Reproduction of the Great Bittern Botaurus stellaris was studied in fishponds in eastern Poland from 2003 to 2009. Study areas were surveyed from March to early July, and the booming period of the Great Bittern was from March to July, varying between 58 to 108 days. The start of booming activity was linked to climatic factors. We visited 143 active nests in the study population, with the majority of females starting to lay eggs at the end of April to the beginning of May. The mean clutch size was 4.8 ± 0.8 (n = 109, range 1–6). There was no significant difference in clutch size between years or within years as the season progressed. Hatching success was high 92% (n = 78 broods). For all years of the study the breeding success (measured as a proportion of successful broods to all broods) was 51% (n = 120). The mean number of fledglings (15-days old chicks) per breeding female was 1.5 (SD ± 1.7, n = 116) and the mean number of fledglings per successful female was 3.0 (SD ± 1.2, n = 58). The main mortality factor for eggs and nestlings was mammalian predation. The high breeding parameters obtained for this population were probably due to extensive fish management and the good and stable food resources in the fishponds surveyed. Long-term monitoring and study of the population structure and breeding parameters of the Great Bittern in relation to known environmental factors are an essential tool for the conservation and management plans for this threatened species.
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