Crows that soar … and paraphyly in ravens

2009 July 7
by David J. Ringer



Visible only as specks to the unaided human eye, Pied Crows (Corvus albus), soar hundreds of feet above busy, crowded Nairobi, rarely needing to flap their black, oar-shaped wings. Their distinctive silhouettes, and perhaps flashes of their white breasts, identify them as they mingle on the thermals with Black Kites and Marabou Storks.

A genetic study published in 2005 found that Pied Crows — a common species throughout much of sub-Saharan Africa — are embedded within the Northern (Common) Raven, Corvus corax (Feldman and Omland 2005). Pied Crows were found to be sister to a ‘Holarctic clade’ of ravens (Eurasia and most of North America), while a genetically distinct raven population centered in California was recovered as sister to the Chihuahuan Raven, Corvus cryptoleucus (map of North American populations). This study backs up an earlier one (Omland et al. 2000) that found similar results but had not included the Pied Crow.

This of course raises the question, if Northern Raven is paraphyletic with respect to Pied Crow, Chihuahuan Raven, and who knows what else (apparently no comprehensive genetic studies of Corvus have been done), should the Holarctic clade and the California clade be split as separate species?

Maybe, but maybe not. The two forms appear to overlap widely in western North America, and whether or not they deserve species status depends on what’s happening in the overlap zone. Are they interbreeding extensively, or are they maintaining separate populations? Nobody knows the answer to that right now.

If they are merging back together after a period of geographic isolation, then, even though other species have split off from the two populations in the intervening time, they will probably still be considered the same species. Paraphyly happens at the species level (and apparently much more often than we realized) — that’s just how it goes. It’s called reticulation, which in visual terms looks less like a tree and more a net — or in mathematical terms, a directed acyclic graph.

In North America, a similar situation exists among several grackle populations. Great-tailed Grackle (Quiscalus mexicanus) is paraphyletic with respect to Boat-tailed Grackle (Quiscalus major) and the extinct Slender-billed Grackle (Quiscalus palustris). The eastern population of Great-tailed Grackle is reproductively isolated from its sister, the Boat-tailed Grackle, where they co-occur along the western Gulf Coast (this co-occurrence is due to a dramatic range expansion of the Great-tailed Grackle in the last century). The eastern and western populations of Great-tailed Grackle have recently met each other again in the southwestern United States, and they do not appear to be reproductively isolated. But again, more study is needed.

So, I find all of that fascinating, but sometimes, it’s enough just look up into the sky and watch the ravens soar:


Related posts:

  1. Island crows
  2. Boat-tailed Grackle: Iris color in Texas
  3. Pipits without a name
  4. Long-lost South Florida photos
  5. Black Kite, Milvus migrans

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