Geographical and seasonal variation in the intensity of sexual selection in the barn swallow Hirundo rustica: a meta‐analysis

Abstract

Sexual selection arises from competition among individuals for access to mates, resulting in the evolution of conspicuous sexually selected traits, especially when inter-sexual competition is mediated by mate choice. Different sexual selection regimes may occur among populations/subspecies within the same species. This is particularly the case when mate choice is based on multiple sexually selected traits. However, empirical evidence supporting this hypothesis at the among-populations level is scarce. We conducted a meta-analysis of the intensity of sexual selection on the largest database to date for a single species, the barn swallow (Hirundo rustica), relying on quantitative estimates of sexual selection. The intensity of sexual selection was expressed as the strength (effect size) of the relationships between six plumage ornaments (tail length, tail asymmetry, size of white spots on tail, ventral plumage colour, throat plumage colour and throat patch size) and several fitness proxies related to reproduction, parental care, offspring quality, arrival date from spring migration, and survival. The data were gathered for four geographically separated subspecies (H. r. rustica, H. r. erythrogaster, H. r. gutturalis, H. r. transitiva). The overall mean effect size (Z = 0.214; 95% confidence interval = 0.175-0.254; N = 329) was of intermediate magnitude, with intensity of sexual selection being stronger in males than in females. Effect sizes varied during the breeding cycle, being larger before egg deposition, when competition for access to mates reaches its maximum (i.e. in the promiscuous part of the breeding cycle), and decreasing thereafter. In addition, effect sizes from experiments were not significantly larger than those from correlative studies. Finally, sexual selection on different sexually dimorphic traits varied among subspecies. This last result suggests that morphological divergence among populations has partly arisen from divergent sexual selection, which may eventually lead to speciation.

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

“…As expected, males with high-quality territories paired earlier, indicating female mate choice based on territory quality (Hasegawa et al 2012a; though there may still be some population differences in the relationship between territory quality and reproductive parameters; see above). Although territory quality is often related to male morphological traits (e.g., throat coloration in this species; Hasegawa et al 2014;Wilkins et al 2015), multivariable analysis indicates that territory quality is an important mate choice criteria than male morphology (in fact, effect size, Zr = 0.60, 95% CI = 0.14-1.06 for laying date and 0.62, 95% CI = 0.11-1.12 for pairing date, computed from R 2 value, is quite high for female preference for territory quality, though sample sizes were small (n = 21, 18, respectively); note that mean overall effect size for plumage ornaments was 0.21; Romano et al 2017a). As swallows choose high-quality (i.e., intact) nests within their territories (Hasegawa et al 2012a) or choose colonies/breeding sites with many high-quality nests (Safran 2004(Safran , 2007; also see Ringhofer and Hasegawa 2014), it is not surprising that females choose territories with high-quality nests, though experimental validation remains to be carried out.…”

“…Thus, males can attract females to nest sites by their plumage characteristics, songs, and enticement calls, and thereafter show off nest sites in their territories. These traits would be potentially important targets of sexual selection via female mate choice (i.e., mate choice criteria), compared to male traits that can be assessed only afterwards (e.g., paternal care behaviors), because sexual selection intensity (or strictly, effect size in each selection episode) decreases with the progress of the breeding season (Romano et al 2017a). Female mate choice criteria In barn swallows, many studies have focused on female mate choice.…”

“…In addition to these tail characteristics, plumage coloration would be important. A red throat patch (color : Ninni 2003;size: Møller 1994a;Hasegawa and Arai 2013a), and other ventral plumage feathers (e.g., breast and vent coloration; Safran and McGraw 2004;Safran et al 2005) are related to several indices of female choice (e.g., mating success, paternity; reviewed in Romano et al 2017a). Because all of these ornaments are sexually dimorphic, one may argue that female preference for these traits are as expected (Andersson 1994).…”

“…The target, direction, and intensity of preference varies among populations at least for plumage ornaments (e.g., Safran et al 2016;Wilkins et al 2016; reviewed in Scordato and Safran 2014;Romano et al 2017a), affecting the geographic variation in male trait expression, although most studies have found positive female preference (i.e., toward exaggerated traits; Romano et al 2017a). Well-ornamented males provide less, rather than more, paternal care compared to less-ornamented males in barn swallows (e.g., de Lope and Møller 1993;Saino and Møller 1995;Maguire and Safran 2010;Hasegawa et al 2014;Hasegawa and Arai 2015a), which may be costly for females even if wellornamented males have fewer ectoparasites and thus lower risk of contagion (Møller 1994a).…”

“…Also, because this is a cosmopolitan species distributed across all continents except Antarctica, and exhibits different phenotypes (e.g., some subspecies have relatively short tails with dark rufous-chestnut underparts ;Turner 2006), the barn swallow is a suitable system for studying geographic variation as well as speciation (sensu Scordato and Safran 2014). Until now, several reviews have been published concerning female mate choice and its geographic variation in this species (e.g., Møller 1994a;Turner 2006;Scordato and Safran 2014;Romano et al 2017a). However, all of them are focused solely on secondary sexual characteristics (and mostly on plumage ornaments), simply because other kinds of sexually selected traits are less studied than secondary sexual characteristics.…”