SEEN - SE European Bird Migration Network
Sympatric partial migrants under evolutionary pressure caused by global warming

Jarosław Krzysztof Nowakowski
Bird Migration Research Station, University of Gdańsk, Przebendowo, 84-210 Choczewo, Poland
j.k.nowakowski@gmail.com

Global climate and environmental changes observed in the last decades cause changes in migratory behaviour of birds and are seen in arrival time, distance of movement etc. They are mainly behavioural and it is not know to what extent are also accompanied by evolutionary changes. Basing on measurements of 73 416 great tits and 43 657 blue tits, which are known to be partial migrants, caught during migration in northern Poland I found that in the period from 1967 to 2003 considerable changes in biometrics of these two species took place. Wings got more rounded and symmetrical which shape is typical for sedentary species. Lean body mass of great tits increased (in males twice as quickly as in females), while that of blue tits decreased (in females twice as quickly as in males). These tendencies could be explained by higher inter- and intraspecies competition pressure due to overcrowding caused by increased proportion of individuals of both species that do not leave breeding ground for winter.

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