight reproductive advantage, with earlier egg-laying and larger clutches, was evident in King Eiders Somateria spect-abilis that wintered closest to the breeding site, compared with those that wintered in a different region further away (Mehl et al. 2004).

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