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

Abstract

According to classical prediction of aerodynamic theory, birds and other powered fliers that migrate over long distances should have longer and more pointed wings than those that migrate less. However, the association between wing morphology and migratory behavior can be masked by contrasting selective pressures related to foraging behavior, habitat selection and predator avoidance, possibly at the cost of lower flight energetic efficiency. We studied the handwing morphology of Eurasian barn swallows Hirundo rustica from four populations representing a migration distance gradient. This species is an aerial insectivore, so it flies extensively while foraging, and may migrate during the day using a 'fly-and-forage' migration strategy. Prolonged foraging flights may reinforce the effects of migration distance on flight morphology. We found that two wings' aerodynamic properties-isometric handwing length and pointedness, both favoring energetically efficient flight, were more pronounced in barn swallows from populations undertaking longer seasonal migrations compared to less migratory populations. Our result contrast with two recent interspecific comparative studies that either reported no relationship or reported a negative relationship between pointedness and the degree of migratory behavior in hirundines. Our results may thus contribute to confirming the universality of the rule that longer migrations are associated with more pointed wings.

Keywords: Hirundo rustica; flight morphology; geographical differences; migration syndrome; population; wing shape.

Figure 1.

Eurasian barn swallow Hirundo rustica populations investigated in the present study. Circles represent the breeding colonies where individual morphological variables were measured. Breeding colonies in Europe belong to the subspecies H. r. rustica (migratory), whereas the breeding colony in Israel belongs to the subspecies H. r. transitiva (resident). Filled diamonds represent wintering centroids of 35, 32 and 12 individuals tagged with light-level geolocators in the Spanish, Italian and Lithuanian-Polish breeding populations (Seifert et al. 2018; Briedis et al. 2018; López-Calderón et al. 2021; Matyjasiak P, López-Calderon C, Ambrosini R, et al., unpublished). The two squares in Lithuania indicate the specific breeding colonies in which Northern European barn swallows were tagged with geolocators (we assume that migratory behavior of such individuals is the same as these from Poland). We estimated wintering areas for each breeding population as 0.5 kernel utilization distributions with R package adehabitatHR (Calenge 2006). In order to avoid replication, we only included one wintering centroid per individual. To increase sample size for the Lithuanian-Polish breeding population, we pooled together the 12 geolocator-tagged swallows with nine winter recoveries of barn swallows ringed in Poland. The latter nine ringing recoveries are denoted as open diamonds. The color of diamonds and kernel utilization distributions correspond to the color of breeding colonies.

Figure 2.

Representations of pointed (A) and rounded (B) wingtips in male and female Eurasian barn swallows. Arrows highlight the differences in relative length of the two outermost primary wing feathers, which mostly contribute to differences in wing pointedness among individuals.

Figure 3.

Variation in (A) isometric wingtip length (C₁ component) and (B) wingtip pointedness (C₂ component) in females (filled circles) and males (open circles) across four Western Palearctic populations of the Eurasian barn swallow (from Israel, Spain, Italy and Poland), representing increasing gradient of seasonal migration distance. The Israeli population is sedentary. Bars represent confidence intervals around the mean.