Egg size decreases with increasing female tail fork depth in family Hirundinidae

Evolution of female colours in birds: The role of female cost of reproduction and paternal care

Female ornamentation is frequently observed in animal species and is sometimes found as more evolutionary labile than male ornamentation. A complex array of factors may explain its presence and variation. Here we assessed the role of female cost of reproduction and paternal care. Both factors have been pinpointed as important by theoretical studies but have not been investigated yet in details at the interspecific level. We worked on 133 species of North temperate Passeriformes bird species for which both the clutch volume - here taken as the proxy of female cost of reproduction - and amount of paternal care are relatively well known. Using spectrometry, we measured the whole-body coloured plumage patches and quantified three metrics corresponding to brightness (i.e. achromatic component), colour chromaticity (i.e. intensity) and colour volume (i.e. diversity). We found a strong association between male and female colour metrics. Controlling for this association, we found additional small but detectable effects of both cost of reproduction and paternal care. First, females of species with more paternal care were slightly brighter. Second, the interaction between the level of paternal care and egg volume was correlated with female colour intensity: females with more paternal care were more chromatic, with this association mostly present when their investment in reproduction was low. Together these results suggest that female cost of reproduction and paternal care are part of the multiple factors explaining variation of female coloration, besides the strong covariation between male and female coloration.

Opposing population trends of fork-tailed swallows and reddish-coloured swallows in our changing world

Sexual selection can in theory lead to positive and negative effect on population-level fitness and hence population increase/decline in our changing world, but the empirical evidence is scarce. Using a phylogenetic comparative approach, we examined whether and how different sexually selected ornaments affect recent population trends and extinction risk in swallows (Aves: Hirundininae). We found that population trends decreased with increasing depth of male tails, that is a well-known sexually selected trait, and increased with increasing score of reddish plumage coloration, another sexually selected ornament. Similar contrasting patterns were observed for extinction risk. These findings indicate ornament-specific population trends and extinction risk, perhaps due to the differential costs and benefits of ornamentation. Previous studies have mostly focused on the overall effects of sexual selection by combining different kinds of traits, presumed to be sexually selected. However, as predicted by theory, sexual selection would not be a process with the same universal effect on population dynamics as we found here. Divergent ecological consequences would occur through minor differences in sexual selection, which should not be dismissed in future studies.

Experimental tail shortening affects feeding rate depending on original tail length in female barn swallows Hirundo rustica gutturalis

Long tail feathers of the barn swallow Hirundo rustica are a classic example of an intersexually selected trait, but previous aerodynamic analyses indicate that the tail feather is only 10–12 mm longer than the aerodynamic optimum even in the nominate subspecies with long tails. Here, by experimentally shortening female tail length, we studied the feeding cost of long tail feathers in Japanese barn swallows, Hirundo rustica gutturalis, which have ca. 10 mm shorter tails than the nominate subspecies. Female feeding rate was explained by the interaction between treatment and original female tail length: feeding rate decreased with decreasing original female tail length in control, but not in tail-shortened females. Because the interaction term was far from significant in the analysis of female incubation investment, the observed pattern would be specific to feeding rate, which is greatly affected by the aerodynamic properties associated with tail length. Differential allocation of paternal feeding investment was not observed in the current data set. Long tails would be costly at least in short-tailed females, supporting differential costs of ornamentation as predicted by sexual selection theory. Female outermost tail feathers are costly ornamentation in short-tailed Japanese barn swallows.

Sexually dimorphic swallows have higher extinction risk

The effect of sexual selection on extinction risk remains unclear. In theory, sexual selection can lead to both increase and decrease extinction probability depending on the ecology of the study system. Thus, combining different groups might obscure patterns that can be found in groups that share similar ecological features. Using phylogenetic comparative analysis, we studied sexual plumage dimorphism in relation to the perceived risk of extinction in hirundines (subfamily: Hirundininae), in which all species are socially monogamous aerial foragers. Among the 72 species studied, five species are facing a perceived threat of extinction. Species with sexually dimorphic plumage had a higher risk of extinction than did species with sexually monomorphic plumage. Likewise, when focusing solely on tail ornamentation, species that exhibit a sexual dimorphism in tail length had a higher risk of extinction than did other species. In Hirundininae, which are affected a great deal by severe weather, sexual selection and the resultant sexual dimorphism would increase extinction risk.