Photographer Phil Rowntree forwarded me these two photos of egg shells taken by a friend, Kirsten, who found the eggs somewhere in the unfenced 17-acre dog park on May 22. What bird laid them?
Three birders more expert than I vote Killdeer. They do resemble the eggs of Killdeer seen online. I hesitate to agree only because I’ve so rarely seen Killdeer around the park. My last spotting came in September 2018 when a solo Killdeer was pecking the mud around the Schoolhouse Creek outfall. Prior to that, the only Killdeer I saw was in 2013, also on gravel around the creek mouth. That’s a long way from the north-center of the park’s land mass. I’ve never seen one on the ground in the park.
Checking eBird, the database where bird watchers can register their sightings, and looking at finds in Alameda County for all years, one watcher has spotted Killdeer, three of them, all at the Elsie Roemer Bird Sanctuary on Alameda Island. There are no reported sightings of Killdeer anywhere in the Berkeley Marina.
That doesn’t mean it absolutely can’t be Killdeer. This bird, according to the Cornell lab’s Merlin site, “often nests near human development and far from water: parking lots, school roofs, road edges and other spots with bare gravel.” So it’s not impossible that a Killdeer flew in, dropped these eggs on the ground in the middle of the off-leash dog area, sat on them long enough to hatch, and then flew off.
Another possibility is that these are Song Sparrow eggs. I’ve seen more than half a dozen Song Sparrows displaying and singing in the park this year, like the one in the short video below. A pair could well have made its nest in one of the coyote bushes that dot the park, and then dropped the shells over the edge when the hatchlings came out. A photo of Song Sparrow eggs posted online resembles these mystery shells, just as the Killdeer egg photo does. It would help if we knew the size of these eggs, but unfortunately there’s nothing in the photo to indicate scale.
So: they remain mystery shells.