J. Avian Biol. 40: 173180, 2009 doi: 10.1111/j.1600-048X.2009.04521.x # 2009 The Authors. J. Compilation # 2009 J. Avian Biol. Received 26 February 2008, accepted 3 July 2008 Extra-pair paternity in the socially monogamous mountain bluebird Sialia currucoides and its effect on the potential for sexual selection Susan L. Balenger, L. Scott Johnson, Herman L. Mays Jr. and Brian S. Masters S. L. Balenger (correspondence), L. S. Johnson, and B. S. Masters, Dept. of Biol. Sci., Towson Univ., Towson, MD 21252, USA. Email: balensl@auburn.edu.  H. L. Mays Jr., Geier Collections and Res. Center, Cincinnati Museum Center, Cincinnati, OH 45203, USA.  Present address of SLB: Dept. of Biol. Sci., Auburn Univ., Auburn, AL 36849, USA. Sexual selection theory posits that ornamental traits can evolve if they provide individuals with an advantage in securing multiple mates. That male ornamentation occurs in many bird species in which males pair with a single female is therefore puzzling. It has been proposed that extra-pair mating can substantially increase the variance in reproductive success among males in monogamous species, thus increasing the potential for sexual selection. We documented the frequency of extra-pair paternity and examined its effect on variation in male reproductive success in the mountain bluebird Sialia currucoides, a socially monogamous songbird in which males possess brilliant plumage ornamentation. Extra-pair paternity was common in our Wyoming study population, with 72% of broods containing at least one extra- pair offspring. The standardized variance in actual male reproductive success (i.e., the total number of within-pair and extra-pair offspring sired) was more than seven times higher than the variation in apparent success (i.e., success assuming that no extra-pair mating occurred). Success at siring within-pair and extra-pair offspring both contributed to the variation in overall male reproductive success. Within-pair success, however, did not predict a male’s level of extra-pair success, suggesting that males do not sacrifice within-pair paternity to gain extra-pair paternity. Calculation of the sexual selection (Bateman) gradient showed that males sire approximately two additional offspring for each extra-pair mate that we identified. Thus, in this sexually dichromatic species, extra-pair mating increases the variance in male reproductive success and provides the potential for sexual selection to act. The presence of male ornamentation in species in which most or all males form long-term pairbonds with a single female presents something of a conundrum given that ornamentation is often thought have evolved because it gives males an advantage in securing multiple mates (Darwin 1871). It has been suggested that extra-pair (EP) mating can substantially increase the variance in reproduc- tive success among males, thus increasing the opportunity for sexual selection (Webster et al. 1995, Møller and Ninni 1998). The potential for sexual selection to operate on individuals of a given sex is proportional to the variance in reproductive success for that sex, to the extent that this variance reflects differences in the number (and/or quality) of mates (Webster et al. 1995, and references therein). However, a high rate of extra-pair mating alone will not necessarily increase the variance in male reproductive success. For example, if males that seek EP fertilizations do so at the expense of within-pair (WP) paternity (e.g., as a result of spending less time guarding their WP mate), then EP mating may ultimately have little effect on the overall variation in male reproductive success (Webster et al. 1995, Freeman-Gallant et al. 2005, Whittingham and Dunn 2005). In fact, several studies of socially monogamous species found that taking into account the effects EP paternity did not substantially increase estimates of the variation in male reproductive success (reviewed in Freeman-Gallant et al. 2005, Whittingham and Dunn 2005; see also Kraaijeveld et al. 2004, O’Connor et al. 2006, Webster et al. 2007). Other studies, however, have shown that EP mating does increase such variation (e.g., Byers et al. 2004, Albrecht et al. 2007, Dolan et al. 2007). The mountain bluebird Sialia currucoides is a socially monogamous songbird with striking male plumage orna- mentation. We documented the frequency of extra-pair paternity in this species and quantitatively assessed the degree to which extra-pair paternity affected variance in male reproductive success and thus the potential for sexual selection to operate. Methods Study species Mountain bluebirds are medium-sized ( 30 g) passerines that breed in western North America at elevations up to 173