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Sexual Selection in a Socially Monogamous Bird: Male Color Predicts Paternity Success in the Mountain Bluebird, Sialia Currucoides

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Sexual Selection in a Socially Monogamous Bird: Male Color Predicts Paternity Success in the Mountain Bluebird, Sialia Currucoides Behav Ecol Sociobiol (2009) 63:403–411 DOI 10.1007/s00265-008-0674-5 ORIGINAL PAPER Sexual selection in a socially monogamous bird: male color predicts paternity success in the mountain bluebird, Sialia currucoides Susan L. Balenger & L. Scott Johnson & Brian S. Masters Received: 3 May 2008 /Revised: 9 September 2008 /Accepted: 19 September 2008 / Published online: 16 October 2008 # Springer-Verlag 2008 Abstract Ornamental traits are thought to evolve because provides males with an advantage in fertilizing the eggs of they give individuals an advantage in securing multiple multiple females. mates. Thus, the presence of ornamentation among males in many monogamous bird species presents something of a Keywords Extra-pair paternity. Mountain bluebird . Sexual conundrum. Under certain conditions, extra-pair paternity selection . Sialia currucoides . Structural plumage color can increase the variance in reproductive success among males, thus increasing the potential for sexual selection to act. We addressed this possibility in the mountain bluebird Introduction (Sialia currucoides), a socially monogamous songbird in which males possess brilliant ultraviolet (UV)-blue plum- In many animal species, one or sometimes both sexes age. Specifically, we asked whether a male’s success at possess exaggerated, apparently costly traits that do not siring offspring within his own nest and within the nests of enhance, and may even detract from, survival. Darwin other males was related to his coloration. In pairwise (1871) argued that such “ornamental” traits can evolve if comparisons, males that sired extra-pair offspring were not they increase the number of matings that an individual more colorful than the males that they cuckolded. However, obtains. More ornamented individuals may be preferred as males that sired at least one extra-pair offspring were, on mates for a variety of reasons; for example, ornamentation average, brighter and more UV-blue than males that did not may signal an individual’s quality or condition (Zahavi sire extra-pair offspring. Brighter, more UV-blue males 1975; Kodric-Brown and Brown 1984; Andersson 1994; sired more offspring both with their own mate and tended Griffith and Pryke 2006;Hill2006; Senar 2006). to sire more offspring with extra-pair mates and thus sired Male ornamentation is particularly common and dramatic more offspring overall. Our results support the hypothesis in species that have polygamous or promiscuous mating that the brilliant UV-blue ornamental plumage of male systems. In such systems, there is ample opportunity for mountain bluebirds evolved at least in part because it sexual selection to act on males due to the potential for large variation in male reproductive success. Curiously, males in a considerable number of monogamous species have also Communicated by S. Pruett-Jones evolved ornamental traits, such as the elongated and colorful plumages seen in many species of birds. This suggests that S. L. Balenger : L. S. Johnson : B. S. Masters Department of Biological Sciences, Towson University, there is greater variance in male reproductive success in Towson, MD, USA some monogamous species than it would first appear. But what causes this variance? Darwin (1871) proposed two * S. L. Balenger ( ) possible mechanisms. First, if males routinely outnumber Department of Biological Sciences, Auburn University, Auburn, AL 36849, USA females, then competition for mates increases and some e-mail: [email protected] males will fail to obtain mates (see also Kvarnemo and 404 Behav Ecol Sociobiol (2009) 63:403–411 Ahnesjö 1996; Dearborn et al. 2001). Second, some males blue plumage coloration over their entire dorsal side, with may attract females that can produce more offspring (see the rump being the brightest and most spectrally pure also Fisher 1958; Kirkpatrick et al. 1990). Such females region of the body. Male wing, tail, and breast feathers are may, for example, be in better condition and hence more also UV-blue. In females, rump, tail, and primary wing fecund. They may also be ready to breed earlier and thus feathers are a pale, dull blue, while head, back, and breast can breed more times in a season than other females. feathers are rusty-brown and gray. Mountain bluebirds are More recently, the discovery that some fraction of short-distance migrants, typically wintering at lower ele- individuals in many monogamous animal species routinely vations in the southern part of the breeding range and mate outside the pairbond (Jones et al. 2001; Griffith et al. returning in late winter or very early spring to the breeding 2002; Leibgold et al. 2006; Mills et al. 2007) has promoted grounds. This species nests naturally in pre-formed cavities the idea that extra-pair (EP) mating substantially increases but readily uses human-made nest boxes. Clutches typically the variation in male mating success in “socially monoga- contain four to eight eggs. Only females incubate eggs and mous” species (Gowaty 1985; Westneat et al. 1990; Webster brood hatchlings, but both sexes feed offspring. Polygyny et al. 1995; Freeman-Gallant et al. 2005). Ornamentation has not been reported in this species and was not observed could evolve if it increases a male’s success at siring off- in this study. spring, both in his own nest (i.e., by affecting the faith- Mountain bluebirds not only form season-long monog- fulness of his own mate) and in the nests of other males. amous pairbonds but also engage in EP mating (Monk Brilliantly colored plumage is one of the most common 1999). In a previous study of this population, we found that forms of ornamentation in birds. Several studies have found 72% of broods contained ≥1 EP offspring, and 36% of all that more colorful males in sexually dichromatic, socially offspring were sired by EP males (Balenger et al. 2008). monogamous bird species experience either increased levels of EP mating success (Yezerinac and Weatherhead Study site and field methods 1997; Sheldon and Ellegren 1999; Thusius et al. 2001; Bitton et al. 2007; Dehley et al. 2007), reduced levels of We conducted this study during the 2004 and 2005 cuckoldry (Estep et al. 2005; Whittingham and Dunn breeding seasons on a population of bluebirds living near 2005), or both (Møller and Tegelstrom 1997). Other and in the Bighorn Mountains of north-central Wyoming, studies, however, have failed to detect such relationships USA. We observed birds at two sites, a low-elevation site (Dunn et al. 1994; Stutchbury et al. 1997; Kraaijeveld et al. (1,258–1,620 m a.s.l.) in the eastern foothills of the 2004; Westneat 2006). mountains (44°38′ N, 107°01′ W) and a high-elevation site We investigated the relationship between ornamentation (2,443–2,582 m a.s.l.) 44 km away on a mountain plateau and paternity success in the sexually dichromatic, socially (44°46′ N, 107°32′ W; see Johnson et al. 2006 for a detailed monogamous mountain bluebird (Sialia currucoides). description of the two sites). All observed pairs used nest Males of this species are characterized by brilliant boxes. In both years, data were obtained for first broods ultraviolet (UV)-blue plumage, which is produced exclu- only; we did not document the occurrence or fate of second sively through modification to feather nanostructure (Prum breeding attempts. In addition, because time constraints 2006; Shawkey et al. 2006). We have shown elsewhere that prevented us from working with every available nest, we EP paternity increases the opportunity for sexual selection excluded from this study those nests in which two or fewer on males in our study population by increasing the variance nestlings survived to bleeding age. At such nests, we in male reproductive success (Balenger et al. 2008). Our typically only trapped, bled, and marked males. objective in this study was to determine whether male We checked boxes every 1–5 days to determine the date coloration is positively associated with male success at that eggs began hatching in each nest. We marked nestlings siring offspring both with his own mate and with females with a numbered aluminum leg band and collected a small mated to other males in the population. blood sample (∼50 uL) from them between 4 and 7 days after hatching began. Blood samples were stored at room temperature in Queen’s lysis buffer (Seutin et al. 1991) until Materials and methods DNA extraction. We trapped adults as they entered boxes to feed nestlings 4–9 days after hatching began. Upon capture, Study species we marked adults with a unique combination of colored leg bands (excluding blue and purple) and a numbered Mountain bluebirds are medium-sized (∼30 g) passerines aluminum band, collected a blood sample, and collected that breed in western North America at elevations up to 12–15 feathers from the center of their rump. Feathers were 3,800 m above sea level (a.s.l.; biology summarized by stored in opaque envelopes at room temperature. We also Power and Lombardo 1996). Males exhibit structural UV- weighed each adult (nearest 0.1 g) and measured the length Behav Ecol Sociobiol (2009) 63:403–411 405 of their right wing cord (nearest 1 mm). As an index of in all analyses. Males with high PC1 scores had hues body condition, we used the residuals from a regression of shifted more toward UV, had more of their color concen- weight against wing length. trated within the UV-blue range (greater UV-blue chroma), and were generally brighter. Color analysis Male color scores differed between elevations and between years at each elevation (Balenger et al. 2007). We measured reflectance spectra for rump feathers across We therefore standardized color scores to a mean of 0 and a the avian visual range (300–700 nm) using an Ocean Optics standard deviation of 1 at each elevation and within each S2000 spectrometer (Dunedin, FL, USA).
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