What do data from birdwatchers notepads tell us? The case of the Bearded Tit (Panurus biarmicus) occurrence in western Poland

Birdwatchers gather significant amount of casual observations which are often used in scientific research. However, accuracy of this data is seldom controlled. The aim of this study was to detect if casual observations reflect real situation in the case of Bearded Tit occurrence. Two sources of data were compared: different birdwatchers casual observations (CO) and results of regular studies (RS) carried out in the same region by the author. In the case of RS, study areas were selected randomly and playback method was used to maximise efficiency of species searching. It has been revealed that birdwatchers explore mainly large lakes and big river valleys. Small marsh patches and streams, where significant part of Bearded Tit sites is located, are usually omitted. According to CO data, the peak of non-breeding birds, which normally occurs in the first half of October, took place a month later. This shift resulted from higher birdwatchers activity connected with holidays in the beginning of November and dynamics of rare waterbirds in the region. Flocks of wintering Bearded Tits found during RS were significantly smaller than in CO. Possible causes of observer biases are presented and discussed.