Aspects of the Thermal Ecology of Bobwhites in North Texas

Abstract: We studied the thermal ecology of northern bobwhites (Colinus virginianus) to better understand the role of temperature in the field behavior of these birds. We obtained descriptive data on thermal aspects of the landscape; bobwhite selection for Normalized Difference Vegetation Index (NDVI; vegetation biomass) classes and cover associations relative to their thermal properties; and thermal conditions at nests, mid-day coverts, and roosts. We collected data on a 796-ha area in the Texas Rolling Plains during May 2000-July 2003 using satellite imagery, black-bulb temperature probes, mortality- and temperature-sensing radiotransmitters, and continuous-recording video cameras for nest observations. Linear models of black-bulb temperature (T_bb) as a function of air temperature (T_a) at a base weather station explained 42–70% of the variation in T_bb in 10 NDVI classes during daylight and 78% during night in summer (all NDVI classes; Jul 2001). During February 2002, T_a explained 38–92% of the variation during day and 89% of the variation at night. The linear models provided a means of qualitatively assessing thermal space on the landscape as T_a changed and of predicting T_bb in NDVI classes. At T_a = 42 °C, 100% of the 796-ha landscape under study had predicted T_bb > 39 °C, the approximate threshold leading to hyperthermia in bobwhites. Based on 9,287 radiolocations of 217 bobwhites, bobwhites selected for all NDVI classes in mixed-shrub cover (sand plum, [Prunus angustifolia]; fragrant sumac, [Rhus aromatica]) on an annual and seasonal basis. If T_a was <35 °C, the approximate upper critical temperature, the operative temperature (T_e) experienced by bobwhites exceeded T_a in 699 of 818 simultaneous readings (n = 24 bobwhites with thermal transmitters) and the difference between T_e and T_a increased as air temperature declined. Data from video cameras indicated thermal stress (i.e., gular flutter) in 25 of 26 incubating bobwhites. Gular flutter began at an average T_a of 30.4 ± 0.2 ±C SE (n = 158) and total bouts of gular flutter averaged 87 minutes/bird/day after 16 June. Data from thermal radiotransmitters indicated 91.3 ± 6.1% of incubating adults were in thermal stress at T_a > 35 °C. Temperature of nest contents averaged about 30 °C and incubating bobwhites appeared to protect nest contents more rigorously from hyperthermia than from hypothermia. Mid-day covert selection (NDVI 6, mixed-shrub cover; n = 58) during summer reduced bobwhite exposure to T_bb > 39 °C by an average of 1,600.7 heating-degree minutes in comparison with random points (n = 58) during 1200–1600 hours. The roosting disc appeared to confer energetic advantages at T_a < 16.2 °C and the advantage increased as T_a declined. Our results were consistent with the heat hypothesis on reproduction variability because we observed thermal stress at low T_a (22.1 °C) in incubating birds and we projected lethal T_bb (53.2 °C) at high T_a (45 °C) in NDVI classes used by nesting bobwhites. Our results reaffirm that temperatures on the landscape are a legitimate concern for bobwhite managers. The goal of habitat management vis-à-vis temperature is to insure well-distributed mid-day coverts for thermal refugia in summer and ground cover of sufficient height and density to prevent heat excess in the near-ground environment.