Ornis Hungarica. vol.19. (2011) p.64-74.
Change in migration phenology and biometrics of Garden Warblers (Sylvia borin) at the Ócsa Bird Ringing Station between 1984-2007
Kovács Szilvia, Csörgő Tibor, Harnos Andrea, Nagy Krisztina, Reiczigel Jenő
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The distribution area and the migration phenology of many bird species – e.g. Gar-
den Warblers (Sylvia borin) – have changed recently, presumably due to climatic factors. In
our study we have used the data of 6760 Garden Warblers caught and ringed at the Ócsa Bird Ringing Station between 1984-2007 in Hungary. We handled the age groups separately. In our study we used more, frequently applied statistical methods (FAD: First Arrival Date, LAD: Last Arrival Date, mean, median, linear regression) and some novel methods (quantile regression, kernel smoothing) to estimate the arrival date of birds. The estimated changes depended on the applied methods, because the different indices were influenced by the outliers and the non-systematic missing of data differently and there could be differences in the interpretation of different methods. We found significant shifts in spring arrival date of garden warblers analysing first arrival date, peak of migration calculeted by kernel smoothing and by quantile regression. According to the results the timing of spring migration shifted earlier. In autumn adult Garden Warblers gain weight faster and hence migrate earlier than juveniles. The timing of autumn migration of adults has not shown any change during the past 24 years measuring by most methods, we found significant shift only in case of 10% quantile regression therefore we can establish that the timing of autumn migration of adults has not changed. All methods showed that juveniles shifted their timing of autumn migration later. During the period of survey the average body mass decreased and the average wing length increased in the migrating population. This phenomenon can be explained by the changing population structure, by the increasing rate of northern breeding birds in the migrating population. Therefore a northward shift in the distribution of this species, as the effect of climate change, is probable. A further effect could be the extending area of the Sahara, because birds have to fly through greater and greater distance without food and this is a selection force toward
longer winged birds.