Bluebird Families in San Pablo Park

I walk the two blocks from my house to San Pablo Park. The birds I'm looking for are right there to greet me, perched on the surveillance camera sign at the edge of the park. Their heads are soft grey, their chests rusty orange flecked with white. Their tails flash cornflower blue. They have a bit of a shaggy look, ends of fluffy, white feathers peaking out on their chests and bellies. These are juvenile western bluebirds. As I walk through the park, I see them on the fence by the tennis court, bathing in the drinking fountain, perched in the chestnut trees, flycatching in the grass by the toddler playground.

Western bluebirds are cavity nesters. They nest in holes in trees made by woodpeckers, or in nest boxes put up by people. They are not normally seen in cities. Non-native, city-loving birds such as house sparrows and starlings usually out-compete bluebirds for nesting spots in cities. Until 2008, bluebirds were extremely rare in Berkeley. That was when Rusty Scalf, a local birder, discovered a pair nesting in San Pablo Park.  We moved to our house by the park that same year. I remember the jolt of pleasure and surprise when I first spotted the bluebirds. The color is so distinctive and bold and had been so firmly associated in my mind with grassy wild-lands like Briones park.

Scalf worked with the park department to install nest boxes, and our small, unusual bluebird population expanded. Now I see them around the neighborhood, not just in the park. One year, a pair even nested in a box in the oak in our front yard. Over the spring and summer, a pair of bluebirds can raise up to three broods of young, with 2-8 birds per brood. Some pair are helped in raising their young by single adult male birds or other pairs without their own nests. This time of year—the end of the summer—the park is aflutter with juveniles. I like to imagine them venturing out through our neighborhood, searching for holes in trees, finding the best fences to catch moths and flies from.

I wonder what is allowing this bluebird population to thrive in Berkeley. There are certainly plenty of house sparrows and starlings here that might compete with them. Have they developed some behavior—perhaps related to the nest helpers—that allows them to fend off the sparrows and starlings? Could they be passing on this behavior as local cultural knowledge? Such passing on of novel behaviors has been documented in certain crow populations, though not as far as I know, among bluebirds. Could there be a genetic difference that allows our birds to survive? Something that allows them to use holes not used by other birds? Could we be witnessing a small example of natural selection? Or is it a matter of chance? If so, perhaps our population will not last. Whatever the answer, it is rare, beautiful, and right around the corner from my house.

LINKS:

Golden Gate Audubon blog post about the Berkeley bluebirds

Berkeleyside article about them

Western Bluebird information from the Cornell Lab of Ornithology