Other research used short term fasting (a few hours before sunset) to explore how food stress can induce torpor (Bech 2007). Bech et al. discovered that torpor could be induced by such minor stresses in some species of Brazilian hummingbirds. This indicates that many species of hummingbirds may use torpor to regulate their energy stores in a much more precise manner, using torpor to prevent changes in day to day energy levels when foraging before nightfall was unsuccessful.

Other studies have used more indirect methods to analyze torpor use in wild hummingbirds. One study by Calder (1994), used differential changes in mass to measure whether or rufous hummingbirds were entering nightly torpor. This research found that rufous hummingbirds will use longer periods of torpor when their masses are lower (see figure 1). This indicates overall that when a hummingbird has been less successful in foraging it is more likely to enter torpor so that it can manage its energy reserves.

Overall, this research indicates that lower than optimum food intake can tends to induce torpor in hummingbirds. Whether starvation is involved or more complex long-term energy management, hummingbirds will enter into torpor to manage their energy reserves.

Figure 1. Noctornal torpor duration of male rufous hummingbirds, indicated by perch thermocouple, as a function of body mass at evening capture (with trendline). Adapted from Calder 1994.