Hummingbird torpor in context: duration, more than temperature, is the key to nighttime energy savings

Torpor is an important energy saving strategy in some small birds, but it has rarely been studied in natural field conditions. We compared torpor use across 43 wild-caught individuals of eight hummingbird species across sites with different natural temperature regimes. Most laboratory studies focus on the relationship between metabolic rate and temperature, but our aim was to evaluate what environmental factors most influence hummingbird nighttime energy management under natural conditions. We found that the probability of an individual entering torpor was weakly correlated with mass but unrelated to nighttime temperature and that hummingbirds at both warm, tropical and cooler, temperate sites used torpor. Energy savings in torpor were maximized as ambient temperatures approached a species' minimum body temperature, consistent with laboratory studies; energy savings ranged between 65-92% of energy per hour in torpor compared to normothermy. However, regardless of the degree of energy savings in torpor, variation in total nighttime energy expenditure was most significantly influenced by torpor bout duration. Lab studies largely assess the effect of temperature on torpor use, but our findings indicate that other environmental conditions are more important in determining hummingbirds' total nighttime energy expenditure under natural temperature cycles. Our results show that a small endotherm's nighttime energy management in its natural habitat is more affected by torpor bout duration, which is linked to photoperiod, than by temperature. This result suggests that in their natural environments hummingbirds are able to save energy in torpor across a range of nighttime temperatures, indicating that they may have sufficient physiological flexibility to tolerate climatic variation.