Laying dates and clutch size of open-nesting passerines in the Czech Republic: a comparison of systematically and incidentally collected data

Data are presented on laying date and dutch size Cn) of Blackcap Sylvia atricapilla (1434), Song Thrush Turdus philomelos (298), Garden Warbler S. borin (260), Yeltowhammer Emberiza citrinella (254), Blackbird T. merula (231), Chaffinch Fringilla coelebs (206), Chiffchaff Phylloscopus collybita (202) Icterine Warbler Hippolais icterina (95) and Dunnock Prunella modularis (89), recorded during an intensive study (since 1993) in woodland habitats of Eastern Bohemia, Czech Republic (49 degrees 54'N, 16 degrees 02'E). Results are compared with two other data sets (a local study and national nest record scheme) collected by Czech volunteer ornithologists during 1945-85 and with other published data from central Europe. There is no evidence that the laying dates and clutch size of the nine species changed at the regional scale over the past decades, and mean values generally match their known geographical variability. Significant species differences were found among the three Czech data sets in both laying dates and clutch size. Most of the differences could be explained as an artefact of seasonally biased sampling of nests, which in turn influenced mean clutch size. Even a systematic study with a standard searching effort on fixed plots provided biased data for some species.