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

March 23, 2011|By Vic Leipzig and Lou Murray

The twig was a bit too large to fit into the round opening,… (Lou Murray, HB Independent )

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.

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