Figure 14.3 The advance of spring, as shown by the average dates that the rising 10°C isotherm reached different parts of Europe during 1971-2000. From T. Sparks, unpublished.

Figure 14.4 The advance of spring, as shown by the mean peak dates of apple flowering in different parts of Europe. From Gatter (2000).

Annual variation in spring migration dates

Because some food supplies depend on weather, many bird species travel more slowly and arrive in their breeding areas later in cold springs than in warm ones. This annual variation is especially apparent in insectivores, such as the Barn Swallow Hirundo rustica and various warblers, whose food is clearly temperature dependent, or in species that depend on snow and ice melt, such as waterfowl and shorebirds (Figure 14.5; Cooke et al. 1995; Huin & Sparks 1998). At particular localities, temperatures in early spring show more annual variation than temperatures in late spring, and correspondingly, early-arriving species typically show significantly more year-to-year variation in arrival dates than do late arriving ones (Gilyazov & Sparks 2002, Tryjanowski et al. 2002; Figure 14.6). For example, at localities in southern England, first arrival dates of the early-arriving Sand Martin Riparia riparia varied by more than 29 days between years, but in the late-arriving Common Swift Apus apus, they varied by about 20 days (Loxton et al. 1998). Disruptions in the normal arrival sequence occur in years when warm and cold periods alternate through the spring, and in some years a spell of bad weather during the arrival period can produce a twin-peaked arrival pattern (see Elkins 2005 for patterns in Sand Martin Riparia riparia).

In some species, the year-to-year relationship between spring arrival and temperature has been studied at more than one locality. In Slovakia, first arrivals of c

Figure 14.5 Arrival dates of Barn J Swallows Hirundo rustica in Britain in different years in relation to mean February-March temperatures. From Sparks et al. (1999).

24 March -

012345678 February-March mean temperature

012345678 February-March mean temperature

Barn Swallows Hirundo rustica advanced about 2.1 days per 1°C temperature rise over February-March, in England about 1.7 days per 1°C, and in Finland about 1.2 days per 1°C (Sparks 1999).1 This suggests a more rapid response at higher latitudes, where arrival dates are much later, and occur over a shorter period. In yet other species, an effect of local food supply on arrival has been detected over and

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