On the relative importance of ecology and geographic isolation as drivers for differentiation of call types of red crossbill Loxia curvirostra in the Palearctic

Abstract

Efforts to understand the process of speciation have been central to the research of biologists since the origin of evolutionary biology as a discipline. While it is well established that geographic isolation has played a key role in many speciation events, particularly in birds, there is ongoing debate about how frequent speciation is in the partial or complete absence of geographical isolation. In the red crossbill Loxia curvirostra, good arguments do exist for sympatric speciation processes. In this species, several classes of calls are clustered in distinct groups, so-called 'call types', which mate assortatively. Often, several call types can be found at a single site, breeding and feeding next to each other. It has been hypothesized that red crossbill call types evolved by specialising in extracting seeds from cones of different conifer species. Alternatively, call types might have evolved in temporal geographic isolation. Within Europe, little is known about the distribution of the various call types and preferences for distinct food resources. In this study, we analysed the temporal and spatial occurrence of red crossbill call types in the Palearctic, investigated potential reasons for call-type composition at a site, and compared the occurrence of call types with the fructification of conifers. Call-type composition changed with site and season but hardly with conifer species. With our data, we could localise range areas of twelve different call types, which cannot be explained by conifer species occurrence. Therefore, we suggest that call types evolved in parapatry in most of the northern Palearctic region, and, although contradictory results exist from Iberia, we argue that differentiation might be driven by the same drivers there as well. Additionally, we discuss the potential influence of anthropogenic changes of forest composition and distribution on call types, which offers a unique possibility for future studies.

Fast cultural evolution of Crossbill (Loxia spp.) calls in the Palaearctic

“…Although, during invasions, groups of these call types are scattered over almost the entire Palaearctic region, sometimes segregated from other groups (comparable to small fragmented populations), this situation usually lasts for only 1 year or a few years. Afterwards, most or all individuals leave the invasion area and may reunite in the original area (Newton 2006, Martin et al 2020.…”

“…For the study, we analysed 11 call types across four species of crossbills (Red Crossbill: N01, N03, N04, N06, N07, N08, N10, N11; Two-barred Crossbill: N19; Scottish Crossbill: N20; Parrot Crossbill: N21), for which we could cover a large temporal and spatial variation of the data. We specifically focused our analysis on Britain and Ireland and therefore on call type N06, which is restricted to the northern part of this region (Martin et al 2020;Figs S1 and S2) Anderson & Taylor 1967, Davies 1979, Woodland Trust 2020 and increased especially in the 1950s (Warren 2009). Breeding of Red Crossbills is presumed to have been rare in the 19th century (Holloway 1996), but numbers increased at the beginning of the 20th century, especially after the 1909/10 irruption.…”

“…Despite the similarity of the FCs of N06 and N02 (cf. Martin et al 2019), we excluded the latter from the evaluation because the ECs differed, FCs differed on average as well, and spatial distribution did not overlap (N02 is distributed especially on the Balkan Peninsula and rarely in central Europe; Martin et al 2020). In the New Forest (Hampshire, UK), we found a population with calls similar but not identical to N06.…”

“…The ability or willingness to learn is reduced in adult Red Crossbills (Sewall 2009), but they can match their mates' FCs during mating (Groth 1993b, Keenan & Benkman 2008). In the Palaearctic, call types are linked to a home region, a so-called 'core breeding area' (Martin et al 2020), a region where they are typically found. Core breeding areas of different call types do not overlap.…”

“…Additionally, the data are spatially well distributed, which is important for studying geographical variation. Secondly, only a few other call types of Red Crossbill cross the English Channel and the North Sea regularly (Martin et al 2020), which reduces the potential impact of other invading call types on local call type N06. Finally, as the Red Crossbill has only been resident in Britain and Ireland since the late 19th century (Holloway 1996), the recent colonization presents a unique opportunity to analyse potential ongoing differentiating systems in calls and identify the potential drivers of call variation.…”