The sympatric breeding of Common and Scottish Crossbills Loxia curvirostra and L. scotica and the evolution of crossbills

Abstract

Within the strongly polytypic Red Crossbill L. curvirostra there are many reports of two or more ‘subspecies’ nesting sympatrically, without interbreeding. This 13‐year study examines one such case, in Scotland, where an endemic form is resident and another was thought to occur after irruptions from its main range in continental Europe. Both forms were present in the study area every year; sympatric breeding was proved in 9 years and probably occurred in the other four. There was no suggestion of interbreeding and the Scottish form should be treated as a separate species, L. scotica, the Scottish Crossbill. This is the only endemic species of bird in Britain and one of very few European endemics. The evolution of crossbills in Europe is discussed in the light of recent palynological evidence and the taxonomic status of the Parrot Crossbill L. pytyopsittacus is re‐examined.

Call Imitation and Call Modification in Red Crossbills

“…In Europe, evidence for reproductive isolation of call types or even recognized species (i.e., L. pytyopsittacus, L. scotica, and L. curvirostra) has also remained scarce (e.g., Knox 1990, Robb 2000, Edelaar et al 2004) until recently (Summers et al 2007). The paucity of such data arises for two reasons.…”

“…The different call types maintain morphological and vocal differences despite widespread sympatry, suggesting that they are reproductively isolated (Groth 1993a; see Knox [1990] for a similar argument for recognizing the Scottish Crossbill [L. scotica] as a distinct biological species). Nevertheless, direct evidence of reproductive isolation and assortative mating among the different call types has remained elusive (e.g., Groth 1993b).…”

Assortative flocking in crossbills and implications for ecological speciation

“…For example, the coefficients of variation of bill depth for call types 1 and 2 [33; see also 20] are less than those for five of six species of finch-like Hawaiian honeycreepers (Drepanididae) [32], a group that shared a common ancestor with the cardueline finches ca 6 Ma [34] and a group in which hybridization is probably rare [32]. The key issue therefore is to understand how high levels of reproductive isolation [12,21,23,35,36] have evolved, especially when sympatric forms overlap extensively in size and are so similar in appearance.…”

Assortative mating also indicates that common crossbill Loxia curvirostra vocal types are species

“…Generalising and extrapolating across these and other, more anecdotal studies (refs. in Adkisson 1996, Groth 1993a, Knox 1990, 1992, most of these vocal types roam over vast areas (sometimes spanning the width of the continent) in search of crops of conifer cones that fit their bill morphologies. Doing so, they regularly occur in sympatry (refs.…”

“…in Knox 1990, 1992, Groth 1993a, Adkisson 1996. For at least some of these types it has been established that interbreeding during sympatry is rare (reviewed in : Knox 1990, 1992, Groth 1993a, b, Smith and Benkman 2007, and that females prefer males of the same call type even when they are identical in plumage and bill size (Snowberg and Benkman 2007). Hence, even though many details are absent and much data still needs to be collected, at this moment the most parsimonious interpretation of all findings is that these vocal types represent evolutionarily separated lineages.…”