Sex ratio and age structure of nomadic Tengmalm's owls: a molecular approach

Sex ratio and age structure of Tengmalm's owls Aegolius funereus crossing the Gulf of Bothnia between Sweden and Finland were investigated by catching owls on the island of Stora Fjäderägg during autumn 1999. Using a molecular sexing technique, we investigated the assertions of previous studies that only adult female and juvenile Tengmalm's owls undertake nomadic movements. Juveniles were generally more frequent than adults, and the sex ratio of juvenile owls was significantly female-biased. However, contrary to expectation, adult males as well as females were caught. Additionally, among juveniles, males migrated earlier than females. Thus we infer that adult males as well as females are nomadic to some extent, and that the difference between the sexes in migratory habits is not as clear as previously thought.