Although truly global in their range (called “Sand Martins” in Eurasia and Africa), Bank Swallows were classed as “threatened” in the state of California in 1989, primarily due to the loss of their required cliff-nesting habitat. Because coastal sand bluffs are so rare, only two known coastal Bank Swallow colony sites exist in California (Fort Funston and Ano Nuevo).
You are more likely to find Bank Swallows nesting inland at a river-bend site, where a steep outer bank has been carved out of friable soil by the river’s rising and falling. These sites are restricted to Northern and Central California, and occur on such widely separated rivers as the Smith, the Feather, the Cosumnes, and the American, to name a few.
But extensive channelization of streams; flood-control projects; and the use of dams, levees, and rip-rap have eliminated many large riverbanks in our state. Southern California is now devoid of Bank Swallow sites, so concern for this species in northern California looms large.
Burrow counts at Fort Funston made from 1993 to 2006 averaged 362 burrows annually, but yearly counts varied from 140 to 924. Since it is difficult to assess whether a burrow is occupied, burrow counts are not equal to the number of Bank Swallow pairs in the colony. In 1987, California Fish and Game biologist Joanie Humphrey counted 417 burrows but estimated that 60% were actually occupied with 250 total pairs.
Bank Swallows return to the Fort Funston cliffs each March to April, and some start building nests right away. Imagine a five-inch swallow clinging by its toes, supported below by its stiff-feathered tail, gouging and flipping aside sand grains with its beak. As the depression becomes hot-dog-sized, the swallow wriggles into the space, gouging with its bill while kicking backward with its feet. For the naïve observer below, she sees a black hole high on the bluff with a mysterious spray of sand shooting out of it every few seconds.
The burrow excavation progresses about four inches per day, with the male and female trading off. A week or so later, the tunnel—tipped gently upward to thwart spring rain run-off—is completed with a fist-sized nest bowl lined with grass, fur, and feathers. If all goes well, swallows fledge by late June or July, and by mid-August they depart southward on their incredible 5,000-mile journey, eventually “wintering” in South America.
To give this threatened swallow the best possible shot at successfully nesting each summer, the National Park Service has imposed a 12-acre buffer zone at the top of the sand bluffs. This reduces the amount of foot traffic near the top of the cliff, which is responsible for small landslides that can collapse burrows. (Nests can be placed within a foot of the top!) Cliff-climbing from both the top and bottom also have caused problems for Bank Swallows, not to mention the occasional rescue operations to save climbers.
Bank Swallows have non-human problems as well. American Kestrels have been witnessed hanging on the cliffs, their claws groping burrows for chicks. Ravens have also predated young swallows, and European Starlings are known to compete for nest burrows with the swallows.
To watch the Bank Swallows yourself, it’s best to wait for a warm day after a storm, when flying insects may be out in force. Walk south along the beach at Fort Funston, staying back well away from the bottom of the cliffs and use binoculars to examine the nest holes from a distance. Nest-digging should be in force by mid- to late-April. Swallow chicks, identified by having buffier highlights on the back and wings than the darker-backed adults, should be on the wing by late June.
By Allen Fish
Director, Golden Gate Raptor Observatory
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