About This Project

When hummingbirds use more energy than they consume, they use deep hypothermia (torpor) at night to lower energy costs. Torpor is not restorative like sleep so extended use can have physiological consequences. Our field work suggests hummingbirds might be capable of controlled shallow hypothermia. This would be a novel tool for periods of low energy intake. We will use thermal imaging to track nighttime body temperature to see if hummingbirds use the more restorative shallow hypothermia.

Ask the Scientists

What is the context of this research?

We have measured hummingbird metabolic rates of both torpid and non-torpid hummingbirds the past two years in Arizona and Ecuador. One striking result was that in both locations some hummingbirds exhibited metabolic rates that were intermediate between torpor and non-torpor. Since nighttime metabolic rate and body temperature are connected, we viewed this as evidence that hummingbirds might be capable of regulating their body temperature a few degrees below their normal body temperature. If true then shallow hypothermia could be used for short periods of time to "sleep" and engage in physiological restoration. During deep torpor, body temperature is so low that this physiological restoration likely does not occur.

What is the significance of this project?

Extensive deep torpor use by hummingbirds has consequences. Torpor is not restorative, so its use for several consecutive nights could disrupt normal physiology resulting in poor condition. In mammals, torpor includes periodic arousals necessary for restorative sleep. Periodic arousals are not an option for hummingbirds because torpor is short-term (night only), and rewarming costs of an arousal would negate overnight energy savings. Use of controlled shallow hypothermia is more compatible with normal sleep, devoid of the rewarming costs of deep torpor, and still provides some energy saving. Broader importance includes understanding hummingbird energy management in response to increasing temperature due to climate change.

What are the goals of the project?

We will measure body-temperature regulation by hummingbirds at night to determine if they are able use shallow hypothermia in addition to deep torpor. While our experiment will involve captive hummingbirds in large cages, the measurements involve non-invasive thermal imaging of individual birds throughout the night. Measurements will be made outdoors so that hummingbirds will experience normal light and temperature variation. This is critical because previous highly controlled laboratory studies of hummingbird torpor might have masked any shallow hypothermic response. Our field site is in Arizona where we can study both territorial and non-territorial species to see if foraging strategy plays a role in how hypothermia is used to manage energy.