The Importance of Nest Box Design

I used to think that where and how you installed a bluebird nest box - the actual box location and pole - are more important than nest box design.

When I first saw the box the Bluebird Restoration Society of Wisconsin (BRAW) recommends, I was skeptical.  It didn't look much like the boxes I'd written about (in the US-Fish and Wildlife Service pamphlet For The Birds).  I didn't think much of the design.

But Dr. Kent Hall and his colleagues at BRAW had thoroughly tested their design and their success is supported by the data they've collected over the years. 

This season, I installed one of the BRAW boxes at Eau Galle Cheese Shop and moved the 2 existing boxes (not quite up to North American Bluebird Society standards) to better locations.  (I did not have permission to replace them with BRAW boxes.)

My experience this spring with the nest boxes at Eau Galle Cheese Factory erased my doubts about the BRAW-design boxes.  The BRAW nest box and pole set-up is by far the best (most productive) for Wisconsin bluebirds.

Here's what I saw in the three nest boxes at the Eau Galle location:

This "home made" box - box #2 - is in a good location - at the edge of the customer parking lot.  It looks good to the human eye (and apparently to House Sparrows and starlings too).  When I drove up to monitor the box this week, a starling was sitting on the roof. 

Unfortunately this attractive box has been deadly for bluebirds.

During the first brood, a pair of bluebirds tried to nest in it.  They were under constant attack by House Sparrows and starlings.  Location wasn't the problem;  it's the design.

According to bluebird researchers, the hole size is too big (making it easy for starlings to get inside and kill the bluebirds), the vent at the top is too generous (making the box vulnerable to wet and cold weather) and the interior dimensions are too small (for maximum bluebird productivity).

Tree Swallows gave it a try.  They failed too.

Finally, a few weeks ago, the bluebirds had built a nest and I thought they'd won the war.

Last week, the female refused to flush when I came to monitor the box.  This week, she was so tenacious I could actually pick her up.  That made me wonder why.

I took a closer look at the nest.  That's what monitoring is all about, right?  Look what I discovered:

Bluebird feathers under the grass nest. 

Then I pulled the nest (with the 5 eggs) out of the box and discovered the dead body of a male bluebird - with the new nest built on top of him.   I suspect the starlings killed the male, and the female found another mate.  But because the entrance hole is too big, the bluebirds are still vulnerable to starling attacks.

Taking the nest out of the box, I discovered the other problems with this box:  its interior dimensions are too small, the ventilation area at the top is too big and the entrance hole is too close to the floor.  It will be interesting to see how many of the 5 eggs will fledge.

I put the other "home made" box - Box #1 -  on a pole out by the highway, near a small tree.   This box was empty for this season' first brood.

I suspect design is to blame.  A drafty box is a deadly box in a chilly and wet spring.   Right now, five Tree Swallow nestlings are crowed in the box.  It's amazing what a difference an inch makes.

The BRAW box - Box #3 - at the entrance to the parking lot has been successful.  The first brood produced 5 fledglings.  They're on their 2nd brood with 5 warm eggs.  No signs of any problems so far.

Someone I met while I was monitoring the nest boxes recently asked:   You see all those nest boxes along the road and in backyards that have perches - your boxes don't look anything like them.  What difference does it make where you put them and what design you use?   The birds don't care.  They use them no matter what they look like and where you put them - regardless of whether you monitor them.  Right?

Yes, the birds will use them, regardless.   And that's the problem.  The difference between the boxes can be measured by their "success."

A successful bluebird nest box will fledge 5 chicks per brood.  If the birds produce 2 or 3 broods, that's a potential of 10-15 fledglings per box.

So far we've had no fledglings (out of a potential 10) and one dead adult bluebird in the 2 "home made"  boxes.  That's a success rate of -1.

We're 5 for 5 (with 5 more eggs) in the BRAW bluebird box.