COSTS OF SEXUAL ADVERTISING IN THE LEKKING JACKSON WIDOWBIRD

Costs of two sexual advertisements, a 20 cm long tail and lek display, were investigated in Jackson's Widowbird (Euplectes jacksoni). Although leks were frequently disturbed by approaching raptors, no predation was recorded. Judging from counts of growth bars, the nuptial tail completes growth in five weeks, and its final length seems to depend equally on growth time and growth rate. A cost of tail formation is inferred from a lower index of body condition (relative body mass) among males in late compared to early stages of tail growth, and from frequent fault bars (translucent bars due to reduced deposition of keratin) in tail feathers. The number and size of faults were larger in the proximal (last formed) half of feathers, and the frequency of faults increased with both feather length and growth rate. A physiological cost of lek attendance and display was indicated by lower condition among full grown courtholders compared to floaters. In two of four seasons, this was also supported by a decline in condition as a function of how long after lek initiation a male was captured. Among all measured males, full grown tail length was positively related to body condition. In a smaller sample of behaviorally observed males this relationship was obscured, while condition instead was negatively related to display rate and lek attendance. Aerodynamic costs of the tail are discussed, as are the possible roles of tail plumes and display as advertisers of male quality.