The Thermal Environment and Its Constraint on Activity of Desert Quail in Summer

I measured the thermal environments experienced by Gambel's Quail (Callipepla gambelii) during the summertime in the Colorado Desert by using estimates of standard operative temperature$(T_{es})$. Simultaneously with these measurements, I monitored the activity patterns of quail. The thermoneutral zone of freshly captured quail extends to 44°C, yet$T_{es}$in sunlit areas may exceed this upper critical temperature for more than 10 h of a hot day. Even in the shade,$T_{es}$is at the upper limits of thermoneutrality for much of the day. At these temperatures the birds maintain a body temperature that is very close to their upper lethal limit. The activity pattern of Gambel's Quail on hot summer days is bimodal, with morning and late afternoon foraging separated by a long quiescent period during midday. The time of onset and the duration of this inactive period varies directly with$T_{es}$and foraging occurs throughout the day on cool, overcast, summer days. This suggests that the thermal environment constrains midday activity. A bimodal activity pattern during the winter is probably a response to predation pressure from avian raptors. During the summer, however, it is advantageous for growing juvenile quail to maximize foraging, and the thermal environment is the most important factor shaping the activity schedule.