Avian Ecol. Behav. 14, 2008: 35-47 Relationship between irruptions of Long-tailed Tits Aegithalos caudatus in the Eastern Baltic and ambient temperature and North Atlantic Oscillation index Leonid V. Sokolov 1 , Anatoly P. Shapoval 1 , Vladislav D. Yefremov 1 , Vladislav V. Kosarev 1 , Mikhail Y. Markovets 1 & Vadim V. Gavrilov 2 Abstract: Sokolov, L.V., Shapoval, A.P., Yefremov, V.D., Kosarev, V.V., Markovets, M.Y. & Gavrilov, V.V. (2008): Relationship between irruptions of Long-tailed Tits Aegithalos cau- datus in the Eastern Baltic and ambient temperature and North Atlantic Oscillation index. Avian Ecol. Behav. 14: 35-47. Analysis of long-term dynamics of numbers showed that frequency of Long-tailed Tit irrup- tions on the Courish Spit has increased in the last 24 years. In 1957-1980 four irruptive years were recorded (when > 250 individuals were captured), whereas in 1981-2004 12 such years occurred. The mean annual totals in the former period were by an order or magnitude lower than in the latter one (138 vs. 1586, respectively). Autumn numbers were significantly related to NAO index during the preceding winter and spring and to spring air temperatures. In the years following mild winter and spring in the breeding area, significantly more birds were captured on the Baltic coast than after cold years. Spring trapping figures on the Courish Spit were significantly related to the numbers recorded during the previous autumn. On the basis of these data, we conclude that pronounced irruptions of Long-tailed Tits in the Eastern Baltic were recorded after the years when winter and spring were comparatively mild in Eu- ropean Russia. The current climate warming in the Northern hemisphere resulted not only in more frequent irruptions of Long-tailed Tits in the study area, but apparently in the general population growth in this species. Key words: passerines, Long-tailed Tit, numbers, irruption, autumn and spring migration, tem- perature, NAO, Eastern Baltic Addresses: 1 Biological Station Rybachy, Zoological Institute, 199034 St. Petersburg, Russia. E-mail: lsok@bioryb.koenig.ru 2 Zvenigorod Biological Station, Moscow State University, Biological Faculty, Vorobyevy Gory, 119992 Moscow, Russia Received 15 April 2006 / Received in revised form 18 May 2006 / Accepted 24 May 2006 1. Introduction Most authors treat the Long-tailed Tit Aegithalos caudatus as a sedentary or partly nomadic species (Dementiev & Gladkov 1954, Cramp & Perrins 1993, Ryab- itsev 2001). However, in Northern and Northeastern Europe (Fennoscandia, Kare- lia, Baltic countries) the Long-tailed Tit behaves as a typical irregular migrant. Its occurrence on migration is subject to significant annual fluctuations (Sokolov et al.