Field and Laboratory Studies of the Thermal Relations of Hatchling Western Gulls

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Abstract
Field and laboratory studies were undertaken on thermoregulatory capabilities during the first 24 h after hatching in chicks of the western gull (Larus occidentalis wymani). Body temperatures ( ) of 12 such hatchlings brooded in their nests on San Nicolas Island, California, averaged 38.0 ± 0.60 SE C between 0900 and 2000 h. Hatchlings unprotected by a parent heated rapidly (up to 1 C/min) in the midday sun and commenced panting at > 40 C. Panting did not stabilize , and coordination and equilibrium were lost at 44-45 C. One chick recovered from a of 46.2 C. Still higher could develop with prolonged exposure to direct sunlight. Unlike new hatchlings, unprotected older chicks (≥ 15 h) moved to the shelter provided by vegetation when risk developed of overheating in the sun. Unbrooded hatchlings (>24 h) became hypothermic at night with air and soil temperatures of 16-17 C. Laboratory observations indicated that chicks in this age class can tolerate as low as 14-18 C. Body temperatures at which various motor patterns reappear during passive rewarming from such were determined. During the first day after hatching, western gulls have insulative capacities below those anticipated for adult birds of similar size, despite the seemingly luxuriant character of the chicks' down. The basal metabolic rate (BMR) for the western gull hatchlings (mean body mass, 58.0 g) is 8.26 mW.g⁻¹, which approximates values predicted for an adult nonpasserine bird of similar size. These less than 1-day-old chicks can increase metabolic rate to 1.9 X BMR at cooler ambient temperatures ( ). They increase evaporative cooling at 42.9-44.0 C sufficiently to dissipate the heat produced in metabolism. Parental attentiveness normally prevents large fluctuations of Of hatchling gulls under cool at night or direct insolation during the day. Such protection is especially important during the latter period, owing to the relatively narrow interval between normal and upper lethal for these young chicks.