One of those delightful chores that signals the beginning of spring is the task of building nest boxes for the birds in my yard. For the past 21 years, I have deployed several nest boxes in the hopes of attracting bluebirds to my yard. This works because I live in an open area with a lot of field and lawn, where the bluebirds feel most comfortable. I generally think of these man-made homes as “bluebird” boxes, but in fact there are four species that will regularly move in and start nesting. More on that in a bit.
I always deploy my nest boxes in pairs because of the well-known relationship between Eastern bluebirds and tree swallows. Both species will readily move into a nest box, and they will aggressively defend the airspace around their homes. However, they are relatively tolerant of one another, which means a pair of boxes will attract both species at the same time. The bluebirds will harass intruders but leave the swallows alone. The swallows will also go after any perceived threat while ignoring the bluebirds. In this way, nesting together gives each species an extra pair of defenders. Win-win!
A complication arises when House Wrens get involved. Tiny as they may be, house wrens can be very aggressive, and they also have the habit of building multiple nests in a particular area. An enterprising male can have several nests in place before the female makes her choice, which can eat up a lot of real estate. The property manager (that’s you) needs to keep an eye on things and take steps to avoid conflict whenever possible. Why have one pair of wrens when you can have wrens, swallows and bluebirds at the same time?
This very situation evolved in my own yard, and all I had to do was put up a few extra boxes to keep the wrens happy. These new boxes were placed on the trunks of trees at the edge of the field, while the “bluebird” boxes remained out in the open. Wrens will rarely use boxes that are more than 100 feet from trees and dense cover, so if you give them options that they prefer, they will leave the other birds alone. I also changed the design for the new wren boxes by decreasing the diameter of the entrance down to just one inch. This keeps any other species from using the boxes — especially house sparrows, which is a vital safety measure.
House wrens have a particular nest-building model that makes their nests easy to identify. Bluebirds use pine needles and dry grasses to construct a shallow cup at the bottom of a nestbox. Into their nests, the females will lay four to five blue eggs. Tree swallows will use dry grasses to build a foundation and then they will add a flourish of feathers. Female tree swallows will then lay four to five pure white eggs.
Wrens, on the other hand, prefer to build a more substantial and bulkier nest out of small twigs. The nest may actually fill the entire interior of the box, leaving only a narrow tunnel through the twigs to reach the grass-lined cup where the eggs are laid. The nest may then be lined with almost anything soft and the female can lay six to eight tiny brown eggs. If you open up a nest box and discover that is packed to the ceiling with small twigs, then you have a wren nest.
This makes nest identification quite easy, but monitoring the nest becomes a challenge. It can be quite difficult (entirely on purpose) to see inside the nest to check on the status of eggs and nestlings. So, this year I tried out a new design feature to see if I could fix that problem. The roof of my new wren box is held in place by a single screw and a single nail.
The screw is in the front left corner of the roof and provides enough security to keep the roof anchored to the rest of the box. I would normally use four screws (one in each corner) when building a bluebird box, but that is what makes wren nests so difficult to monitor. A single screw in the front left corner allows the entire roof to pivot, with the screw serving as an axle. To prevent this from happening accidentally, I added an additional pre-drilled hole in the back left corner through which I could introduce a nail. Fitting loosely in the pre-drilled hole, the nail is easy for a human to remove, but not easy for a Blue Jay or a squirrel. All you have to do is remove the nail, swing the roof out of the way and peer inside the nest to see what’s going on.
I checked the wren nest that was built inside the newest box, and the results were tremendous. The little male had built a somewhat conservative nest with a foundation of small twigs. He had then added grasses, pine needles, feathers and even what looks like some squirrel fur. Then, the healthy female had added the final touch of seven tiny brown eggs. Success!
If you would like to try to build a wren box, then all you have to do is visit the Readers Corner page of my website. There you will find information on identifying all sorts of nests and eggs as well as plans for building my new wren box. Working with wood to build a home for a small creature that lives in your yard can be a very relaxing and rewarding way to spend your time. This is especially true if a little tenant moves in quickly and starts a little family. Give it a try!
Bill Danielson has been a professional writer and nature photographer for 28 years. He has worked for the National Park Service, the US Forest Service, the Nature Conservancy and the Massachusetts State Parks and he currently teaches high school biology and physics. For more information visit www.speakingofnature.com, Speaking of Nature on Facebook, or the Speaking of Nature Podcast.