Climate is one of the most important ecological factors that affect the activities of many organisms, populations, and species in their annual life cycle (Huntley, 1991; Walther et al., 2002; Root et al., 2003; Aasa et al., 2004; Noges & Jarvet, 2005). Climate appears to directly determine, for example, the timing of particular phenological events in the seasonal development of nature. Favourable weather in spring generally advances the start of the growing season and the timing of animal migration, while adverse weather conditions (freeze, blizzard, anomalously low air temperature, late night frosts, etc.) may delay these and, in birds, can even cause reverse migration (Sokolov & Kosarev, 2003; Newton, 2010). The effects of weather conditions are especially important for migratory birds because they determine the habitat quality, especially the food resources, not only in their breeding areas but also at the wintering and stopover sites. This affects the progression speed and territory acquisition, as well as the timing and success of breeding (Newton, 2004, 2010; Drever & Clark, 2007; Moller et al., 2010). Thus, one of the threshold questions in avian ecology is how weather conditions design migration strategies of species or populations both in the case of spring and autumn migration.
The effect of climate on the spring migration phenology of birds is relatively well studied (e.g., Lehikoinen et al., 2004; Leech & Crick, 2007; Moller et al., 2010). It has been suggested that species generally respond differently to climate. This is most clearly visible in differences between short- and long-distance migrants (Stervander et al., 2005; Hubalek & Capek, 2008; Palm et al., 2009; Newton, 2010). The former are largely affected by exogenous factors while the latter are more dependent on endogenous ones, which lead to lower correlations with climate variables and thus lower variability in the spring arrival time (Palm et al., 2009). For instance, the wintering population of the Mute Swan Cygnus olor, a classical short-distance migrant or 'weather bird' or 'facultative migrant' (Berthold, 1971, 1993), increased considerably in Estonia in the 1980s and early 1990s because of mild winters and almost ice-free conditions in the Baltic Sea (Leibak et al., 1994). In contrast, although the spring arrival of the Eurasian Crane Grus grus, a partly short-distance and partly long-distance migrant, has shifted considerably forward in Estonia in the last 50 years due to climate warming (Leito et al., 2006; Palm et al., 2009, 2011), the cranes have not started to winter in Estonia.
Studies of spring migration phenology of birds usually employ single climatological or meteorological characteristics, such as North Atlantic Oscillation (NAO) indices or air temperature (Gordo, 2007). However, most of the biological processes (including bird migration) are driven by a combination of meteorological variables and thus it is advisable to use integrated climatological variables, such as atmospheric circulation types (CTs). The integrated influence of climatic and weather conditions on migration has, however, been poorly treated (Richardson, 1978; Stervander et al., 2005; Gordo, 2007; Newton, 2010).
The main principle of synoptic climatology methods applied in the present study is that the variety of atmospheric circulation can be classified into a relatively small number of CTs (Barry & Perry, 1973; Yarnal, 1993; Huth et al., 2008; Huth, 2010). For instance, classifications of atmospheric circulation elaborated by COST Action 733 'Harmonisation and applications of weather type classifications for European regions' (COST 733, 2010) contain 9, 18, or 27 CTs (Huth, 2010; Philipp et al., 2010). The idea of CT classifications is based on empirical experience of weather forecasters, who noticed that circulation processes share some spatial and temporal variables, such as repeated locations of pressure areas and cyclone trajectories, as well as other variables that describe weather. …