The Uncommon Raven of the Northern Canadian Shield Taiga

I’ve just made my first trip to the Northern Canadian Shield Taiga Ecoregion (NA0612), specifically a winter visit to the capital city of Yellowknife, Northwest Territories. France and I were searching the skies for the famed Aurora borealis and on our last day we did indeed see a great show, standing on a frozen lake at -30°C, with a Great Horned Owl hooting nearby.

The hard-core denizens of an ecoregion are those that are the year-round residents. In this area, bitterly and unreasonably cold in winter by our southern standards, there are few bird species that make the cut. In fact, for our first several days in Yellowknife, the ONLY bird to be seen was the Common Raven, abundant all over town. Later we added a handful of other species (House Sparrow, Black-billed Magpie, Willow Ptarmigan, Hairy Woodpecker, Hoary Redpoll, Great Horned Owl).

This blog must therefore be a tribute to the amazing Common Raven, Corvus corax. It is often overlooked, but this is a bird of superlatives and interesting characteristics:

– it is the largest passerine (perching bird) in the world, the Passeriformes being the order that constitutes about half of all birds
– it is one of the most widespread species in the world, occurring from the furthest reaches of arctic Canada south to Nicaragua and is also widespread in Europe, Asia, and northern Africa
– it thrives in almost every imaginable habitat (deserts to tundra to dense urban environments), except tropical rainforests

Having been unable to survive outside here for more than an hour (fully wrapped in the best goose down coat available!), I’m particularly fascinated by their miraculous temperature regulation abilities. Ravens seem to do just fine swooping around in temperatures down to -50°C, because of a high metabolic rate. But they also manage in desert temperatures up to 45°C, thanks to an ability to shed a lot of heat through the bill, eyes, and legs. The latter heat shedding must somehow be strictly controlled in the winter — I’m not sure how.

The raven evolved primarily as a scavenger, tagging along beside the top carnivores of each ecoregion where it is found. Perhaps not surprising that it has adapted so well to living alongside humans and speeding vehicles, between them now the ultimate top carnivores and sources of scavengable messes. The raven is however not just a carrion eater – it is more correctly considered a highly adaptable omnivore, and can hunt small mammals and birds and also consumes invertebrates, seeds, and fruits. 

Just like people, through DNA testing, are finding embarrassing surprises in their family trees, so it is with birds (and every other organism for that matter). It turns out that Common Ravens of the southwest US (called the “California clade”) are genetically quite distinct from all other ravens (the “Holarctic clade”) of North America and the Old World. Further, the “Holarctic clade” ravens, which include those of Yellowknife, have a greater genetic similarity to the Pied Crow of Africa, than they do to some ravens of California! I featured the Pied Crow in an earlier Lake Tanganyika blog from an African ecoregion. (Now that I look at my photo of the “crow”, I see it really does look like our raven, except for the band of white on the nape.)

What’s going on? 

Ravens evolved in the Old World. About 2 million years ago, some of them crossed over into North America and since then, the isolated New World population evolved and are now smaller and thinner-billed. Perhaps at one time they were widespread in North America but glaciation likely forced them into refugia in what is now the western US. Let us call them “California Ravens”. A further side branch about a million years ago from the “California Raven” resulted in the Chihuahan Raven, Corvus cryptoleucus, now considered a full species, characteristic of the Chihuahua Desert Ecoregion. Shortly after that invasion of ravens into the New World, the Pied Crow Corvus alba split off from the Eurasian ravens, and spread into almost all of Africa.

Much more recently, perhaps about 15,000 years ago, ravens spread a second time into North America from Asia, crossing over the Bering land bridge. These ravens crossed over into the New World at about the same time and place as the first Amerindians did – no wonder so many First Nations tribes have a strong mystical and cultural attachment to these enchanting birds which have been their companions since time immemorial, and on their greatest migrations. This new population of North American ravens has been quite successful and has spread out to occupy virtually all of the species’  current range in North and Central America. 

It might have been expected that when they reached the areas occupied by the “California Raven”, who had been living in isolation from other ravens for about 2 million years, they would be sufficiently distinct that they would not interbreed. Instead, they still basically looked the same; they were able to overlook their differences, and just get on with breeding. The two groups of ravens in North America are therefore actively merging and we consider them now all part of a single species. Its doubtful that any pure “California Ravens” still exist, but some populations in the southwestern US, notably those of the Mojave Desert, are mostly derived from the original immigrants of 2 million years ago and are genetically quite distinct from all other ravens, as noted above. This also explains why Yellowknife ravens, fairly pure representatives of the “second wave”, have a genome that is closer to the Pied Crow of Africa than to some “California ravens”, which are the oldest offshoot from the original raven stock.

Taxonomy can be pretty messy, and fun…

The next time we are back in this area, to try again to see Naka, the Dene name for the Northern Lights, I’m going to bring my photos and videos of Pied Crows from Africa and see if I can elicit some tender emotions in the local ravens!