Morphological differentiation in a migratory bird across geographic gradients in mountains of southern Europe

Abstract

Aim In temperate mountain ranges, sharp spatial variations in habitat heterogeneity and climate provide a perfect study setup to assess genetic and phenotypic differentiation in bird populations. In this paper, we analysed morphological divergence patterns across geographical and environmental gradients, in correlation with genetic differentiation and geographical isolation, in the breeding grounds of a long‐distance migratory passerine. Location Northwestern Iberian mountains. Taxon The Iberian bluethroat, Luscina svecica azuricollis. Methods We collected a sample of 625 Iberian bluethroats across their whole breeding range to measure body weight and tarsus length, as well as wing length and pointedness. Morphological differentiation across geographical (latitude and elevation) and environmental (climate and vegetation) gradients was assessed using generalized linear mixed models. Additionally, the role of genetic distance and geographical isolation as drivers of morphological differentiation was evaluated with Mantel tests. Results Bird morphology varied significantly with latitude and elevation, but not with climate or vegetation. In the case of latitude, morphological differences in body size were contrary to Bergmann's rule statements. All biometric and morphometric variables diverged among localities. A similar trend was found for genetic clusters, except for wing shape. Body weight and wing length were both correlated with geographic distance, while only the former varied with genetic differentiation. The greatest genetic and phenotypic differentiation was detected in the southernmost mountain range, that holds the most geographically isolated genetic group. Main conclusion Evidence suggests that the strong morphological differentiation observed in the Iberian breeding bluethroat across geographical gradients results from the combination of unambiguous deterministic forces, such as ecological selection mechanisms, and other factors, such as geographical isolation, that can be either deterministic, stochastic or both, all acting at different scales.

Variation in wing morphology is related to breeding environment in a high‐elevation specialist bird

“…All individuals may indeed be adequately adapted to flight conditions at high elevations, and possible small advantages represented by longer or more pointed wings may be overridden by several more relevant factors during breeding habitat selection. Alternatively, the pattern we observed could be common among birds, as suggested also by some previous studies (Boyce et al 2019, García et al 2021). Further studies considering elevation in combination with other potentially relevant environmental factors are needed to better clarify the relationship between intraspecific variation in bird wing morphology and elevation.…”

“…A detailed investigation of the escape strategies from different predator species in this bird species would be needed to verify this hypothesis (Burns and Ydenberg 2002). Previous studies on passerine birds have alternatively assessed the occurrence (Mérő et al 2020) or the lack (García et al 2021) of association between variation in wing morphology and vegetation structure. While the number of studies about this issue is still reduced to raise any conclusions, our results and the previous findings suggest that such association is probably species‐ (and possibly context‐) dependent.…”

“…Variation in wing morphology also occurs at the intraspecific level, across as well as within populations (Berthold 1996, Arizaga et al 2006, Rolshausen et al 2009, Baldwin et al 2010, Delingat et al 2011, Neto et al 2013). Such variation is often linked with sexual dimorphism (Szekely et al 2007), but can also be related to age (Alatalo et al 1984, Figuerola and Gutiérrez 2000, García et al 2021), to within‐population differences in migration strategy (Rolshausen et al 2009) or to environmental factors such as vegetation structure (Vanhooydonck et al 2009, Saino et al 2017). The relationship between intraspecific variation in wing morphology and environment has been scarcely investigated and is therefore still poorly understood.…”

“…The latter is a pattern regularly observed through mountain chains worldwide (Gentry 1988, Ozenda 1988, Luna‐Vega et al 2001, Zhu et al 2005). In addition, while most authors considered wing size, variation in wing shape across elevational gradients is still almost unexplored (but see García et al 2021). Therefore, the role of elevation in shaping wing morphology is still unclear, and studies along elevational gradients considering both elevation and other environmental factors are needed.…”

Song complexity is associated with habitat quality in an upland passerine

“…We found that song frequency varies significantly among localities and genetic clusters. This result can be partly explained by the covariation of genetic and biometric differentiation in Bluethroats (Turcokov a et al 2010, Garc ıa et al 2021), as has also been observed in other bird species (Ryan & Brenowitz 1985, Martin et al 2011, Illera et al 2014. Allometric relationships represent a mechanistic explanation useful for interpreting the genetics-acoustics association and body size-song frequency relationship that have been reported among and within species in a large number of studies (Ryan & Brenowitz 1985, Podos 2001, Price 2008, although exceptions exist (Patel et al 2010, Illera et al 2014, especially in species with learnt and highly variable songs (Forstmeier et al 2009), as in the case of the Bluethroat.…”