Scott Severs / Courtesy photo Baby bank swallows clamor for food from their hole-in-the-wall nest.
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It’s a paradox that bank swallows, birds that epitomize aerial agility, actually nest in a hole in the ground. These members of the Hirundinidae family thumb their beaks at another tradition: They mostly nest in early summer, not spring.

It makes sense for these insectivorous birds to nest at the peak of insect activity. More mosquitoes, flies, beetles, butterflies and other bugs — which the swallows capture in flight — means more and healthier chicks.

Adept aerialists, bank swallows dart and bank with speed and ease.

But why a burrow? Possibly because temperature is more constant in a cozy cavity and because vertical cliffs, their preferred bit of ground, offer more protection from predators such as snakes.

The male uses feet, wings and his small, conical bill to dig a burrow that sometimes reaches 5 feet in length. The female then hovers in front of proffered homes before choosing her mate. Is she choosing the best burrow, rather than the most beautiful bird? After the male enlarges the tunnel to include a nest chamber, his mate builds a grassy mat and lays three to seven eggs, which hatch in a couple of weeks. The breeding season usually extends through mid-July, with some fledglings observed into early August.

Other burrow-nesting birds in Boulder County include northern rough-winged swallows, belted kingfishers and burrowing owls.

As their scientific name suggests, Riparia riparia prefer banks above rivers, creeks or other bodies of water where they nest in colonies ranging from 10 to more than a thousand. These social birds huddle together to keep warm in cold weather and will even huddle with different swallow species.

Bank swallows, our smallest swallow, are identified by a dark breast band that contrasts with white underparts. They usually arrive in late April and leave in mid-September, heading to Central and South America and the eastern Caribbean. Formerly common throughout much of their range, they are declining worldwide, mainly due to loss of nesting habitats when sandy bluffs are destroyed by erosion, often caused by flood control and construction projects. Only scattered populations nest in Colorado.

Northern rough-winged swallows, which lack the distinctive breast band, nest in embankment burrows. Iridescent green and white tree swallows and violet-green swallows lay their eggs primarily in tree cavities or human-provided nest boxes. Highly social and locally common barn and cliff swallows typically choose cliffs, bridges and building ledges.

Baby bank swallows wait for the return of their parent.

One of the best places to see nesting bank swallows are the sandy banks above the St. Vrain River in Longmont, north of Fairgrounds Lake and west of the bridge. In the past, we’ve also watched them near Walden Ponds, where they nested in the big gravel piles. Because their nesting sites are prone to erode away, there is little nest site fidelity from year to year. So, consider yourself lucky if you sight such a site.

Other July Events

• Above treeline, pikas make hay piles on the tundra as marmots sunbathe on mountain boulders. White-tailed ptarmigan chicks hunt for insects, and American pipits, white-crowned sparrows and horned larks hatch.

• Late winter and spring storms have resulted in an unusually lush flower bloom this year, and we hope the tundra will be as lavish as the foothills have been.

• Fireflies glitter and glow early in July in grassy wetlands near Sawhill Ponds and in a few other muggy marshes as mosquitoes whine and dine on firefly watchers.

• Increasing numbers of bats come out for an evening drink, dipping down for a sip from lakes and ponds.

• Butterfly populations peak, and annual counts are held around July 4.


 

Ruth Carol Cushman and Stephen Jones are authors of “Wild Boulder County” and “The North American Prairie.”

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