Evolution of tail fork depth in genusHirundo

Abstract

A classic example of a sexually selected trait, the deep fork tail of the barn swallow Hirundo rustica is now claimed to have evolved and be maintained mainly via aerodynamic advantage rather than sexually selected advantage. However, this aerodynamic advantage hypothesis does not clarify which flight habits select for/against deep fork tails, causing diversity of tail fork depth in hirundines. Here, by focusing on the genus Hirundo, we investigated whether the large variation in tail fork depth could be explained by the differential flight habits. Using a phylogenetic comparative approach, we found that migrant species had deeper fork tails, but less colorful plumage, than the other species, indicating that migration favors a specific trait, deep fork tails. At the same time, tail fork depth but not plumage coloration decreased with increasing bill size – a proxy of prey size, suggesting that foraging on larger prey items favors shallower fork tails. Variation of tail fork depth in the genus Hirundo may be explained by differential flight habits, even without assuming sexual selection.

Convergent evolution of the tradeoff between egg size and tail fork depth in swallows and swifts

“…This perspective is consistent with another phylogenetic study in which fork depth decreased with increasing prey size index in the genus Hirundo (i.e. those breed in cup-shaped nests, which would not constrain the evolution of fork tails, differed from other nest shape such as enclosed nests: Hasegawa et al 2016). Furthermore, the negative association between egg size and female fork depth is consistent with sexual selection theory, in which females must trade off between ornamentation and maternal investment because females need to invest considerable resources into reproduction (Fitzpatrick et al 1995, Chenoweth et al 2006.…”

“…In species those have square tails rather than fork tails, fork depth approximated zero, as in Hasegawa and Arai (2017b). Because of the large variation in measurements within species (and due to the lack of decimal points in some measurements), trait sizes were rounded down to the nearest integer so as not to overfit the model (Hasegawa et al 2016, Hasegawa andArai 2017b). Likewise, relative fork depth was grouped into ten subscales (per 20%: i.e.…”

“…Swifts might mitigate the costs of fork depth by having a long wing as a cost-reducing trait (sensu Chantler andDriessens 1995, Møller 1996; Supplementary material Appendix 1 Table A1). Or, fork depth might be adaptive in swifts if fork tails are suitable for capturing weak-flying insects rather than strong-flying large insects (Hasegawa et al 2016).…”

Beauty alone is insufficient: female mate choice in the barn swallow

“…Although there are some notions that the aerodynamic advantages (i.e., maneuverability, here) of deep fork tails can explain the evolution of this trait (e.g., Thomas 1993;Norberg 1994;Evans 1998; also see e.g., Aparicio et al 2012;Pap et al 2015 for objection to this explanation: reviewed in Hasegawa et al 2016b;Hasegawa and Arai 2017a), population mean tail length should be aerodynamically costly, indicating that female preference for long-tailed males should account for the evolution of long tails, at least in part (e.g., Buchanan and Evans 2000;Rowe et al 2001). Because only highquality (e.g., viable) males can bear the cost of long-tails, females obtain high-quality mates by preferring longtailed males (reviewed in Møller 1994a;Turner 2006; also see Romano et al 2017b).…”

“…Still, it is often unclear whether these patterns reflect evolutionary covariation without a well-designed experiment (e.g., Wagner et al 2012;van Noordwijk and de Jong 1986;Stearns 1992;Andersson et al 2002;Saino et al 2003), due to individual variation in resource availability (e.g., when the allocation between the traits varies less than the total investment, a positive correlation can be predicted even if there is a trade-off between two ornaments). For example, colorful ventral plumage and tail ornaments vary inversely across regions, subspecies, and even species within the genus Hirundo (Turner 2006;Hasegawa and Arai 2013b;Hasegawa et al 2016b), indicating that within-population patterns do not always predict evolutionary relationships in swallows (also see Vortman et al 2015 for heritable variation in the relative investment of the two ornaments). An exception is the case in which traits are developmentally and functionally (or genetically) integrated (Andersson et al 2002), in which case the evolution of one trait accompanies those of others (e.g., long, symmetrical tails; Møller 1990, 1993b, Balmford and Thomas 1992.…”