Several field studies have shown that ornamented males of both polygynous and monogamous animals gain an advantage in terms of mate acquisition. Male reproductive success is strongly positively related to the number of mates acquired, but differential quality of female mates may also play an important role. Darwin (The descent of man, and selection in relation to sex (1871)) and Fisher (The genetical theory of natural selection (1958)) suggested that preferred males of monogamous species should experience a reproductive advantage because of differences in female phenotypic quality. Good condition is supposed to allow females to breed early and to grant them high fecundity. Preferred males should acquire early-breeding females in good condition and for that reason have greater reproductive success. Several genetic models confirm the possible significance of this sexual selection mechanism. Alternatively, preferred males may acquire mates that invest more in reproduction because they have acquired males of higher quality. Field data on birds suggest that preferred males do acquire early-breeding mates, but it has never been shown that such mates are of a higher phenotypic quality in terms of condition. Long-tailed males of the monogamous swallow, Hirundo rustica, are preferred by females. In this study, observations using males with naturally varying and experimentally manipulated tails show that females mated to long-tailed males were heavier, bred earlier, produced more offspring per season and were much more likely to survive to the next breeding season that were females mated to short-tailed male swallows. This is the first demonstration that preferred males actually obtain mates of high phenotypic quality. Preferred males had a direct reproductive benefit of mating with long-lived females, because pairs consisting of mate-faithful individuals reproduced earlier and produced more offspring than did pairs without previous common experience.