In northern California, USA, western Burrowing Owls (Athene cunicularia) use burrows in active California ground squirrel (Otospermophilus beecheyi) colonies for nesting and protection. Ground squirrels have well-developed, anti-predator alarm-calling behavior, but the potential benefits of such alarm calling to Burrowing Owls have not been examined. The objective of this study was to assess the rate and types of predator interactions experienced by Burrowing Owls and the extent to which they may benefit from alarm calls given by California ground squirrels. We studied interactions of California ground squirrels and Burrowing Owls at Moffett Federal Airfield in urban Santa Clara County, California, during two Burrowing Owl breeding seasons, in 2012 and 2013, using cameras and direct observations. We detected 177 approaches by predators, four of which resulted in predation events on owls. The rate of predator approach during diurnal periods, as determined by direct observation, was 0.93/hr. Ground squirrels called in response to predator approaches before owls did 66% of the time, which was approximately proportional to the abundance of ground squirrels and Burrowing Owls. When squirrels called first, an estimated 75% of owls exhibited alert responses, including alarm calling, running to the burrow, and scanning, indicating that owls benefited from ground squirrel alarm calls in response to approaching predators. Our research suggests that healthy ground squirrel populations may provide important predator alert services to Burrowing Owls, especially in the context of increasing populations of urban predator species.