Effect of atmospheric circulation types on spring arrival of migratory birds and long-term trends in the first arrival dates in Estonia

Abstract

We studied the spring arrival of 42 migratory bird species in Estonia in relation to integrated climatological variables (atmospheric circulation types). We found that all early arrivals were short- distance migrants, which were more directly affected by cyclonic conditions. Those conditions design a tailwind system that favours or hinders the migration of birds to their breeding areas. All late arrivals were long-distance migrants, whose spring arrival took place mostly in anti-cyclonic windless or also eastern or south-eastern wind conditions, i.e. tail- or crosswinds. Our hypothesis that different arrival times of the same species in western (Kuressaare) and eastern (Tartu) Estonia may have been caused by the birds' different circulation type preferences was not confirmed - birds preferred similar types. Differences in the first arrival dates between western and eastern Estonia were related rather to climatic differences as well as differences in flyways and migratory routes. The spring arrival date significantly advanced during the period 1958-2002 in several species from both guilds but the rate and significance of the advances were in general higher in early arrivals than in late arrivals. We concluded that weather conditions and climate change affected the spring arrival of both short-distance and long-distance migrants. Among circulation classifications, the effects on migratory birds were most often detected in the case of classifications with a low number of circulation types (such as SANDRAS or CKMEANSC09), suggesting that birds are generally not waiting for very specific types of circulation. The results indicate that methods of synoptic climatology are useful for studying bird migration phenology and migration patterns, especially at a large scale.