As my wife and I emerged from the woods a few evenings ago, I noticed a patch of blue on the trail ahead. In fact, I almost stepped on it.
It was a pure blue robin’s egg in seemingly pristine condition. We immediately scanned the branches over ahead, but could not find a nest. So we searched the ground. I thought perhaps wind from a recent storm had blown down a nest from an overhead crotch. Nothing.
Then I picked up the egg and examined it more closely. It was perfect, except for a small puncture on the side. It was as if someone had plunged an ice pick through the shell.
This was the work of a house wren. House wrens are extremely territorial and aggressive. While most birds limit their aggression to members of their own species, house wrens are interspecifically antisocial. Within their small territory of an acre or so, they don’t like any other birds nesting nearby.
When house wrens find a nest in another cavity near their own, they often enter the cavity while it’s unoccupied and puncture the eggs. This kills the developing embryos and forces the “intruders” to nest again elsewhere.
Sometimes they then remove the egg from of the nest and drop it some distance away. House wrens usually confine this nasty behavior to other cavity nesters. It’s a way to “own” all the cavities within their territory. But sometimes they even puncture eggs in nests of open, cup nesting species. In addition to the robin egg I described earlier, I’ve seen eggs of phoebes, towhees, cardinals and song sparrows similarly destroyed.
I don’t know that other species avoid building nests near active house wren cavities, but they should. I suspect many people would agree with my wife’s opinion of house wren’s aggressive territorial behavior. She finds it “despicable.”
What house wrens do
But it is neither good nor bad. It’s just what house wrens do. It is just one of many tactics they use to gain an advantage in the intense competition for the relatively few nesting cavities available.
From a biological standpoint, I admire their ability to confront and frustrate larger competitors. House wrens are the most widespread of the nine species of wrens that inhabit the United States. They live in brushy old fields, shrubby backyards, and forest edges. Often they are the first occupants of a backyard bird house.
(If house wrens occupy nest boxes intended for bluebirds, the boxes are probably too close to dense vegetation. Remember, bluebirds prefer more open habitat.)
Drab and brown, house wrens are nondescript, but their explosive, bubbly song is distinctive, especially when sung at 5:30 a.m. outside the bedroom window.
House wrens usually attempt their first nest in early May, shortly after arriving in the spring. And nesting continues well into August. I’ve found active nests as late as Aug. 26.
House wren nests are easy to recognize in a nest box. At first it seems the nest is simply a jumble of small twigs. But deep in a back corner rests a tiny cup lined with rootlets, grass, leaves, animal fur, and a feather or two.
Here the female lays five to eight eggs and incubates them for 13 days. After the eggs hatch, both parents feed the nestlings for about 17 days until they are ready to fledge.
Two broods per season are typical, and some females raise three. Though house wrens are common and widespread, they are sometimes confused with Carolina wrens. House wrens are small, drab, explosive singers, and they arrive in early May. Carolina wrens are bigger, light below, chestnut brown above, have a prominent white eyeline, sing “Tea-kettle, tea-kettle, tea-kettle!” and do not migrate.
And speaking of Carolina wrens, mine have disappeared. After about 10 years of increasing numbers, my Carolina wrens vanished in April.
In early April, I had three singing males around the yard. By the end of the month, they were all gone, and I haven’t found a single Carolina wren nest this year. Perhaps predators have had a good year and taken them all, but I wonder if any readers have observed a similar disappearance of Carolina wrens this year.
If you have, please let me know.
Send questions and comments to Dr. Scott Shalaway, RD 5, Cameron, WV, 26033 or by email via his website at http://scottshalaway.googlepages.com