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A tale of 2 wrens

Bewick’s wrens inhabit areas around Bear Creek all winter. [Allaboutbirds.org photo]

It’s the third week of August, and much of the bird world is on the move.

Migration of our backyard and forest birds kicked into high gear this week ramping up from a trickle that started this month. Tanagers, orioles, black-headed grosbeaks, vireos and hosts of warblers are about in profusion, usually in flocks that spend a couple of days in one place before a jump of hundred miles or more to the next layover.

This all happens in near silence. Just the occasional call note draws attention to these ambitious travelers.

Not all our birds depart for points south. A bike ride along the Greenway likely will find the silence of August punctuated by the singing of Bewick’s wrens. Every mile or so, one is likely to encounter the enthusiastic song of another individual filtering up from the tangle of blackberries.

Mexico is not for these birds. They are content to cruise their small section of Bear Creek all winter.

This contrasts with another wren, the house wren. They have no intention of remaining in the valley. On undersized wings but with great heart, they challenge mountains and deserts for milder climes.

I have encountered them at 6,200 feet among a thicket of chinkapin. Not the bird I expected to see at this altitude. It was a short refueling stop on a several-week adventure.

Why the difference in these two species that are so similar?

They both have long, thin bills used to extract insects and spiders from crevices. They both forage low in dense vegetation and often on the ground. Both nest and roost in cavities or nest boxes. And yet, one species finds it in its best interest to remain and tough out the winter while the other tackles the rigors of migration.

The Bewick’s wren remains to eke out a meager existence of limited food and sometimes bitterly cold weather. Tiny birds have an especially hard time finding enough resources to sustain their small body. The smaller the organism, the more difficult it is to retain body heat.

But the familiarity of refuges and potential hazards by remaining in place is a plus. Another plus is that the male already owns a property and does not need to seek out and fight for a new territory the following spring.

On the other hand, the house wren strikes out on an odyssey that often exceeds 1,000 miles and is fraught with the hazards of unanticipated predators, windows and the task of finding proper habitat and food in unfamiliar stopovers. Mortality rates in migration for small birds have been measured at 20 times the rate of remaining in place. But the weather will be warmer and food more plentiful if they can reach their winter home.

Such is the calculus that goes into the strategy practiced by each species. For one, the balance sheet says stay. For the other, the numbers indicate a trip to sunny Mexico.

I wish all well, but I am grateful the math works out that a few species remain to enrich the valley with song in several otherwise very quiet months.

Stewart Janes is a retired biology professor at Southern Oregon University. He can be reached at janes@sou.edu.