At issue are cliff swallows, which had long built their mud nests on a pair of concrete bridges near Highway 101 in the Sonoma County community of Petaluma. When Caltrans decided it was time to rebuild those bridges during a highway widening project, contractors installed bird netting to keep the swallows away from the construction area. But that netting ended up snaring the small birds, and more than a hundred of them died.
Cliff swallows are so-named for their habit of building mud nests in cavities on cliffs and protected rock outcroppings. They’re one of the wildlife species that has adjusted well to human influence, mainly because of our habit of building thousands of vertical “cliff faces” everywhere we go. Concrete bridges and similar structures are especially attractive to the swallows because they tend to have concrete sills that overhang the vertical faces, which allow the birds to build their nests more efficiently.
When Caltrans agreed to end the use of netting during the construction project, its contractor C.C. Myers deployed a slick material called “Bird Slide” over the concrete. When the swallows learn after a few tries that they can’t get their carefully gathered beaks full of mud to stick to the Bird Slide, they give up and build their nests somewhere else.
That’s undeniably a better alternative than the netting: rather than getting injured or killed through entanglement, the birds shrug their shoulders and go off to build a nest somewhere else. And they may not have to travel far: C.C. Myers has left some of the concrete on the bridges bare, where swallow nests won’t get in the way of construction.
But according to Sommers, those patches of open concrete are more exposed to the elements than is ideal for the longevity of the nests, which are, after all, made of mud and spit. Swallows have nested on the bridges this season, with about 80 nests total. But that’s well under the historic 500-800 nests the bridge colonies built before construction.
Where did the remaining swallows go? They may well have dispersed throughout the nearby community looking for more suitable nesting sites. And that’s a problem: the highly gregarious birds fare better in huge colonies with up to 6,000 nests. Breaking up a colony may well mean a population decline in the long run.
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