Crossing Paths With Washington's Wildlife
Some Swallows Have Really Adapted To Us!
August 1, 2015
It doesn’t take much looking across the state this month to see huge aggregations of birds – this year’s production of many migrant species that are gathering now and will be moving south soon.
Some of the most obvious are swallows, which readily collect on power and telephone lines along roads in open areas where they’re swooping after insects.
Earlier this summer our Snohomish Basin area habitat biologist Jamie Bails especially noticed a particular nesting colony of cliff swallows, one of the seven species of swallows that include Washington in their breeding range. It got her thinking about and researching how some wildlife species, like the cliff swallow (Petrochelidon pyrrhonota), have adapted to us and our structures more than others. Here’s Jamie’s account:
Tucked high underneath the State Route 2 trestle, between the towns of Everett and Snohomish in Snohomish County, are approximately 100 gourd-shaped mud nests. These nests are the work of cliff swallows, native passerines that actually prefer man-made structures over cliffs and caves for nest location.
Originally, cliff swallows nested on cliffs and inside canyons throughout North America. But the building of concrete structures, and replacement of wooden highway bridges and trestles with concrete, has expanded their nesting habitat options, increased their range, and even created an unexpected evolution of body size and wing length.
Over ten years ago, the Washington State Department of Transportation replaced the aging wooden State Route 2 trestle with a one mile long concrete overpass. Under that trestle today, I watch adult swallows slip in and out of their nests pasted to the concrete. Disturbed, they call with a faint twitter or squeak that warns potential predators like me to move away.
Even without binoculars, from 50 feet below visitors to the Washington Department of Fish and Wildlife (WDFW) Snoqualmie Wildlife Area’s Ebey Island Unit, located upstream of Spencer Island between the towns of Everett and Snohomish, can watch these birds’ lives. A parent bird tends three to four eggs while its partner swoops and soars for insects across the spacious grass field. Little white beaks stick out of the nests, waiting for meals from the adults. It’s exciting to watch a hundred adults flying to and from the nests.
The colony faces south, just west of Ebey Slough, a side channel of the Snohomish River. This prime location provides access to soft river mud, fine silts and clays which are perfect for nest building, as well as a field full of flying insects for daytime foraging.
According to local author and Professor John Marzluff of the University of Washington, cliff swallows are a generalist species that have managed to successfully adapt to human-made conditions, buildings and structures. In his recent book, “Suburbia,” he describes species like the swallows that have thrived in spite of human impacts, and adapted and survived.
But cliff swallows have adapted at another level, too. Over decades they have actually evolved with larger bodies and shorter wings that help them better avoid speeding motor vehicles in our urban and suburban landscape.
This adaptation was documented by Mary Bomberger Brown of the University of Nebraska over 30 years of studying birds near Lake McConaughy in western Nebraska. Across the landscape, she noted that swallows were moving from traditional cliff homes to highway bridges as the old structures were upgraded to concrete. Initially, she found an increase in the number of road killed birds at the new bridge locations. But over time, the number of birds killed decreased at each study site.
When a freak May storm in the winter of 1996 killed 1,800 birds, 70 percent of the local population, it allowed her to measure the body size and wing length of killed birds versus those that survived. She found that dead birds were shorter with longer wings, but birds who survived were larger with half-inch shorter wings. Bomberger Brown concluded that birds with shorter wings had been more agile in avoiding collisions, and passed down a genetic trait to offspring, a simple evolutionary adaption to new environmental threats.
These adaptations in both behavior and size, gave me a new appreciation for cliff swallows. I learned that they migrate here each spring in large groups from as much as 6,000 miles away in Brazil, Argentina, and Chile. When they reach their traditional nesting area, they begin gathering mud pellets to build nests. If last year’s nest survived, the first to return have dibs on them, quickly refurbishing and declaring occupancy. Later arrivals, or those not originally from this colony or yet unmated, begin to build anew.
Down on the mud they land, scooping up a ball of mud in their beak and carrying each one to the nest, as many as 1000-1400 per nest, one per trip. Whether new or refurbished, the nest building is a task both sexes share, taking up to two weeks for the mud to dry and harden. Nest dimensions vary from 5 to 10 inches in length and 5 to 8 inches in base width. Egg laying may begin before the nest is finished with one egg laid per day until the clutch of three to four eggs is completed.
If a nest is destroyed by weather or another bird, the pair will re-build and try again. Cliff swallows are even known to place eggs in another’s nest, possibly as insurance policy in case their own nest fails.
For 15 or 16 days, both sexes incubate the eggs, which hatch in mid-June. The adults are kept busy feeding the nestlings by foraging over an area two to four miles from the nest. A sign of a successful nest is white excrement rimming the nest entrance, indicating the presence of young swallows inside. Then, 20-25 days after hatching, the chicks take their first flight from the nest. After their first flight, they remain near the colony for about a week.
After leaving the nesting colony, cliff swallows remain in the area for several weeks. By mid-August many begin moving south, and by the end of September few remain.
Cliff swallows are beautifully colored, with chestnut, cinnamon, buff and black feathers. They are distinguished from similarly-colored barn swallows by a square tail rather than a forked tail.
While a colony of swallow nests may seem unsightly around the house or barn, it’s important to think about the hundreds of mosquitos and other bothersome insects that these birds are eating daily while they’re here.
And after all, if they can adapt to us, surely we can adapt to them!