Convergent evolution in social swallows (Aves: Hirundinidae)

Why the long beak? Phylogeny, convergence, feeding ecology, and evolutionary allometry shaped the skull of the Giant Cowbird Molothrus oryzivorus (Icteridae)

Cowbirds are a successful group of obligate brood parasites in the Neotropical passerine family Icteridae that offer an interesting model to explore the factors behind the evolution of the bird craniomandibular complex. The Giant Cowbird, Molothrus oryzivorus, stands out from its congeners, among other features, in diet (feeds mostly on fruit, nectar, and arthropods, instead on seeds), its larger body size, and longer, more robust beak with a much broader bony casque than in other cowbirds. In turn, Giant Cowbirds show a remarkable resemblance in these features to the distantly related caciques and oropendolas (some are its breeding hosts). However, the causes behind the latter resemblance and the distinctiveness among cowbirds have not yet been elucidated. We aim to explore the factors involved in the diverging morphology of the Giant Cowbird from its congeners and the convergence with caciques and oropendolas, surveying their skull and lower jaw under an explicit evolutionary framework. Using geometric morphometrics and comparative methods, we assessed the signal of phylogeny, convergence, feeding ecology, and size in skull shape. Our results indicated that evolution of the craniomandibular complex of icterids in general, and of the beak morphology in the Giant Cowbird in particular, are shaped by multiple factors, with phylogeny being largely overridden by changes in size (evolutionary allometry), primarily, and feeding ecology, secondarily. However, the evolution of a broad bony casque in the Giant Cowbird, otherwise a hallmark of caciques and oropendolas, does not appear to have primarily been ruled by evolutionary allometry. Instead, taking into account the unique extreme convergence between Giant Cowbirds and some of its caciques hosts, it might be consequence of selective regimes associated with parasite-host interactions acting on top of other evolutionary processes. This suggests chick mimicry as a reasonable explanation for this peculiar morphology that would require further investigation.

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.

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.

Convergent evolution in social swallows (Aves: Hirundinidae)

Behavioral shifts can initiate morphological evolution by pushing lineages into new adaptive zones. This has primarily been examined in ecological behaviors, such as foraging, but social behaviors may also alter morphology. Swallows and martins (Hirundinidae) are aerial insectivores that exhibit a range of social behaviors, from solitary to colonial breeding and foraging. Using a well-resolved phylogenetic tree, a database of social behaviors, and morphological measurements, we ask how shifts from solitary to social breeding and foraging have affected morphological evolution in the Hirundinidae. Using a threshold model of discrete state evolution, we find that shifts in both breeding and foraging social behavior are common across the phylogeny of swallows. Solitary swallows have highly variable morphology, while social swallows show much less absolute variance in all morphological traits. Metrics of convergence based on both the trajectory of social lineages through morphospace and the overall morphological distance between social species scaled by their phylogenetic distance indicate strong convergence in social swallows, especially socially foraging swallows. Smaller physical traits generally observed in social species suggest that social species benefit from a distinctive flight style, likely increasing maneuverability and foraging success and reducing in-flight collisions within large flocks. These results highlight the importance of sociality in species evolution, a link that had previously been examined only in eusocial insects and primates.