Petaluma’s Cliff Swallows – one year later
By April Rose Sommer
Cliff swallows love a good overpass. Unfortunately, last spring Caltrans construction to widen Highway 101 in Petaluma ran afoul of two colonial Cliff Swallow breeding sites. Caltrans contractors installed netting that was aimed at discouraging swallows from building nests there, but instead trapped and killed over 100 birds. When the agency refused to remove the netting, Golden Gate Bird Alliance joined a coalition of other conservation groups in filing a lawsuit.
Several month ago, the stakeholders reached a settlement that included the installation of less deadly exclusionary measures. Last week, GGBA joined its fellow plaintiffs and Caltrans representative to tour the sites, view the exclusionary measures, and check up on the swallows.
Instead of the lethal netting, Caltrans has installed a barrier material called Bird Slide directly over the concrete faces of the bridge and overpass. The exclusionary mechanism is simple – the installed materials are slick, and the swallows are unable to get mud to stick to the surface.
Petaluma highway construction – with white and beige exclusionary material on right, and area left clear with nests on left /Photo by April Rose Sommer
Broader view of the site. Again, the white exclusionary material is on the right and the area left clear for nests is on the left. / Photo by April Rose Sommer
Cliff Swallow nests are built drop by drop, with a typical nest containing 900 to 1,400 bits of hardened mud. Mud is collected by both male and females at ponds, puddles, ditches, and other sites up to 1/2 mile away from the nest. Mud gathering and nest construction are social activities; even unmated swallows will start nests! Most colony sites are close to a water and mud source and open fields or pastures for foraging. In this case, the nearby Petaluma Wetlands provide ample mud for nesting, and the Petaluma River provides a desired water source.
In natural habitats, Cliff Swallows live up to their name, building their nests on cliff faces and overhangs. But urban habitats provide a range of attractive nesting places, including buildings, bridges, and overpasses. The swallows especially favor 90-degree angles such as the supports on the Petaluma Bridge and nearby Lakeview overpass, both part of the highway widening. A corner nest requires less mud, thus decreasing the time needed to build.
Cliff Swallow gathering material for a nest / Photo by Bob Lewis
But all the mud, water and right angle in the world can’t help if there’s no good place to deposit the mud drops!…