Jayson in Castle Pines writes, What’s driving you crazy? “What is this black netting under the C-470 bridge over Acres Green Drive? It doesn’t look like it can catch anything so why is it there?”

Well Jayson, (cough,cough) there are many hazards when construction workers repair or expand a bridge like the many bridges being worked on as part of the C-470 express lanes project. One hazard that most people may never have considered is nesting migrating birds. That is the reason for that black netting under the bridge. In fact, the netting has been there since the Spring to deter nesting birds from making that bridge their home.

The contractor, Flatiron Construction Corp, doesn’t want to be fined for removing or destroying migrating bird nests and eggs. A contracting company in Kansas found out the hard way how expensive removing nests with eggs inside can be. Wildcat Concrete Services was fined $372,750 in January 2014 after destroying 818 eggs and removing approximately 1,491 nests from a bridge they were repairing. One of the employees was sentenced to three months’ probation and 10 hours of community service.

There are numerous examples of these migrating birds slowing or stopping bridge projects. A construction crew in Washington state found a single nest and had to delay bridge repairs until the swallow chicks were old enough to fly away. A bridge maintenance project in Louisiana was outright canceled because of the birds. A bridge project in Ottawa was put on hold until the city and contractor moved some nests. Birds delayed a bridge demolition in northeast Arkansas. And in California, swallows disrupted a highway widening project after returning to their nesting area during the middle of construction. Swallows pose such a problem in Oklahoma that their Department of Transportation commissioned a research project to find the best ways of working around the birds. The Colorado Department of Transportation paid for a similar study in 2010.

If you want to see a healthy family of these nesting swallows just look up at the I-25 bridge over University. There you will see what looks like hundreds of nests built in the girders with birds coming and going like travelers through Denver International Airport. Swallows are not endangered but federally protected because they’re migratory birds.

All species of swallows are protected in the U.S. under the Migratory Bird Treaty Act of 1918, which makes disturbing active nests of the birds named under the Act illegal, whether they are destroyed intentionally or not. An active nest is defined as one with eggs or a brooding adult bird living in it. This is particularly difficult for any companies planning bridge repairs or other infrastructure projects. Once the swallows move in, there is no way to get rid of them until they fly off themselves, which could be up to four months. If a nest has been abandoned or there are no eggs present, it can be removed and destroyed as needed. Nests of invasive bird species, those not native to the U.S., such as house sparrows or European starlings, are not covered by federal laws.

The easiest way for contractors to avoid the nesting problem is to stop the nesting in the first place. That is what the netting under the bridge is supposed to prevent. There are several companies that specialize in this nesting prevention netting. The contractor installs the net and then is reasonably assured there will not be any nesting birds that will delay construction when the contractor is ready.

Another way contractors deter these birds from nesting is using bioacoustic deterrents. They are basically electronic sonic devices that broadcast several unique recordings of alarm and distress calls that the birds don’t like. There are also less common non-toxic chemical like methyl anthranilate that can be used to deter the birds from nesting in specific areas.

When the contractor is ready to start work on that bridge they will check for any birds, remove the netting and continue with their project.

Typically, after all the construction work is done the netting is removed and the birds return year after year.