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.

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.

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! With Bird Slide and other slick surfaces covering the bridge and overpass, we did not observe any swallows nesting in the off-limits area.
On the Petaluma bridge, Caltrans left some areas of the outer-facing surfaces clear for swallow nests.   Unfortunately, swallow mud nests are highly susceptible to environmental conditions, easily destroyed by an unexpected storm or strong winds. We and our conservation allies are concerned that the areas left for nesting are widely exposed to the elements, and the nests are at high risk for destruction.  Nonetheless, we were able to enjoy an acrobatic show put on by about 50 swallows over the Petaluma River that appear to have opted to nest in the area despite the disruption.  We observed about 50 nests in progress on the Petaluma Bridge and about 20-30 nests in progress on the Lakeview overpass.
It remains an open question whether the Petaluma Bridge and Lakeview overpass will ever regain their traditional colony of 500-800 nests. When Cliff Swallows migrate to California from Central and South America, they often return to ancestral colonial breeding sites – the same sites where their parents and grandparents raised their young.  But if an ancestral site is not successful for a swallow, it’s likely to abandon the site in the next breeding season.
As preeminent cliff swallow expert Charles R. Brown explains,

“Cliff swallows are similar to many other bird species in that they all have one simple rule: if unsuccessful at one place, avoid it in the future.  If birds lose their nest, eggs, or young to predators, parasites, or storms, they immediately abandon the site for good.  Natural selection favors those cliff swallows who never forget, whether in western Nebraska or San Juan Capistrano, Calif.”  (See How Cliff Swallows Choose Where to Live for more info.)

This seems to be what happened with Mission San Juan Capistrano’s famous swallow colony. For over 100 years, on or near March 19, the Capistrano swallows returned from their wintering range 6,000 miles away in Argentina. But the Mission swallow colony has dwindled ever since nests were removed and excluded during preservation efforts in the early 1990s.  While the swallows haven’t gone too far — colonizing nearby structures at a golf course, college, and various overpasses — some fear the Mission colony will never return to its prior numbers.  Trying to bring back its famous birds, the Mission has even installed speakers that play a recording of the swallows’ courtship calls!
Similarly, the Petaluma Bridge colony may suffer a permanent decrease in its numbers as a result of the construction and last year’s swallow kills. And while individual swallows have been observed nesting in the area, there does not appear to be an en masse movement of the colony, which may harm the population in the long run. Cliff Swallows are social creatures that prefer to nests in colonies as large as 6000 nests.  Colonial living helps swallows find the swarms of insects that make up their diet: A colony member will watch out for neighbors that are successfully hunting and then follow the leader to the swarm.  The more neighbors to serve as swarm-finders, the better!
How will Petaluma’s Cliff Swallows fare during the rest of this nesting season? And will the colony recover in future years? GGBA and its fellow plaintiffs will return to the site for another visit in a couple of months to see how the nests-in-progress have fared. We hope we will be able to view thriving swallow babies.

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April Rose Sommer is an avid outdoorswoman, animal-lover, and conservationist born and raised in the East Bay. She practices environmental law at her firm Sommer Public Interest Law and advocates for birds and habitat protection on the GGBA Conservation Committee.  Her birding skills are a work in progress but she enjoys caring for raptors and raising baby songbirds as a volunteer in the Lindsay Museum Wildlife Hospital.