These birds may live in colonies of thousands of nests, but you might not even know they’re nearby

Cliff swallows are said by ornithologists to show the highest degree of colonization of any of the 89 swallow species in the world. Colonies can number from 200 to more than 2,000 nests. They could be nesting in your neighborhood without being obvious about it. That happened to me this spring. More on that later.

Believed to be limited to the southwestern part of the United States 100 to 150 years ago, cliff swallows today breed almost everywhere in North America.

Most North American bird species have been impacted by humans one way or another, some for better, others for worse. These swallows are a species that has benefited significantly from efforts never intended to involve birds at all.

We built bridges and buildings and culverts. Those structures provide the very particular site requirements these birds need for their self-constructed mud nests.

In choosing a nesting site, the swallows look for a vertical face with a surface to which mud will adhere. Ninety degrees is one of our basic construction strategies, so our work meets the birds’ criteria very well. Cement is a perfect foundation.

We moved this spring to an apartment not far from our old neighborhood. We have chosen what could be called a purple martin housing strategy, with elevators.

There is water here, two small lakes linked by manmade channels crossed by concrete bridges. The water attracts insects, the shorelines provide mud.

Walking here one day I noticed swallows coming and going from beneath the bridges. The concrete bridge surfaces are perfect for nest construction, numberless dabs of mud shaped to enclose a gourd-like nest, a small entry hole for access.

You might have encountered cliff swallows without knowing it, perhaps a bridge of any construct as long as the mud sticks. Watch for air filled with swooping swallows. Large culverts are good.

(Since all five of the swallow species found in Minnesota are aerial insectivores — catching bugs on the wing — other swallow species also can be found swarming at bridge sites. They also are foraging for insects.)

The first nests begin at that critical right angle. Other nests are attached to early nests, and eventually there may be hundreds of nests stuck together at one breeding site.

The birds will fly miles to find mud. A pair of birds with good access can deliver as many as 44 mud pellets to the construction site in 30 minutes, a study has shown. The average nest contains from 900 to 1,200 pellets. Sharing walls with a neighbor cuts construction time. The interior of the nest is lined with grass.

Breeding birds form a weak pair bond described in ornithological literature as more of a “mutual tolerance.” When gathering mud for nests, the birds flutter their wings vigorously above their backs.

That had long puzzled me. Researching this column I found the answer: Fluttering wings foil male birds seeking extra-pair copulation — a chance to breed with a female paired with another male.

Males want to spread their genes as broadly as possible. Females will accept the effort because the new suitor might have better genes than the accepted mate.

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com.

Minnesota is also home to other swallow species, with barn swallows and tree swallows being abundant, according to the Minnesota Breeding Bird Atlas. (Cliff swallows are listed as common here.)

The Northern rough-winged swallow and bank swallow both count as regular Minnesota breeding residents and migrants, but are listed as uncommon during the Minnesota Breeding Bird Atlas. Ditto with the purple martin, also in the swallow category.

There are eight species of swallows that nest regularly in the United States. According to the U.S. Fish & Wildlife Service, swallows provide us with an ecological service as insect controllers. They consume swarming insects such as bees, wasps, flies, damselflies, moths, grasshoppers, crickets and more.