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. Received 13 May 1993, accepted 2 July 1993. SEXUALLY SELECTED characters, such as the elongated tails or the exaggerated displays of some birds, are expected to evolve until their mating advantages are balanced by costs (Fisher 1930), like predation or physiological constraints (Harvey and Bradbury 1991, Andersson 1994). At this dynamic equilibrium, morphological or behavioral adaptations that reduce the costs (hence, improve survival and allow further exaggeration of the trait) can be expected to evolve. Two such indirect effects of sexual selection on avian morphology have recently been reported. Hedenstrom and M6ller (1992) used aerodynamic theory to predict morphological adaptations to the demands of song flight. Consistent with their predictions, they found greater sexual dimorphism in wing span and wing area in eight passerines with song flight than in related species without this (supposedly sexually selected) behavior. Evans and Thomas (1992) estimated the aerodynamic costs of elongated tails in three species of sunbirds (Nectarinia spp). An increased tail area was shown to affect the aerodynamics in several ways, but most importantly increase the drag on the bird, which is a major component of flight cost (Norberg 1990). This cost should be particularly strong for birds with graduated tail ornaments (i.e. in I Present address: Department of Biology 0116, University of California at San Diego, La Jolla, California 92093, USA. which all rectrices are elongated) compared to pin tails and fork tails (Balmford et al. 1993). As predicted based on higher energy costs of flight, sunbirds with experimentally elongated tails spent less time flying than controls (Evans and Hatchwell 1992). Based on a correlation between wing length and tail length in one of the species (Nectarinia johnstoni), Evans and Hatchwell (1992) suggested that the sunbirds compensate for the energetic costs of a long tail by increasing wing span. Likewise, intraspecific relationships between ornamental tail length and wing length have been found in the Long-tailed Widowbird (Euplectes progne; Craig 1989) and Jackson's Widowbird (E. jacksoni; Andersson 1992a), suggesting similar compensations for the cost of carrying long tails. In particular, the exceptionally long wings of the Long-tailed Widowbird have made it an outlier in comparative studies of sexual size dimorphism based on wing length (Payne