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Figure 21.3 Rate of change in first arrival dates (days per year) of 56 bird species in Lithuania (Zuvintas Strict Nature Reserve) in 1966-1995, in relation to their average arrival dates. Regression (day 1 = 1 January): change= -0.0003xday2+0.071 xday-4.933, r2=55.2%. Negative values indicate earlier arrival. From Lehikoinen et al. (2004, based on Zalakevicius & Zaleveciute (2001).

• Greater changes have occurred in the arrival dates of small bird species than of large ones. This is possibly because the smaller species are more sensitive to annual temperature differences and their effects on food supplies (although their shorter generation times would also favour more rapid genetic change than is possible in large, longer-lived species).

• Year-to-year arrival dates of short-distance migrants generally showed a correlation with spring temperatures in the breeding locality, but such correlations were less obvious in long-distance migrants (Tryjanowski et al. 2002). Moreover, where it has been explored, weather back down the route often showed a better relationship with arrival dates than did weather at the arrival location itself (e.g. arrival dates of Barn Swallows Hirundo rustica in Britain were better related to weather in France-Spain than to weather in Britain, Huin & Sparks 1998).

• Spring weather has not changed everywhere in the same way. Correspondingly, the degree of change in bird arrival dates in breeding areas has varied across Europe, with arrival dates in most areas getting earlier as springs have warmed, but in some areas getting later as springs have cooled. In the Mediterranean region, springs are now cooler than in the past, which may be another factor slowing the return of long-distance migrants from tropical Africa to the mid-and higher latitudes of Europe.

• Most species still arrive in breeding areas earlier in warm springs than in cool ones.

Three explanations may account for the fact that more short-distance migrants than long-distance migrants now arrive earlier in spring, and in closer correlation to temperatures on breeding areas. First, the stronger endogenous control of migration in long-distance migrants might inhibit a rapid reaction to a changing environment (Gwinner 1986, Berthold 1996). Short-distance migrants are typically more flexible (facultative) in their response, and more able to alter their behaviour in relation to prevailing conditions (Chapter 12). Second, the closer a species winters to its breeding areas, the more closely correlated are the day-to-day weather changes in the two areas, enabling short-distance migrants to react more rapidly and appropriately. Third, as mentioned above, weather is more variable early in the spring, when most short-distance migrants arrive in their breeding areas, than later in the spring, when most long-distance migrants arrive.

Because, in most bird species, males arrive in breeding areas before females (Chapter 15), studies of migration timing based on first arrival dates concern males only. But the two sexes may not necessarily respond in the same way to climate change. A long-term study of arrival dates of male and female Barn Swallows Hirundo rustica in Denmark revealed that only males responded to an amelioration of weather conditions during migration, while females did not (M0ller 2004). Therefore, the sex difference in arrival dates increased as a consequence of climate change, giving no change in breeding date, because females arrived no earlier than they did 30 years previously.

Earlier arrival in breeding areas in response to warmer weather there could be brought about by birds: (a) increasing the speed of spring migration, (b) leaving wintering areas earlier but migrating at the same speed, (c) wintering closer to the breeding area and migrating at the same speed, or (d) a combination of these various possibilities. More rapid progress in warm than cold springs has been recorded in many migrants from the dates they pass through successive observation sites in different years. Only facultative responses could account for the year-to-year variation in arrival dates seen in many migrants, but this need not exclude the possibility of genetic change in response to longer-term environmental trends, such as climate warming (for evidence of genetic variation in timing of spring migration, see Chapter 20).

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