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

Abstract

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.

2020 Editor’s Choice Award and Editor’s Choice Articles

“…Masaru Hasegawa et al (2020). Experimental tail shortening affects feeding rate depending on original tail length in female barn swallows Hirundo rustica gutturalis (Volume 38, Issue 2, pp 179-184).…”

Male mate preference against tail-elongated females in the barn swallow

“…Many correlational studies support the sexual selection explanation for the evolution of long female tails (e.g., Møller 1993aMøller , 1994see Hasegawa et al 2017 for Japanese barn swallows), but, in contrast to these correlational studies, manipulative experiments on female tails do not support sexual selection for long tail feathers (Cuervo et al 1996; also see Hasegawa et a. 2018Hasegawa et a. , 2020 for manipulative experiments showing no detectable differential allocation). However, all these correlational and experimental studies used indirect measures of mate preferences (i.e., breeding onset date, assortative mating, paternal investment, annual reproductive success; reviewed in Romano et al 2017), and thus cannot be conclusive (sensu Amundsen & Pärn 2006; see above).…”

“…The barn swallow Hirundo rustica is a socially monogamous bird and a model species of sexual selection, in which females have been shown to behaviorally prefer tail-elongated males, explaining their long outermost tail feathers (e.g., acceptance/rejection of extra-pair copulation; Møller 1988; reviewed in Møller 1994; Turner 2006; Romano et al 2017; also see Hasegawa & Arai 2020a for comparative support for the evolution of tail fork depth in relation to high opportunities of extrapair paternity in hirundines). Compared to males, female barn swallows have somewhat shorter tails, but their tails are nonetheless costly to produce and maintain as is the case for male tails (e.g., Cuervo et al 2006; Cuervo & de Ayala 2014; Hasegawa et al 2020; also see Hasegawa & Arai 2017 for comparative support); thus, a selective advantage is needed to explain their existence (Amundsen 2000). In fact, in hirundines (family: Hirundinidae), male and female tails evolved differently (Hasegawa & Arai 2020b) and hence sexual dimorphism in tail length varies greatly across species (Turner & Rose 1994; Hasegawa & Arai 2020a), indicating that the correlated response hypothesis alone cannot explain the expression of female tail length.…”

“…However, all these correlational and experimental studies used indirect measures of mate preferences (i.e., breeding onset date, assortative mating, paternal investment, annual reproductive success; reviewed in Romano et al 2017), and thus cannot be conclusive (sensu Amundsen & Pärn 2006; see above). In highly aerial birds like barn swallows, indirect measures of mate choice should also be affected by the aerodynamic cost of long female tails (e.g., Buchanan & Evans 2000; Hasegawa et al 2020), preventing us from elucidating the function of long tails in females. In fact, “any experimental shortening and lengthening of the outer tail feathers is likely to upset an original co-adapted character set…” (Norberg 1994, p. 231), which would then affect female physiology and behavior, possibly confound with indirect measures of mate choice.…”

“…We conducted this “sequential choice” test rather than the “simultaneous choice” test, because the former is more appropriate than the latter in the barn swallow, a protandry migrant species, in which males encounter females sequentially rather than simultaneously as in many other species (Barry & Kokko 2010). Because long-tailed females are high-quality females in terms of arrival date, survivorship, breeding experience, and parental and reproductive investment in barn swallows including Japanese populations (e.g., Møller 1993a, 1994; Brown & Brown 1999; Hasegawa et al 2016b, 2017, 2018, 2020), and because previous correlational studies support sexual selection for female long tails in Japanese barn swallows (Hasegawa et al 2017), we predicted that males would prefer tail-elongated female models to control female models. We also examined individual mate preference for female tail length in relation to male tail length, which is virtually impossible to examine when using indirect measures of mate preference (e.g., breeding date, assortative mating; see above).…”