Female- and male-specific signals of quality in the barn owl

Most bird studies of female signalling have been confined to species in whi ch females display a male-ornament in a vestigial form. However, a great de al of benefit may be gained from considering phenotypic traits that are spe cific to females. This is because (1) sex-specific traits may signal sex-sp ecific qualities and (2) females may develop a male-ornament not because th ey are selected to do so, but because fathers transmit to daughters the und erlying genes for its expression (genetic correlation between the sexes). W e investigated these two propositions in the barn owl Tyto alba, a species in which male plumage is lighter in colour and less marked with black spots than that of females. Firstly, we present published evidence that female p lumage spottiness reflects parasite resistance ability. We also show that m ale plumage coloration is correlated with reproductive success, male feedin g rate and heart mass. Secondly, cross-fostering experiments demonstrate th at plumage coloration and spottiness are genetically correlated between the sexes. This implies that if a given trait value is selected in one sex, th e other sex will indirectly evolve towards a similar value. This prediction is supported by the observation that female plumage coloration and spottin ess resembled that of males, in comparisons at the level of Tyto alba alba populations, Tyto alba subspecies and Tyto species. Our results therefore s upport the hypothesis that sex-specific traits signal sex-specific qualitie s and that a gene for a sex-specific trait can be expressed in the other se x as the consequence of a genetic correlation between the sexes.