Natural Perspectives: Mr. and Mrs. Wren find home in backyard

The big excitement in our yard is that a pair of house wrens has set up house in our bluebird nest box. Vic and I got the nest box from Friends of Shipley Nature Center last year, but I put it up too late in the season to attract any bluebirds. A male bluebird examined it several times over the summer, but no female joined him.

I was hoping that the male bluebird would come back this spring and find a mate. Sorry, bluebird, the house wrens beat you to the box. Or did they?

House wren males are known to build dummy nests, sometimes several of them. Our nest box now holds a loose arrangement of twigs that pretty much fills the box. I peeked in the box at night, and there doesn't seem to be any fine material that would line a functioning nest. So I'm not sure that what we have really qualifies as a true nest. It may simply be a dummy nest.

Although two wrens were going in and out of the nest box regularly late last week, the female may have rejected it as an actual nesting site. I haven't seen much activity at the nest box since the big storm this weekend.

Both male and female house wrens work on building the actual nest that is used for eggs, but only the female incubates the eggs. Incubation is hardly an all-day task. Eggs are more sensitive to heat than to cold, so a female wren may sit on the eggs for only a few minutes at a time. The amount of time that she spends incubating depends on the air temperature. For example, studies have shown that the female will sit on the eggs for an average of 14 minutes when the temperature is 59 degrees. But she sits on the eggs for only seven minutes if the air temperature is 86 degrees.

Birds build nests not only to keep the eggs from rolling around, but to help insulate the eggs from variations in temperature. Many bird species have receptors in the skin of their brood patches that help them to sense the temperature of the eggs. To help distribute the heat more evenly, birds turn their eggs frequently. This also helps prevent embryonic membranes from sticking to the shell.

Some bird species use other approaches to maintain uniform egg temperature. Ducks line their nests with copious amounts of down (fine feathers) that they pull from their breasts. They will pull some of the down over the eggs when they leave the nest. This can help keep heat in or prevent overheating if the eggs are exposed to sun.

Killdeer and some other shorebirds soak the feathers on their bellies to dampen the eggs. This can help cool the eggs by evaporation. While this adaptation to extreme heat may not be terribly important at coastal locations like Bolsa Chica, those birds nesting at the Salton Sea have to use such cooling strategies. Temperatures there often soar to 120 degrees in the summer.

Some bird species mate for life. Others mate only for the season. Most species form pair bonds, but house wrens are among those species where the male may have more than one mate in a given season. Female house wrens tend to choose males with superior territories even if that male is already mated to another female. They will reject bachelor house wrens with poor territories in favor of mated males with good territories. Since the male doesn't help with egg incubation, that works out fine for the wrens. Well, except for the poor bachelor male who ended up with the poor territory and no mate at all.

Naturally, we like to think that our yard is a superior territory. We have an organic yard so all of the insects, worms, caterpillars and spiders are safe for the birds to eat. We have shrubs and fruit trees around the periphery of the yard for cover, a nice pond in front and birdbaths front and back. The house wrens should find everything that they want in our yard, including no outdoor cats. We're happy that all cat owners in our neighborhood keep their cats indoors. With coyotes wandering about, it's the best thing for the cats as well.

We keep several bird feeders filled with sunflower seeds for the house finches, but the house wrens ignore those. We also have trays of mixed seed for mourning doves and house sparrows. Again, the house wrens have no interest. They are meat eaters and love millipedes, spiders and snails.

Fortunately for us, we don't have any snails. Our yard is planted with drought-tolerant plants, and there just isn't enough water in summer to keep the snails happy. They died out long ago. Maybe the wrens can find some snails in our neighbors' yards. But if they want bugs, they've come to the right yard. We have plenty of nice, juicy bugs.

Male house wrens tend to remain loyal to their chosen nesting territories, returning year after year if they were successful there. Since they are not involved in the rearing of the young, I guess they mark success by whether or not they mated with a female or females. This loyalty to a nesting location is called site tenacity.

An advantage for birds returning to the same site is that they know where to find food and how to escape predators at that location. Birds look for physical stability of the site, protection from predators and harsh weather, and a readily available food supply. We're hoping that Mr. and Mrs. Wren find all of that in our yard. We'll let you know.

VIC LEIPZIG and LOU MURRAY are Huntington Beach residents and environmentalists. They can be reached at

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