The Auk 111(1):80-86, 1994 TAIL ORNAMENTATION, SIZE DIMORPHISM AND WING LENGTH IN THE GENUS EUPLECTES (PLOCEINAE) STAFFAN ANDERSSON 1 AND MALTE ANDERSSON Department of Zoology, University of Gb'teborg, Medicinaregatan 18 S-413 90 G•teborg, Sweden AlaSTRACT.--Sexual wing dimorphism in relation to tail ornaments and body size was studied in the strikingly sexually dimorphicwidowbirdsand bishops (Euplectes) of the African tropics. Sevenwidowbirdsgrow long tails,varying from 7 cm in Fan-tailed Widowbird (E. axillaris) to 0.5 m in the Long-tailedWidowbird (E. progne). Aerodynamic drag increases with tail length, and adaptations to compensate for this costmight be expected (e.g. increasing wing length), a prediction that was supported among the widowbirds. After controlling for overall size dimorphism (estimatedby tarsuslength), 70% of the variation in residual wing dimorphismamong 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 usingwing length alonefor interspecific comparisons of sexual size dimorphism. Based on tarsuslength, the lekking E. jacksoni is more size dimorphic than the average of its congeners,in contrast to what has been concluded in previous studies basedon wing length. Received 13 May 1993,accepted 2 July1993. SEXUALLY SELECTED characters, such as the elongatedtails or the exaggerated displaysof some birds, are expected to evolve until their mating advantages are balanced by costs (Fisher 1930), like predation or physiological con- straints (Harvey and Bradbury 1991, Andersson 1994). At this dynamic equilibrium, morpho- logical or behavioral adaptations that reduce the costs(hence, improve survival and allow further exaggeration of the trait) can be ex- pected to evolve. Two such indirect effects of sexual selection on avian morphology have recently been re- ported. Hedenstr6m and M611er (1992) used aerodynamictheory to predict morphological adaptations to the demands of songflight. Con- sistent with their predictions, they found great- er sexualdimorphism in wing span and wing area in eight passerines with song flight than in relatedspecies without this (supposedly sex- ually selected) behavior. Evans and Thomas (1992) estimatedthe aerodynamiccosts of elon- gated tails in three speciesof sunbirds (Nectar- inia 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 graduatedtail ornaments (i.e. in • Present address: Department of Biology0116, Uni- versityof Californiaat SanDiego,La Jolla, California 92093, USA. which all rectricesare elongated) compared to pin tails and fork tails (Balmford et al. 1993). As predicted based on higher energy costsof flight, sunbirds with experimentally elongated tails spent lesstime flying than controls (Evans and Hatchwell 1992). Based on a correlation be- tween wing length and tail length in one of the species (Nectarinia johnstoni), Evansand Hatch- well (1992) suggestedthat the sunbirds com- pensate for the energeticcosts of a long tail by increasing wing span. Likewise, intraspecific relationshipsbetween ornamental tail length and wing length have been found in the Long-tailed Widowbird (Eu- plectes progne; Craig 1989)and Jackson's Widow- bird (E. jacksoni; Andersson 1992a), suggesting similar compensations for the cost of carrying long tails. In particular, the exceptionallylong wings of the Long-tailedWidowbird havemade it an outlier in comparative studies of sexual size dimorphism basedon wing length (Payne 1984) In this paper, we investigate the possibility that sexualdimorphism in wing length in eight widowbird species is a sexually selectedadap- tation to compensatefor the aerodynamic costs of tail ornamentation. We also discuss the re- lationship between sexual size dimorphism es- timated from wing-length data and that esti- mated from tarsuslength, and the relationship between size dimorphism and body size. Sexual selectionand, hence, sexualdimorphism are be- lieved to be particularly strong in lek mating 80