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

Abstract

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.

Compensatory traits can explain concave cost function of purely sexual traits

“…In fact, concerning the function of long tails in barn swallows, Norberg (1994) mentioned that “ any experimental shortening and lengthening of the outer tail feathers is likely to upset an original co-adapted character set …” (p. 231). Also, macroevolutionary studies of swallows have supported sexual (rather than viability) selection explanation (e.g., Hasegawa et al 2016; Hasegawa & Arai 2017, 2018, 2020a,b, 2021), demanding further experimental studies, possibly using a smaller amount of manipulation (so that overcompensation can be avoided) in many species with various ornamentation. An alternative approach, the manipulation of compensatory traits (sensu Møller 1996) would in theory be valuable to show the effect of overcompensation, but is impractical due to the multi-dimensionality of compensatory traits (e.g., manipulations of wing size and shape are often difficult to conduct).…”

“…Also, all macroevolutionary studies of these aerial insectivores so far have supported sexual (rather than viability) selection explanation (e.g., Hasegawa et al . 2016; Hasegawa & Arai 2017, 2018, 2020a,b, 2021, 2022), demanding further experimental studies, possibly using a smaller amount of manipulation (so that overcompensation can be avoided) in many species with various ornamentation (i.e., phylogenetic comparative “experiment”). An alternative approach, the manipulation of compensatory traits (sensu Møller 1996; see also Fig.…”

Maneuverable flight evolved with forked tails and opportunities for extrapair mating in swallows and martins (Aves: Hirundininae)

“…Information on morphology (wing length, as a measure of body size, and relative fork depth), prey size (large or small), and migratory habits (migrant or not) of hirundines were obtained from Turner & Rose (1994), as described before (e.g., Hasegawa & Arai 2020, 2021, 2022; Hasegawa et al 2022). Relative fork depth was calculated as (fork depth)/(central tail feather length), where fork depth is the difference between outermost and central tail feather lengths.…”

“…1, light blue line; Hasegawa 2023). Therefore, these experiments were not conclusive, and the evolutionary force favoring forked tails remains controversial (e.g., Barbosa 1999; Møller & Barbosa 2001; Matyjyasiak et al 2009 for early disputes; Hasegawa & Arai 2020, 2021, 2022 for recent discussions).…”

“…Under this hypothesis, we predicted that flight performance, notably maneuverable flight, should be enhanced by forked tails, at least to some points (see above). Because tail fork depth covaries with indices of sexual selection (Hasegawa & Arai 2021, 2022), we also analyzed these potentially confounding factors. Finally, based on the assumption that high aerodynamic performance improves foraging ability (e.g., Norberg 1994; Rowe et al 2001), we tested whether foraging ability increases with our measures of flight performance.…”