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 Departmentof Zoology,University of Gb'teborg,Medicinaregatan 18 S-41390 G•teborg,Sweden AlaSTRACT.--Sexualwing dimorphism in relation to tail ornaments and body size was studiedin the strikingly sexuallydimorphic widowbirds and bishops(Euplectes) of the African tropics.Seven widowbirds grow long tails, varying from 7 cm in Fan-tailedWidowbird (E. axillaris)to 0.5 m in the Long-tailedWidowbird (E. progne).Aerodynamic drag increaseswith tail length, and adaptationsto compensatefor this cost might be expected(e.g. increasing wing length), a prediction that was supportedamong 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. Bishopswere more dimorphic in wing length than expected,which may be related to their slow display flight. The resultssuggest caution in using wing length alone for interspecificcomparisons of sexual size dimorphism. Basedon tarsuslength, the lekking E. jacksoniis more size dimorphic than the average of its congeners, in contrast to what has been concluded in previous studies basedon wing length. Received13 May 1993,accepted 2 July 1993. SEXUALLY SELECTED characters, such as the which all rectricesare elongated) compared to elongated tails or the exaggerateddisplays of pin tails and fork tails (Balmford et al. 1993). some birds, are expected to evolve until their As predicted based on higher energy costs of mating advantagesare balancedby costs(Fisher flight, sunbirds with experimentally elongated 1930), like predation or physiological con- tails spent less time flying than controls (Evans straints (Harvey and Bradbury 1991, Andersson and Hatchwell 1992). Based on a correlation be- 1994). At this dynamic equilibrium, morpho- tween wing length and tail length in one of the logical or behavioral adaptations that reduce species(Nectarinia johnstoni), Evans and Hatch- the costs (hence, improve survival and allow well (1992) suggestedthat the sunbirds com- further exaggeration of the trait) can be ex- pensatefor the energetic costsof a long tail by pected to evolve. increasing wing span. Two such indirect effects of sexual selection Likewise, intraspecificrelationships between on avian morphology have recently been re- ornamental tail length and wing length have ported. Hedenstr6m and M611er (1992) used been found in the Long-tailed Widowbird (Eu- aerodynamic theory to predict morphological plectesprogne; Craig 1989) and Jackson'sWidow- adaptationsto the demandsof songflight. Con- bird (E. jacksoni;Andersson 1992a), suggesting sistentwith their predictions,they found great- similar compensationsfor the cost of carrying er sexual dimorphism in wing span and wing long tails. In particular, the exceptionallylong area in eight passerineswith song flight than wings of the Long-tailedWidowbird have made in related specieswithout this (supposedlysex- it an outlier in comparative studies of sexual ually selected) behavior. Evans and Thomas size dimorphism based on wing length (Payne (1992) estimated the aerodynamiccosts of elon- 1984) gated tails in three speciesof sunbirds (Nectar- In this paper, we investigate the possibility inia spp). An increased tail area was shown to that sexualdimorphism in wing length in eight affectthe aerodynamicsin severalways, but most widowbird speciesis a sexually selected adap- importantly increasethe drag on the bird, which tation to compensate for the aerodynamic costs is a major component of flight cost (Norberg of tail ornamentation. We also discuss the re- 1990). This cost should be particularly strong lationship between sexual size dimorphism es- for birds with graduated tail ornaments(i.e. in timated from wing-length data and that esti- mated from tarsuslength, and the relationship • Presentaddress: Department of Biology 0116, Uni- between size dimorphism and body size. Sexual versityof Californiaat SanDiego, La Jolla,California selection and, hence, sexual dimorphism are be- 92093, USA. lieved to be particularly strong in lek mating 80 January1994] SexualDimorphism in Widowbirds 81 systems, but the comparative evidence is in- through female choice, but in some casesmay conclusive,and exceptionsabound (Payne 1984, also function asagonistic signals. The long tails H6glund 1989, Trail 1990, Oakes 1992). For ex- are of the graduatedtype that should handicap ample, the only lek breeder in this genus,Jack- flight considerably(Balmford et al. 1993).In this son'sWidowbird, was reported as no more size paper we test the prediction that adaptationsto dimorphic than its resource-defensepolygy- reduce such costs have evolved in the widow- nous congeners (Payne 1984, HtSglund 1989, birds. Oakes 1992). These comparisonswere basedon wing lengths, and we discussthe validity of METHODS these conclusions and the new patterns that Table 1 gives the biometric data used for each spe- emerge from using tarsuslengths instead. cies. Data were obtained by S.A. primarily from the The widowbirds (formerly Coliuspasser)to- collection of bird skins in the National Museums of gether with the bishops form the genus Eu- Kenya, Nairobi, and (for E. hartlaubipsammocromius) plectes,a group of 16 small (13-45 g) granivorous in the British Museum of Natural History, Tring. weaverbirds (subfamily Ploceinae) of the Af- Specimensthat could not unequivocally be assigned rican tropics(Delacour and Edmond-Blanc1933, by sex or specieswere excluded. As some samples, Hall and Moreau 1970). All speciesare similar particularly of females,were very small or equivocal in general biology and behavior (Craig 1980) (uncertain sexing and/or speciesidentification), we included data from the literature (Maclean 1988, Craig and, obviously, closely related as hybridization 1989). For tarsus and tail length of albonotatusand occurs in captivity (Colahan and Craig 1981). macrourus,MacLean (1988) did not give sample sizes. All Euplectesare strikingly sexually dimorphic Measurementsof aferand fransiscanuswere provided in plumage. Breeding males have black body by A. J. F. K. Craig. The smallestsamples included plumage with patchesof bright red, yellow, or are for gierowiifemales (2), and macrourusmales (4) white. The male bishopsare the mostbrilliantly and females(4). All other samplesare larger than five. colored and have a short tail, while male wid- Due to lack of data, three species (Golden-backed owbirds replace the tail feathersduring the pre- Bishop [E. aureus],Fire-fronted Bishop [E. diadematus] breeding molt, and grow graduated ornamental and Red Bishop[E. orix])were excludedfrom the anal- tails of various lengths. Females are in most ysis.Most Euplectesare widespreadin Africa and some have geographically isolated populations and sub- casesnoticeably smaller, streakybuff or brown, species(Hall and Moreau 1970,Mackworth-Praed and and difficult to distinguish in the field. Non- Grant 1985). When applicable, the full names of the breeding malesresemble the femalesbut in some subspeciesincluded in this analysisare given in Table speciesretain part of the nuptial coloration on 1. For one species,the Long-tailed Widowbird, there rump or wings. were large sample sizes for more than one of the Habitat preferencesrange from reed bedsand geographicallyisolated populationsin Craig (1989), savanna with sparsebushes through to open of which we randomly selected one (E. prognedela- grassland,and probably all speciesare polyg- merei)by throwing a die. However, we alsomade sure ynous (Craig 1980).Except in the lek-breeding that inclusion of any of the other two did not signif- Jackson'sWidowbird, males defend nesting ter- icantly affect the results.The scientificand common names used follow Sibley and Monroe (1990). ritories and participate to some extent in nest On the museum skins, tarsuslength was measured building, but not (or very rarely) in incubation with calipers as the distance from the middle of the or feeding. intertarsal joint to the distal edge of the last complete The tail ornamentsvary greatly in size, from tarsal scale (i.e. before the toes begin). Wing (chord) the slightly elongated7-cm tail in the male Fan- length was measuredwith a ruler as the distance be- tailed Widowbird (E. axillaris) to the 50-cm tween the bend of the wing and the tip of the longest plumes trailing behind the male Long-tailed primary. The dry wing chords of the museum skins Widowbird--more than four times his body could not be flattened against the ruler to the extent length and one of the most extreme avian that is common practice when measuring live birds adornments. For the Long-tailed Widowbird (Svensson1975), producing a slight underestimate(1- 2 mm) of absolutewing length, but not affecting the (Andersson 1982) and the lekking Jackson's sexualdimorphism as the sexeswere treated equally. Widowbird (20-cm tail; Andersson 1989, 1992b) For tail length a thin ruler was positionedunder the there is evidence that tail length is a cue used upper tail covertssuch that it stoppedat the baseof by femalesin mate choice.It seemsreasonable the feathers, thus providing a measurement of dis- to assumethat the long tailsof male widowbirds tance from there to the tip of the longest rectrix. generally are sexually selected, probably We calculated relative dimorphism as the natural 82 ANDERSSONAND ANDERSSON [Auk, Vol. 111 the genus, jacksoni,is interesting with respect 0.4