Heat Exchange Influence on Foraging Behavior of Zonotrichia Flocks

Abstract

The heat exchange of individuals in a foraging mixed population of Zonotrichia leucophrys and Z. atricapilla in western Oregon is analyzed for its influence on the direction of stance and feeding location. Values for heat gain or loss in the field were generated from a model that estimates the equivalent blackbody temperature (Te) and from previous laboratory findings for the effects of air temperature, solar radiation, and wind speed on metabolism. Observations were recorded at 1—min intervals throughout the daylight hours with samples at weekly intervals from January to March 1978. A summation of 1500 observations on direction of stance subjected to a g test for randomness supports the hypothesis that spatial orientation of individuals is not random. Maximum differences in Te for different stances with respect to the wind direction were only 1°C because of the low winter values for incoming shortwave radiation. More important factors than heat exchange influencing the direction of stance were food availability, social interactions, and the location of the hedge in the area used for cover. Placing a trace line of food perpendicular to the most frequently observed direction of stance caused a significant change in percentage of time spent facing each direction for that location. Individuals were facing the same direction as the majority of their neighbors within a 2—m radius during 68% of the total number of recorded observations for a nonrandom flock orientation. Heat loss was affected by changes in the microhabitats at the site. Differences in wind speed, air temperature, and solar radiation were large enough to increase metabolic rate as much as 20% for the open area foraging as compared to the hedge. Instead of feeding in the open area during midday where the shortwave radiation is greatest, the flock would often move into the hedge, thus occupying each location when the thermal conditions were least advantageous. Factors that may influence this feeding behavior could be a decreased cost—to—gain ratio for caloric intake in the open area in conjunction with circadian rhythms for metabolic rate and energy needs.