The energy animals spend in a day can be a useful currency to link their daily ecology to their evolutionary history. Animals need to maintain energy
balance to survive, deriving their daily energy requirements either from the environment or their energy stores. Maintaining daily energy balance is especially
challenging for hummingbirds, which are at the small size extreme for endothermy and use an expensive foraging method—hovering—that drives their energy
expenditure up. Unlike many larger endotherms, hummingbirds do not rely on endogenous energy stores through the day, so they must forage frequently, causing
activity costs to dominate their energy budget. By modeling their daily energy budget, I found that activity is indeed the largest component of their energy
budget (relative to thermoregulation, nighttime energy expenditure and basal metabolism). These high activity levels, and high muscle function, rather than
factors more impacted by surface area (e.g. thermoregulation), not only drive an individual’s daily energy budget but they also appear to determine the
allometric scaling of daily energy expenditure in hummingbirds. I found, by assessing 17 species over eight of the nine hummingbird clades, that hummingbird
daily energy expenditure varies with body-mass (M) as daily energy expenditure = 2.04*M0.95. This unusually high exponent (0.95) can be explained by their high
daytime activity levels, relative to all birds (0.67), whose scaling is likely influenced largely by surface-area related heat and water loss. Their high
exponent also hints that they are perhaps released from some of the evolutionary constraints that other birds face.

Even though nighttime energy expenditure is only a small component of daily energy expenditure, it can be a limiting factor to survival, because with no
nighttime energy intake, hummingbirds need to carefully ration their limited nighttime energy stores. I discovered that hummingbirds can manage NEE on a finer
scale than originally thought, by using a combination of shallow and deep torpor, while other bird species have only been described to use one or the other.
Hummingbirds’ ability to use torpor at varying levels, and the flexibility in their daily energy budgets, suggest the importance of even small adjustments in
energy expenditure. Collectively, these data suggest that hummingbirds’ energetic demands in their nectarivore niche require constant and precise management that
might make them flexible to rapid and short-term environmental changes, but could make them susceptible to prolonged environmental shifts.