FEMALE REPRODUCTIVE EFFORT DEPENDS ON THE DEGREE OF ORNAMENTATION OF THEIR MATES.

Sexual selection theory assumes that secondary sexual characters do not influence female reproductive effort. Female animals may invest relatively more in reproduction if they acquire mates of high phenotypic quality, because offspring sired by preferred males may be relatively more viable than offspring sired by less preferred males. Here we report for the first time in a field study that females of the monogamous barn swallow Hirundo rustica adjust their reproductive effort to the attractiveness of their mates. Experimental manipulation of male tail length, which is a trait currently subject to a directional female mating preference, affected the reproductive effort by females in single broods as well as their decision on the seasonal number of clutches. These results, and those of previous experiments, demonstrate that female barn swallows assess the quality of their mates throughout the reproductive season and adjust their reproductive decisions accordingly. This result has important implications for the theory of sexual selection and for the possibility of testing current models of female mate preferences, because the viability of offspring will be confounded by differential reproductive effort. © 1993 The Society for the Study of Evolution.