Meeting Abstract
Hummingbirds have a high daily energy requirement that makes energy storage difficult and balancing their energy budget daily a necessity. When daily energy intake falls below demand, hummingbirds can use torpor (nocturnal hypothermia) to balance their energy budget by reducing nighttime energy expenditure. Broad-billed Hummingbirds (Cynanthus latirostris) in SE Arizona used torpor less frequently when nighttime temperatures were well above (~8 °C) their minimum body temperature, suggesting that higher nighttime Ta might reduce the energetic benefit of torpor use. We tested the broad impact of higher nighttime temperature on torpor use by measuring torpor frequency in several hummingbirds at two mid-elevation sites on the western slope of the Andes Mountains in Ecuador during Summer 2014. We studied a combined 5 species at our lower elevation (1300m; mean nighttime Ta=23.4°C) and higher elevation (1900m; mean nighttime Ta=20.9°C) sites. We studied 3 species each at our low and high elevation sites. Throughout the study the difference between minimum and maximum nighttime Ta did not exceed 4-6°C at both sites. Torpor was observed in 5 of 8 (63%) hummingbirds at low elevation, and 3 of 12 (25%) hummingbirds at high elevation. Since torpor was used more frequently at the warmer of the two sites it is unlikely that temperature influenced torpor use in these tropical habitats. Instead, there may be taxonomic differences between species, or differences in energy demands between sites. It is also possible that tropical hummingbirds, which are generally resident, might be physiologically adapted to maximize energetic benefit from relatively shallow hypothermia.
