Egg turning behavior is an important determinant of egg hatchability, but it remains relatively understudied. Here, we examined egg turning rates and egg temperatures in Forster’s terns (Sterna forsteri). We used artificial eggs containing a data logger with a 3-D accelerometer, a magnetometer, and a temperature thermistor to monitor parental incubation behavior of 131 tern nests. Overall, adults turned their eggs an average (±SD) of 3.8 ± 0.8 turns h-1, which is nearly two times higher than that of other seabirds. Egg turning rates increased with nest initiation date. We also examined egg turning rates and egg temperatures in relation to egg mercury contamination. Mercury contamination has been shown to be associated with reduced egg hatchability, and we hypothesized that mercury may decrease egg hatchability via altered egg turning behavior by parents. Despite the high variability in egg turning rates among individuals, the rate of egg turning was not related to mercury concentrations in sibling eggs. These findings highlight the need for further study concerning the potential determinants of egg turning behavior.

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Several avian species exhibit reduced hatching success as a result of mercury contamination, but the mechanism by which this occurs is unknown. We examined egg turning rates and egg temperatures, two important determinants of egg hatchability, in relation to egg mercury contamination of Forster's terns (Sterna forsteri) in San Francisco Bay. Here, we used artificial eggs containing a data logger with a 3-D accelerometer, a magnetometer, and a temperature thermistor to monitor parental incubation behavior (sampled at 1 Hz) of 186 tern nests. Overall, adults turned their eggs an average of 3.8 ± 0.8 SD turns h !! , which is nearly two times higher than that of other seabirds. Egg turning rates also increased with nest initiation date. Changes about the yaw axis (side-to-side egg movements) were the most prominent aspect of egg turning (224 ± 4.7 degrees h !!), though roll and pitch (up-and-down movements) also showed substantial changes (149 ± 2.5 degrees h !! and 89 ± 1.1 degrees h !! , respectively). Despite the high variability in egg turning rates among individuals (SD = 0.8 turns h !!), the rate of turning was not correlated with mercury concentrations in surrogate eggs collected from the same nest. Our results indicate that egg turning rates in Forster's terns are high, but unrelated to mercury contamination. v ACKNOWLEDGMENTS I would like to thank Scott Shaffer, for his guidance and advice every step of the way. Many thanks to Josh Ackerman, for his collaboration on this project and for sharing his expertise. Dr. Shelley Cargill, for her helpful feedback.