Curious Iowa: What are the boxes mounted on the back of Iowa highway signs?

Eagle-eyed motorists on Iowa’s interstate highways may have noticed what appear to be bird boxes mounted on the back of DOT signs. What are they for? And who put them there?

That’s what one Cedar Rapids resident wanted to know. He posed his question to The Gazette’s Curious Iowa, a series that answers readers' questions about Iowa, its people, and the culture.

To answer this question, we looked into the decades-old history of the houses, and also asked why it was seemingly discontinued.

The American Kestrel

  Buy Photo

Pat Schlarbaum, retired wildlife diversity technician with the Iowa Department of Natural Resources, said the American Kestrel — North America’s smallest falcon — experienced population fluctuations in Iowa over the past 40 years. Specifically, the 1980s saw a massive decline in the kestrel population, he said.

The American Kestrel is a bird of prey and is consequently an important part of the ecosystem.

Schlarbaum, who retired in 2017 after 33 years with the DNR, was present when the first kestrel conservation efforts began. He volunteered for a few years before accepting a position with the DNR in 1985. His first task was to construct 88 kestrel nesting boxes.

These nesting boxes were part of the DNR’s proposed solution to curb the birds’ population decline. The Iowa DNR and Department of Transportation joined together to put the nesting boxes on the backs of highway signs throughout Iowa. Schlarbaum described the partnership as a “bridge” between the two agencies, and one of the first of its kind.

  Buy Photo

The primary motivator behind the nesting boxes as a means of conservation, he said, was to provide safe nesting areas for the kestrels, which are cavity nesting birds, meaning they rely on holes in trees or other structures for nesting.

In the 1980s, there was a lack of healthy trees with cavities that could be used for nesting, Iowa State University professor and avian ecologist Stephen Dinsmore, said. So, the nesting boxes were intended as a boost to the shortage of nesting options for the bird.

Safety was another issue, Schlarbaum said, that the nesting boxes were able to address.

“Predation is a big deal for these cavity nesting birds,” he said. “The raccoons and snakes are a major predator. The safe nesting is huge. There weren't really any predation issues, and there was no danger with roadkill or young being hit.”

Why were the boxes placed so close to highways? Because of the signs’ proximity to grasslands, the borders of which Schlarbaum said are kestrels’ natural habitat.

Population changes and program evolution

In the 2010s, the DNR observed the kestrel population returning to healthy levels. Additionally, Schlarbaum said, the amount of traffic along the highways where the boxes were located generally increased in the decades since the program began. Thus, the safety of DOT workers when working on the boxes came into question.

The DOT and DNR discontinued the program in the mid 2010s, with the DOT prioritizing removal of the boxes during nearby road construction in 2016, Schlarbaum said.

However, the DNR continues nesting box efforts in smaller ways outside of the DOT-affiliated highway nesting boxes. Dinsmore said many boxes have popped up across Story County, and individual counties have taken steps to install nesting boxes on lower-traffic county roads.

  Buy Photo

Boone County has a program dedicated to installing kestrel boxes on power poles in opportune nesting areas.

While the DOT and DNR program ended, the DNR and ecologists like Dinsmore continue to encourage the practice to maintain these predating birds’ population.

“That bird is still much, much reduced in population,” Dinsmore said. “There's plenty of room for it to continue to grow. So I think boxes are still something that's a worthwhile effort.”

Have a question for Curious Iowa?

Tell us what to investigate next.

Read weekly answers to readers’ questions — along with behind-the-scenes observations — by signing up for the Curious Iowa newsletter.

Comments: 641-691-8669; evan.watson@thegazette.com