Diurnal and nocturnal avian antipredator behavior in thermally challenging conditions

Diurnal avian antipredator behavior has been the focus of much past research, yet the influence of the thermal environment on such behaviors is often overlooked. Far less is known about nocturnal avian antipredator behavior, including how these behaviors are influenced by challenging thermal environments. The first portion of my research focused on how the thermal environment influences the diurnal antipredator behavior of wintering birds while (i) exposed to high wind speeds, (ii) foraging in sunlit and shaded microhabitats, and (iii) when using thermoregulatory postures to conserve body heat. In addition to increasing convective heat loss, high wind speeds increase the prevalence of background movements in the environment. My research demonstrated that wintering sparrows exposed to a moving stimulus are less likely to flush to cover on windy days than on calm days, suggesting that wind-driven visual noise may interfere with predator detection. Predator detection may also vary when feeding in sunlight and shade, and the thermal benefits of foraging in direct sunlight on cold winter days may also play an important role in dictating microhabitat choice. Regardless of the thermal benefits of foraging in sunlight, wintering sparrows preferred to feed in shaded microhabitats even at ambient temperatures well below thermoneutrality. However, these birds foraged in sunlight more frequently as ambient temperatures fell, suggesting a trade-off between thermoregulation (solar input) and predation risk. Additional evidence of such a thermoregulation-predation trade-off was evident in the use of heat-conserving thermoregulatory postures by wintering sparrows. Fluffing the feathers or standing on one foot will reduce the amount of heat lost to the environment. However, such postures slow take-off time and likely result in an increase in predation risk. As such, these risky postures were only used when feeding at relatively low ambient temperatures and when near protective cover. In general, these results indicate that characteristics of the thermal environment play an important role in dictating diurnal antipredator behavior.;To address how the thermal environment influences nocturnal avian antipredator behavior, I examined the predation-related costs of using energy-saving nocturnal hypothermia. Many species of birds reduce their nighttime body temperature, thus reducing metabolic rate and conserving energy. Such drops in body temperature may be quite substantial and likely influence a bird's ability to respond to a potential threat during the night. To examine the potential costs of hypothermia, I conducted nocturnal flight tests on hypothermic mourning doves (Zenaida macroura). In general, doves that cooled by more than 5 °C flew poorly or were unable to fly, but were able to fly well once re-warmed to near their normal daytime body temperatures. Thus, low body temperatures during energy-saving hypothermia likely result in an increase in the risk of nocturnal predation. Nocturnal antipredator behavior was also examined in ruby-throated hummingbirds (Archilochus colubris). These hummingbirds frequently use nocturnal torpor (i.e., deep hypothermia), with significant reductions in body temperature and corresponding inability to respond behaviorally to external stimuli. Although hummingbirds altered torpor use seasonally and over the course of the observation period, hummingbirds did not consistently reduce their use of torpor following an experimental increase in perceived predation risk. Thus, although hypothermia is behaviorally costly, further studies are needed to clarify the role of predation on nocturnal behavior in birds.