MORPHOLOGICAL ADAPTATIONS TO AN EXTREME SEXUAL DISPLAY, STONE-CARRYING IN THE BLACK WHEATEAR, OENANTHE LEUCURA

Males of the black wheatear (Oenanthe leucura) carry on average almost 2 kg of stones to cavities inside caves before the laying of each clu tch, a display involved in postmating sexual selection as determined f rom female adjustment of timing and rate of reproduction to experiment ally manipulated numbers of stones carried. A large wing area in relat ion to body mass would allow males to carry many and heavy stones. The number and mass of stones carried were inversely related to wing load ing, suggesting that a large wing area has evolved as an adaptation to stone carrying. We tested this functional hypothesis in a field exper iment with three treatments: (1) two primaries removed from each wing (manipulation), (2) the tips of two primaries removed from each wing ( sham-manipulation), and (3) the male just captured and handled (contro l). The number and mass of stones carried were inversely related to or iginal wing area, as predicted by the hypothesis, and males with initi ally large wing areas were better able to cope with the experimental t reatment than others. These results are consistent with stone carrying being a reliable signal of maximum working ability of males during sh ort bursts of stone carrying, and intraspecific differences in wing mo rphology therefore allow males to display at different levels.