Identification of nest predators and reproductive response of the Modesto Song Sparrow, Melospiza melodia mailliardi, to experimental predator removal

I measured the reproductive response of Modesto Song Sparrow (Melospiza melodia mailliardi) to the experimental removal of an introduced predator, the black rat (Rattus rattus), and identified nest predators using video cameras at the Cosumnes River Preserve in the Central Valley of California. Nests were monitored before (2003) and after (2004) black rat removal on both an experimental removal site (Tall Forest) and a reference site (Orr Forest) in riparian habitat. I measured Song Sparrow reproductive response to black rat removal in four ways: 1) seasonal fecundity, 2) total productivity, 3) daily nest survival rates and 4) variation in clutch size. Seasonal fecundity and total productivity increased more on the experimental site than on the reference site after rat removal, but these differences were not statistically significant. I found no effect of rat removal on clutch size. I used an information-theoretic approach to evaluate support for different models of daily nest survival rate. The two top models included an experimental effect (year by site interaction) indicating that rat removal did have a positive effect on daily nest survival. Two nest predator species were identified using video monitoring: four nests were depredated by Brown-headed Cowbirds (Molothrus ater) and four by black rats. In addition, I witnessed a black rat depredating a Song Sparrow nest that was not recorded. My results suggest that these two introduced species are important nest predators in natural riparian habitat at the Cosumnes River Preserve. Rat control may be useful as a management tool to increase songbird nesting success, however, due to the limitations of my study more research needs to be conducted to evaluate the efficacy of this tool in riparian systems.