Spring dates

From 983 Eurasian bird populations in which first arrival dates in breeding areas were monitored over periods of years, 59% showing no significant change, but 39% had become significantly earlier, while only 2% had become significantly later (Lehikoinen et al. 2004). Both short-distance and long-distance migrants showed the same trends. From 222 populations for which mean passage dates could be calculated over periods of years, 69% showed no change, while 26% had become significantly earlier, and only 5% had become significantly later. The average change of first arrival date over all species and sites was -0.37 days per year, while the equivalent figure for mean passage dates was -0.10 days per year, both figures being statistically significant. It is not obvious why the two figures differed, but in general the mean migration dates were based on larger, more standardised data-sets, and were less susceptible to increase in observer numbers.

1For examples, see Moritz (1993), Loxton & Sparks (1999), Vogel & Moritz (1995), Sparks (1999), Sparks & Mason (2001), Fiedler (2001), Inouye et al. (2000), Jenkins & Watson (2000), Sokolov (2001, 2006), Sokolov et al. (1998, 1999), Bairlein & Winkel (2001), Zalakevicius & Zalakeviciute (2001), Huppop & Huppop (2003), Tryjanowski et al. (2002), Bradley et al. (1999), Root et al. (2003), Lehikoinen et al. (2004), Vahatalo et al. (2004), Mills (2005), Stervander et al. (2005).

Year

Figure 21.1 Mean first arrival dates of Common Chiffchaff Phylloscopus collybita and Blackcap Sylvia atricapilla at eight coastal bird observatories in Britain during 1960-1996. In both species the response to temperature is in the order of 2-3 days earlier per 1°C rise in mean spring temperature. From Sparks (2000).

Year

Figure 21.1 Mean first arrival dates of Common Chiffchaff Phylloscopus collybita and Blackcap Sylvia atricapilla at eight coastal bird observatories in Britain during 1960-1996. In both species the response to temperature is in the order of 2-3 days earlier per 1°C rise in mean spring temperature. From Sparks (2000).

Despite the long-term trends, arrival and migration dates still fluctuated from year to year in line with local temperatures (Figures 14.5, 21.1 and 21.2).2 Typically, most birds arrived about 2.5-3.3 days earlier for every 1°C increase in spring temperature (based on 203 regression analyses for different Eurasian bird populations, Lehikoinen et al. 2004). The smaller number of studies available from North America revealed some similar trends (Bradley et al. 1999, Inouye et al. 2000, Butler 2003, Marra et al. 2005, Mills 2005, Murphy-Klaassen et al. 2005), although in some eastern parts of the continent, long-term temperature change has been less marked than in western Europe. In general, earlier arrival of migrants in spring leads to earli er breeding, as described as a recent trend in a wide range of species (Crick & Sparks 1999, Sokolov 2006), and earlier breeding often gives rise to better success at the population level (Chapter 14; Thingstand 1997, Sokolov 1999, 2000, Bairlein & Winkel 2001).

Despite strong correlations between arrival dates and temperature on the breeding area, much of the variance in arrival dates remains unaccounted for. Arrival dates are also influenced by weather further down the migration route

2Most researchers have used annual temperatures from localities on the migration route or breeding area, while some have used the winter-spring index of the North Atlantic Oscillation (NAO), a large-scale climate phenomenon influencing weather in this region (e.g. Huppop & Huppop 2003, Vahatalo et al. 2004, Stervander et al. 2005, Sokolov 2006, Zalekevicius et al. 2006). It is calculated as a difference in normalised values of atmospheric pressure between the Azores and Iceland for each month. Positive values indicate warmer and wetter winter-spring weather (followed by earlier spring migration) in northwest Europe and the opposite weather conditions and later arrival dates than usual in southern Europe.

Figure 21.2 Mean first arrival dates of Barn Swallows Hirundo rustica averaged for seven bird observatories in Britain, 1970-1997, during which time mean arrival dates advanced by about one week in association with gradual spring warming. From Sparks et al. (1999).

or in wintering areas (Sokolov 2006), as well as by other aspects of weather, such as wind, and by different factors such as food supply, as yet largely unstudied in this context. Moreover, poor weather on one part of the route can hold up migration, even though conditions may be favourable further along the route. The variations in response between species, recorded in every relevant study, could well have been partly related to diet, and the dates that their different foods become available, but further investigation of species variation is needed. Moreover, not all studies have shown a single sustained long-term trend in migration dates. For example, at the Rybachy Bird Observatory on the Courish Spit in the southeastern Baltic, warmings in the 1930s and 1940s, and then in the 1960s and 1980s, were associated with significantly earlier spring migration in many species of passerines, while colder periods during the 1950s and 1970s were associated with later passage (Sokolov et al. 1998).

In comparing the changes that have occurred in the spring migration dates of different species, several fairly general patterns have emerged:

• Greater changes have occurred in the migration dates of early-migrating species than of later migrating ones (Figure 21.3). This is associated with weather (including temperature) being more variable earlier than later in the spring (for passage dates see Sokolov et al. 1998, for arrival dates at breeding sites see Slagsvold 1976, Loxton & Sparks 1999).

• Greater changes have occurred in the arrival dates of short-distance migrants than of long-distance migrants - presumably because short-distance migrants generally arrive earlier in spring (same point as above), and have more flexibility in their migration timings (Tryjanowski et al. 2002, Butler 2003).

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