Orientation toward the sun combined with postural changes minimized the net radiation gain on the adult herring gull during the breeding season. The decrease in absorbed solar radiation was due to reduced surface area exposed to direct sunlight and to reduced absorptivity since, when the bird orients toward the sun, the angle of incidence is such that only the white surfaces receive direct sunlight. At low wind velocities (0 to 0.6 meter per second) and ambient temperatures above the lower critical temperature, the birds rotated 180 degrees during the day, always facing the sun.
The interaction of insolation, wind, and ambient temperature on the energetics and behavior of juvenile Herring Gulls was studied. At low ambient temperatures gulls can reduce their energy expenditure for thermoregulation as much as 29% by basking. Insolation at ambient temperatures above 15°C was stressful to the gulls because the thermal gradient was into the bird. The feet were found to be an important means of heat loss in birds at high and moderate ambient temperatures receiving insolation. Under natural conditions the use of the microhabitat is extremely important in minimizing the effects of radiation both during the day and at night.
Field and laboratory studies were conducted during the winter of 1975—76 to assess and quantify the energetic determinants of the communal roosting behavior of the Starling (Sturnus vulgaris) in a pine woods. Of the microclimate factors measured, only the wind velocity varied significantly between the recordings obtained outside of the pine woods and in areas within the woods that were used extensively as roost sites. Subjection to wind velocities of 2.9, 7.3, and 15.1 km/h at ambient temperatures of —10.0°, 0.0°, and 5.0°C caused significant increases in metabolism of the 30 Starlings tested. Selection of microclimates where wind velocity is virtually eliminated confers upon roost inhabitants the energetic advantage derived from minimized forced convective heat loss. The reduction in daily existence metabolism is estimated to range from 12% to 38% under environmental conditions that are normally presented in temperate latitudes during winter nights.
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