Males prefer ornamented females: a field experiment of male choice in the rock sparrow

Recent evidence suggests that female ornaments in birds not only are genetic correlates of selection on males but may also have evolved by selection acting directly on females. However, most evidence comes from studies in captivity and it is therefore still debated whether male choice is important under natural conditions. The rock sparrow, Petronia petronia, is a monomorphic passerine in which both sexes show a yellow breast patch, although females have a smaller patch than males. Breast patch in females is positively correlated with body mass and fecundity (number of broods per year). We tested whether males preferentially mate with more ornamented females in this species. We examined male mate choice by reducing the size of the yellow breast patch of free-ranging females during the pair formation period in a colony in southeast Spain. We found that females with an experimentally reduced breast patch were courted less intensely and chased less frequently, than control females were. Furthermore, females with an experimentally reduced ornament were less likely to mate and, when mated, did so at a significantly later date than controls. These results are supported by correlational observations from another rock sparrow population, for which a larger data set was available, showing that pairs mated assortatively with respect to breast patch size. Altogether, these results therefore suggest that female ornamentation may be sexually selected in this species.