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Effects of Weather Conditions, Time of Breeding, Brood Size and Hatching Order on Eurasian Bittern Nestling Growth in a Food-Rich Fishpond Habitat

The development of nestlings depends on both biological and weather factors. However, their combined effect differs among bird species. In this study, the impacts of three temperature variables, precipitation, wind speed, timing of breeding, brood size and hatching order on the growth of Eurasian bittern Botaurus stellaris chicks were analysed. Measurements of 183 nestlings from 57 nests were made at fishpond complexes in eastern Poland. Relative growth rates (RGR) were calculated on the basis of tarsus length and body mass. Generalised linear mixed models showed that brood size, hatching order and precipitation were the most important factors. Nestlings in broods of two and three grew faster than those from broods of four and five. In the largest broods, the fifth-hatched chicks had lower growth rates; this also applied to the oldest chicks in all brood-size categories. Nestlings from late-season nests showed enhanced mass and tarsus growth, while heavier precipitation and strong winds depressed growth rates. The study emphasises that even among bitterns breeding in food-rich habitats like fishponds, the chicks in largest broods run the risk of lower growth rates.