Thermal Environment and Tolerance of Embryonic Western Gulls

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The thermal environment and tolerance of eggs and embryos of western gulls (Larus occidentalis wymani) on San Nicolas Island, California, were determined. Incubated egg temperature measured by telemetry ranged between 30 and 36 C and averaged 33.4 and 34.2 C for two different nests; the former underwent pronounced cycles of heating during the day and cooling at night. Exposed eggs underwent a daily thermal excursion between 6 and 50 C. Embryos could maintain heartbeat between 11 and 46 C. Eggs which are exposed to solar radiation heat slowly (about 5 C/h). Diurnal exposure during parental absence does not, therefore, result in embryonic mortality unless exposure persists for several hours. Embryos recover completely after overnight exposure to relatively cool temperatures. Short-term exposure does not, therefore, constitute an immediate threat to embryonic survival. Adult gulls in this colony do not closely defend their nests and will leave them exposed in the presence of an intruder. The behavior of the parent gull and physiological tolerance of the embryos in reference to the thermal environment form an adaptive suite of characters contrasting with those of other gulls nesting under hot and arid conditions.

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