| Summary: | <p> 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 (<i>Sterna forsteri</i>) 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<sup>-1</sup>, 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<sup> -1</sup>), though roll and pitch (up-and-down movements) also showed substantial changes (149 ± 2.5 degrees h<sup>-1</sup> and 89 ± 1.1 degrees h<sup>-1</sup>, respectively). Despite the high variability in egg turning rates among individuals (SD = 0.8 turns h<sup>-1</sup>), 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.</p>
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