Of the following species, which is the most successful: American crow, American robin or house wren?
The question is really unfair without defining what I mean by “successful.” I mean which has the broadest distribution and lives in the widest variety of habitats?
The clear loser is the American crow. Its range barely extends beyond the United States and southern Canada, and there are vast portions of the West where they do not occur.
American robins are clearly more successful. You can find them from Nome to Mexico City, and they can be found in almost every habitat, which in Oregon includes juniper country, the rim of Crater Lake, coastal dunes and almost every place in between.
The winner, in my contest (and with my rules) is the house wren. You can find them almost anywhere you go from Canada south for thousands of miles until you run out of land. They are in the oaks on the Table Rocks. They are in the clear cuts of Buck Prairie. They are in my backyard. They are in the mountains of Arizona. They inhabit Costa Rica, Trinidad, Peru and most land south to Tierra del Fuego. They frequent the margins of the tropical rainforests up into the high Andes above 15,000 feet. It’s almost easier to describe where they aren’t found.
OK, some authorities have split the house wren into two species, the house wren and the southern house wren. But not all agree, and I can’t see (or hear) any difference.
What makes this wren so successful? What makes any species successful? Is it a better strategy to be a specialist or a generalist? Is it better to have great powers of dispersal or to remain close to home?
To be a specialist is a good thing. If you can do something better than any other organism, life is good ... until that one thing changes. And nature is always changing. Ask the ivory-billed woodpecker that depended on old-growth forests along the rivers in the southeast or the Kirtland’s warbler in Michigan that depends on fire to produce young jack pine with limbs that reach the ground to protect their nests.
Looking at the house wren, there appears to be little remarkable about this tiny ball of fluff. There is nothing to suggest that it has any special talent except maybe a tendency to sing endlessly (and loudly). Are they food or foraging specialists? I don’t see it. They forage in trees and shrubs, but more often you will see them on the ground scurrying around in the tangle searching for spiders and insects.
As far as powers of dispersal, they appear to have none. And yet they do. House wrens in Oregon migrate. This amazes me. They are hardly a vision of elegance on the rare occasion one attempts to fly across my yard. Yet, our birds retire to Southern California or Mexico for the winter. And they have even managed to fly across 300 miles of ocean to colonize the Falkland Islands (Malvinas if you prefer). Impossible! Or so it would seem. I’m certain this is part of the answer, but still.
I just have to add the success of house wrens to my long list of avian mysteries for which I have no answer.
Stewart Janes is a biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.