North African (or Atlas Crossbill) and Balearic crossbills are the most genetically divergent among all Common (Red) Crossbill subspecies in the Western Palearctic according to a new study. This divergence is even greater than between the Common and Parrot crossbills.
Parmi toutes les sous-espèces du Bec-croisé des sapins (Loxia curvirostra) dans le Paléarctique occidental, le Bec-croisé du Maghreb (ssp. poliogyna) et des îles Baléares (ssp. balearica) sont les plus différenciés génétiquement. Cette divergence génétique est encore plus grande que celle entre le Bec-croisé des sapins et le Bec-croisé perroquet (Loxia pytyopsittacus).
The distinctiveness of the North African and some Mediterranean subspecies of the Common Crossbill (Loxia curvirostra) is already known from some old studies. For example, Bruno Massa concluded his article about the variations in Mediterranean crossbills published in 1987 by the following: “the process of speciation is advanced in the populations in Cyprus, Mallorca and North Africa, but less so in Calabria, Sicily and Corsica”.
However, before the recent study of Parchman et al. (2018) reported here, there were no appropriate genetic studies that could have tested this phenotypic distinctiveness. The only exception is the study of Björklund et al. (2013) that showed that the Balearic subspecies (balearica) is also genetically different from the Iberian and North European subspecies. Unfortunately however, this study did not include samples from North Africa or from other Mediterranean islands.
Food stability, sedentariness and crossbill evolution
Research in North America has shown that a type of crossbill in southern Idaho has diverged and became reproductively isolated from the other Common Crossbills occurring in the same region mainly because it relied on a stable food resource: the cones of lodgepole pine (Pinus contorta latifolia). This population of crossbill, known as the South Hills Crossbill, became sedentary. Also important in this context, this crossbill did not share the stable food resource with the red squirrel (a competitor for pine seeds) which is absent from the area. Essentially, the South Hills Crossbill co-evolved alone with the lodgepole pine in that area of southern Idaho.
Years of research led finally to the recognition of the South Hills Crossbill as a new species under the new name of Cassia Crossbill (Loxia sinesciuris). The new species has been accepted by both the American Ornithological Society and IOC Checklist. So we have a new species that evolved in a very short period of time, evolutionary speaking, which is about 6000 years. Importantly also, this happened without any geographical isolation (which is known to aid speciation). For more details see these papers: Benkman et al. (2009) and Parchman et al. (2016). The main keywords from here are “sedentariness” and “resource stability”, which will be useful below.
One may ask, what is the relationship of all this development in North America with crossbills in the Western Palearctic? Well, the same team involved with the Cassia Crossbill has teamed with European researchers to found out if the association between resource stability and greater genetic divergence documented in North American crossbills is also found in Western Palearctic crossbills. And sure they did found just that (however, this time geographical isolation was also involved, alongside food stability).
Crossbills associated with Aleppo pine are the most divergent
The new study by Parchman and his coauthors (2018) found that the crossbills associated with Aleppo pine (Pinus halepensis) – which provide a stable food resource – are the most genetically divergent among all the Western Palearctic crossbills included in the study. These are the North African crossbill which is also known as the Atlas Crossbill (L. c. poliogyna) and the Balearic Crossbill (L. c. balearica). These two populations are resident, and apart from their association with the Aleppo pine, they are also geographically isolated from the other subspecies which “has undoubtedly contributed to their genetic divergence”.
The subspecies L. c. hispana is also associated with Aleppo pine (stable food). And sure enough, it’s also genetically differentiated from the northern populations (from the Pyrenees and Scotland). However, this differentiation is not significant as in the case of the North African and Balearic crossbills. Unlike North Africa and the Balearic Islands, southeastern Spain where hispana breeds is not geographically isolated (so it can easily be reached by nomadic curvirostra). This could explain the lower levels of genetic divergence of hispana compared to balearica and poliogyna.
I think it won’t take long to add the Atlas Crossbill to the list of North African endemic birds. Likewise, the Balearic Crossbill can be added to the islands’ endemic species.
What about the vocal types of the Common Crossbill?
In short, there is no – or very little – genetic differentiation between the different vocal types within curvirostra. The stability of food resources and “lifestyle” (resident or not) are at play here as well. All these crossbills occur in regions with less stable food resources, and most of them are nomadic (in fact, all populations except one are nomadic).
Genetically, Parrot Crossbill is not divergent
The study results showed that the Parrot Crossbill did not form a single monophyletic cluster, but instead the six individuals included in the study were scattered within curvirostra from the Pyrenees and Scotland (see figure 3 reproduced below). However, after excluding the strongly divergent North African and Balearic subspecies, the authors managed to detect very slight genetic divergence between curvirostra and Parrot Crossbill.
The marked morphological and vocal divergence of the Parrot Crossbill and the strong assortative mating (animals prefer to mate with animals with similar phenotype, thus reinforcing the original distinctiveness) are probably enough to keep this species as it is.
Based on these studies, I feel a split or two are coming soon. So, get ready to add at least one, may be two, new crossbill species to your list.
Benkman, C.W., Smith, J.W., Keenan, P.C., Parchman, T.L. & Santisteban, L. 2009. A new species of the red crossbill (Fringillidae: Loxia) from Idaho. Condor 111: 169–176. doi: 10.1525/cond.2009.080042
Massa, B. 1987. Variations in Mediterranean crossbills Loxia curvirostra. Bull. Br. Ornithol. Club 107: 118–29.
Parchman, T.L., Buerkle, C.A., Soria-Carrasco, V. & Benkman, C.W. 2016. Genome divergence and diversification within a geographic mosaic of coevolution. Molecular Ecology 25: 5705–5718. doi: 10.1111/mec.13825
Parchman, T.L., Edelaar, P., Uckele, K., Mezquida, E.T., Alonso, D., Jahner, J.P., Summers, R.W. & Benkman, C.W. 2018. Resource stability and geographic isolation associated with genome divergence in western Palearctic crossbills. Journal of Evolutionary Biology 31: 1715-1731. doi: 10.1111/jeb.13367