A study on the Northern Pintail (Anas acuta) females with masculinized plumage: their prevalence, morphological and behavioral traits, and reproductive organs

Abstract

We report here on the prevalence, morphological and behavioral traits, and gonadal condition of anomalous Northern Pintail (Anas acuta) females found at the Hyo-ko Waterfowl Park in Niigata, Japan. A total of 15 anomalous birds were banded and examined; these had variably masculinized plumages that could be tentatively classified into three phases: A (extensively masculinized), B (moderately masculinized), and C (partially masculinized). Unbanded birds with the same external features were also found at the same location. Based on these birds and data from a previous study, the prevalence of anomalous birds at the study site during the past 10 winters (1998–2008) was 0.01–0.18% (mean 0.07%) of the local population per winter, or approximately 3,800–8,500 individuals. Anatomical examination of the eight birds sacrificed showed that their ovary had remarkably degenerated, although their oviduct and syrinx were morphologically normal; two of these were 8 years old). Long-term observation of one of the focal birds showed that the external masculinization had commenced at a certain adult stage, progressing slowly thereafter. In term of behaviors, these anomalous birds were sexually inactive, although one of them showed temporarily and exceptionally the burping behavior, which is a male-type courtship display. We consider that all of these masculinized traits seen in the Northern Pintail females were caused by spontaneous ovarian degeneration after sex differentiation.