Keeping warm is costly, especially when paired with one of the world's fastest lifestyles. During the day, hummingbirds zip about feeding on sugar-rich nectar to fuel their high-octane existence, but at night, the aeronautic acrobats must conserve energy, slipping into a miniature form of hibernation, known as torpor; many drop their body temperature to around 10°C and some get even chillier. But animals that use torpor are in danger from predators, so some minimise the risk by entering a shallower version from which they can emerge more rapidly. However, it wasn't clear to Anusha Shankar (Cornell University, USA), Catherine Graham (Swiss Federal Research Institute) and Donald Powers (George Fox University, USA) whether birds are also capable of using both shallow and deep types of torpor, either to avoid incurring the costs of deep torpor or as an intermediate step along the way. To find out, Shankar with Powers, Isabelle Cisneros and Sarah Thompson, also from George Fox University, headed to the Chiricahua Mountains of Arizona, USA – home to many migrating hummingbirds during spring and summer – to find out how the energetic birds deal with cold nights.

‘We studied three species of hummingbird: tiny black-chinned hummingbirds [3 g], the blue-throated mountain-gem [8 g] and Rivoli's hummingbird [7 g]’, says Shankar, who trapped the wild birds by carefully dropping a net over them as they feasted at a feeder during the hour before sunset. Then, the team gently placed each hummingbird in a chamber positioned outside on a table next to a window, allowing the bird to settle before recording the temperature of its eye every 10 min using an infrared camera located in the window to find out how the animal's body temperature varied as it conserved energy overnight.

Monitoring the air temperature, which ranged from 3 to 23°C, the team even recorded one particularly cold night when there was frost and the mercury fell to −1°C. They then analysed the body temperature of the birds and could see that they were quite warm as they settled down for the evening, with eye temperatures stabilising between 29 and 31°C. However, as the night progressed and the air temperature fell, the birds’ body temperature began to decline also, with all of the tiny black-chinned hummingbirds (Archilochus alexandri) dropping into deep torpor for approximately half of the night, with body temperatures below 20°C. However, the larger blue-throated mountain-gems (Lampornis clemenciae) maintained their warm evening body temperature for two-thirds of the night, only allowing their bodies to decline slightly below 30°C as they fell into shallow torpor for a quarter of the night; they rarely fell into deep torpor. Meanwhile, the Rivoli's hummingbirds (Eugenes fulgens) seemed to take an equally nonchalant approach to conserving energy. Although half of the birds resorted to deep torpor, with body temperature around 10°C for ∼20% of the night, 10 of the 12 birds dipped into shallow torpor for about a third of the night.

So, hummingbirds are capable of selecting how torpid they become, suggesting that the costs associated with falling into deep torpor may sometimes outweigh the energy conservation benefits. ‘For example, torpid birds in trees might be more conspicuous than a mammal concealed in a burrow that can use deep torpor all the time’, says Shankar, explaining that hummingbirds in shallow torpor can warm up and become fully alert within a matter of seconds to evade predators, in contrast with hummingbirds that have descended into deep torpor, which may take up to half an hour to recover. ‘This makes shallow torpor more effective than deep torpor in some situations’, Shankar concludes.

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