Geographical variation, sex and age in Great Bittern Botaurus stellaris using coloration and morphometrics

Abstract

Biometrics, plumage and bare‐part colour of 87 Great Bitterns Botaurus stellaris from the UK, France, Italy, Poland and Belarus, of known sex (sexed by behaviour, DNA or dissection), were analysed to provide reliable sexing and ageing criteria for this little‐known species, and to investigate geographical variation in biometrics. Four parameters were analysed: bill length, tarsus, wing length and body weight. We found little (though significant) geographical variation across Europe, but this was not clinal and we cannot exclude variation in measurement technique among observers. Males were significantly larger than females for all biometric parameters: a discriminant function based on these parameters was able to sex correctly all individuals. Body weight varied with season, especially for males. During the breeding period, lore colour was also a reliable sex‐criterion. Ageing proved more difficult and required a combination of iris colour (which darkens with age) and flight‐feather moult. From this, the largest sample size of known‐age Great Bitterns, we conclude that most previously published criteria for age determination of this species were wrong.