Tail Ornamentation, Size Dimorphism and Wing Length in the Genus Euplectes (Ploceinae)

Abstract

Sexual wing dimorphism in relation to tail ornaments and body size was studied in the strikingly sexually dimorphic widowbirds and bishops (Euplectes) of the African tropics. Seven widowbirds grow long tails, varying from 7 cm in Fan-tailed Widowbird (E. axillaris) to 0.5 m in the Long-tailed Widowbird (E. progne). Aerodynamic drag increases with tail length, and adaptations to compensate for this cost might be expected (e.g. increasing wing length), a prediction that was supported among the widowbirds. After controlling for overall size dimorphism (estimated by tarsus length), 70% of the variation in residual wing dimorphism among the widowbirds was explained by tail dimorphism. Bishops were more dimorphic in wing length than expected, which may be related to their slow display flight. The results suggest caution in using wing length alone for interspecific comparisons of sexual size dimorphism. Based on tarsus length, the lekking E. jacksoni is more size dimorphic than the average of its congeners, in contrast to what has been concluded in previous studies based on wing length.