Determining Great Bittern Botaurus stellaris laying date from egg and chick biometrics

Abstract

Capsule Great Bittern breeding phenology can be estimated from egg and chick biometrics. Aims To estimate egg or chick ages in order to back-calculate egg-laying dates. Methods Bittern nests were searched for in six French and three Belarussian sites between 1999 and 2004. Eggs and chicks were measured at each visit. By using a subsample of nests with known egg-laying (or hatching) dates, regression equations are determined using egg density and tarsus length in order to estimate, respectively, egg and chick ages. Additionally in Belarus, the ‘water test’ was used to estimate the incubation stage of the clutch. Results A total of 141 Bittern nests were found. Egg density decreased linearly from 1.063 at laying to 0.915 the day before hatching. A regression equation therefore allows estimation of egg age from its density. A scale was also constructed to estimate egg age from its position in water, and the accuracy of the two methods is compared. Chick growth rates were similar between the two countries. Before the age of 25 days, chicks are best aged by tarsus length compared to other measurements (weight, bill length). No data were available after that age because chicks were no longer found on nests. Conclusions Egg-laying date can be estimated to within ±3 days using egg density, and to within ±5 days, using the ‘water test’. Tarsus length can be used until the age of 25 days to age chicks to within ±2 days. These simple measurements provide efficient and accurate methods to record the breeding calendar of this endangered species.