Sex‐ and age‐dependent morphology and selection on wing shape in the barn swallow Hirundo rustica

Abstract

Wings have evolved in phylogenetically distant organisms with morphologies that depend on the combined effects of diverse, potentially contrasting selective forces. In birds, long pointed wings boost speed and energetic efficiency during cruising flight but reduce manoeuvrability. Migratory behavior is believed to lead to the evolution of more pointed wings, but selection on pointedness has never been estimated. Because annual routines of migrants are tightly scheduled, wing pointedness may be selected for because it allows for earlier arrival to the breeding grounds. In long‐distance migratory barn swallows Hirundo rustica we showed that selection via breeding date and thus annual fecundity operates on wing pointedness, but not on other wing traits, among yearling females but not among older females or males. Selection on wing pointedness specifically in yearling females may result from climatic effects, which favour earlier arrival from migration, and from yearling females being the sex‐by‐age class with the latest migration and the smallest wing pointedness. Wing morphology differed between sexes and age classes because of change in size of the outermost but not the innermost wing feathers. Hence, sex‐ and age‐specific selection on wing pointedness operates in a species with sex‐ and age‐dependent variation in phenology and wing morphology.

Wing morphology covaries with migration distance in a highly aerial insectivorous songbird

“…The mean differences in length of these feathers (P9 minus P8) in males and females from the highly migratory Polish population vs. the less migratory Spanish population were (mean ± SE) as follows: males 0.9 ± 0.1 mm, females 0.3 ± 0.2 mm vs. males 0.2 ± 0.2 mm, females ‐0.1 ± 0.2 mm (sample sizes are given in Table 1 ). The differences between the two populations appear small, but in magnitude they correspond to similarly quantified differences in handwing pointedness between sex and age groups of barn swallows (see calculations above and Figure 1 in Saino et al 2017 ).…”

“…The difference between the results of our individual-based study and those of the comparative research by Huber et al (2017) , reporting that highly migratory hirundines have more rounded wingtips compared to more sedentary ones, is remarkable. However, it should be noticed, as Saino et al (2017) pointed out earlier, that in Huber et al (2017) differences between sexes in wingtip morphology were not taken into account and, consequently, the effect of sexual selection on wingtip morphology was not assessed. Male barn swallows have longer and more pointed wings than females ( Saino et al 2017 ).…”

“…However, it should be noticed, as Saino et al (2017) pointed out earlier, that in Huber et al (2017) differences between sexes in wingtip morphology were not taken into account and, consequently, the effect of sexual selection on wingtip morphology was not assessed. Male barn swallows have longer and more pointed wings than females ( Saino et al 2017 ). In addition, the isometric length and pointedness of the wingtip significantly covaries with the length of the tail (this study).…”

“…Even the opposite relationship has been shown—that migratory hirundines possess more rounded handwings than sedentary ones ( Huber et al 2017 ). However, the results of other studies suggest that in the barn swallow Hirundo rustica wing pointedness is favored by selection because birds with pointier wings arrive earlier from spring migration ( Matyjasiak et al 2013 ) and breed earlier ( Saino et al 2017 ), thus achieving greater seasonal reproductive success.…”

Sex and age-specific differences in wing pointedness and wing length in blackcapsSylvia atricapillamigrating through the southern Baltic coast

“…Later migration timing of juveniles relative to adults may be an adaptive strategy to avoid the high costs of fast migration and competition for breeding territories with experienced adults ( McKinnon et al 2014 ). Selection on wing pointedness may also result from climatic effects, which favor earlier arrival from migration ( Saino et al 2017 ), and indeed that may be the case in adult blackcaps. Moreover, we documented a profound difference in wing length between adult male blackcaps and juvenile males and females.…”