“And the crow once called the raven black.”
— George R. R. Martin
“As the crow flies” is a common expression to refer to the linear distance between two points. When you consider the current distribution of several crow species (family Corvidae), you will notice that they can fly a long way. Indeed, many corvids have wide distributions, sometimes ranging from Western Europe all the way to Eastern Siberia. This makes corvids excellent study systems for phylogeography, the study of the historical processes responsible for the contemporary geographic distributions of individuals. A recent paper in the Russian Journal of Genetics and Breeding nicely reviews the recent work on several members of the Corvidae.
Carrion and Hooded Crow (x2)
Let’s start with a textbook example: the hybrid zone between Western Carrion Crow (Corvus corone) and Hooded Crow (C. cornix). This contact zone runs from Scotland, through Denmark, Germany, the Czech Republic and Hungary, to Italy. Genomic analyses of this species pair revealed that they differed in just a few genomic regions, representing less than 1 percent of the genome. One of these differentiated regions contains contains genes involved in pigmentation and visual perception which might explain why these species mate assortatively (i.e. choose a partner with the same plumage pattern).
Interestingly, a similar hybrid zone occurs in Siberia. Here, Hooded Crow interbreeds with the Eastern Carrion Crow (C. orientalis). Genomic analyses of this hybrid zone showed that different genomic regions were differentiated compared to the European situation. This finding indicates that the genetic basis of speciation is highly species-specific and context-dependent. You can read this blog post for more information about this hybrid zone comparison.
Previous work on the Siberian hybrid zone also revealed that the Collared Crow (C. pectoralis) had the same mitochondrial haplotype as the southeastern Carrion Crow population. Perhaps it acquired this haplotype through hybridization?
An interesting hypothesis concerns the Azure-winged Magpie (Cyanopica cyanus). This beautiful bird has a peculiar distribution: one population occurs on the Iberian Peninsula while the other population is restricted to Eastern Asia. Some researchers argued that Portuguese and Spanish sailor might have transported these birds from Asia during Medieval times. Analyses of the mitochondrial gene cytb showed that these populations diverged more than three million years ago. This finding rejects the hypothesis of human-mediated transport. These birds probably had a wider distribution that was broken up during the Pleistocene ice ages.
The two populations are currently recognized as distinct species: the Asian Azure-winged Magpie (Cyanopica cyanus) and the Iberian Azure-winged Magpie (Cyanopica cooki).
Azure-winged Magpies are not the only corvids that show million-year-old divergences. A comparative study uncovered similar patterns in other species, such as the Rook (Corvus frugilegus), the Jackdaw (Corvus monedula and Corvus dauuricus) and the Eurasian Magpie (Pica pica). All of these corvids show significant divergence between eastern and western populations. Such divergence was absent for other species, including the Raven (Corvus corax), the Siberian Jay (Perisoreus infaustus) and the Nutcracker (Nucifraga caryocatactes). What could explain these patterns? The author of this review – Alexey Kryukov – thinks ecological differences might hold the key:
All species of the first group preferably nest in semi-open habitats and forest edges, while the second group (single-group species) live mainly in forests. The raven (C. corax) is an ubiquist. We arrived at the conclusion that a prominent factor influencing the pattern of genetic differentiation seems to be the preference for either open to semi-open habitats (the west-east pattern) or forest dominated habitats (the single group pattern). Separated refuge areas (western and eastern) during cold periods led to accumulation of diversity.
Kryukov, A. P. (2019). Phylogeography and hybridization of corvid birds in the Palearctic Region. Journal of Genetics and Breeding, 23(2), 232-238.
This paper has been added to the Corvidae page.