Does the thermal environment influence vigilance behavior in dark-eyed juncos ( Junco hyemalis)? An approach using standard operative temperature

Abstract

One might expect that increased thermal stress would cause wintering birds to forage faster in order to meet the increased metabolic demand. Faster foraging should, in turn, lead to a reduction in vigilance, since feeding and vigilance are mutually antagonistic activities. We examined these intuitive behavioral expectations using newly developed standard operative temperature sensors designed specifically to characterize the thermal effect of the microclimate environment (rain excluded) on wintering dark-eyed juncos ( Junco hyemalis). These sensors allowed us to distinguish the behavioral effects of thermal stress from non-thermal effects associated with micrometeorological conditions (e.g. wind noise). Our analysis indicated that neither thermal nor non-thermal aspects of the physical environment influenced the proportion of time spent vigilant by juncos. However, the rate of food ingestion (measured as pecking rate) exhibited a negative correlation with thermal stress per se. This unexpected result may reflect the effect of thermal stress on feeding posture, peripheral muscle cooling, or both. The effect of thermal stress on pecking rate was nevertheless minor in comparison to the effect of flock size, which exerted by far the largest effect on both vigilance and pecking rate. Our overall results suggest that birds experiencing thermal stress will not necessarily lower their vigilance, but rather increase feeding bout length to compensate for the greater metabolic demand. This interpretation is consistent with theoretical models of vigilance in a non-time-limited environment, and may help explain the contradictory results to date on the effect of thermal stress on vigilance.