Mention food warming, and most of us conjure up images of slow Sunday roasts or convenient microwave dinners. For Chris Lotz and his colleagues at the University of Wyoming, however, this phrase holds another meaning entirely. Expert in the field of animal energetics, Lotz's most recent study reveals the high price that rufous hummingbirds pay for a warm drink. All warm-blooded animals need to heat the food they have eaten to maintain their body temperature. But when pickings are low in energy, vast amounts of food must be consumed, at a hefty energetic cost. Nectar-feeding birds are a case in point - consuming up to five times their body mass each day when feeding on low-calorie liquids. And to make matters worse, mountain-dwelling rufous hummingbirds dine on chilly nectar that is often 10°C or less.
Determined to shed light on the cost of food warming, Lotz and his team created a mathematical model to investigate the energetic costs of nectar heating in the rufous hummingbirds of the Rocky Mountains. The scientists worked out that the amount of energy required to heat nectar depends upon three things: how much nectar is consumed, the specific heat of the sugar solution and the difference between body temperature and nectar temperature. Armed with theoretical answers to questions of food warming costs, the researchers then tested the model's predictions by measuring changes in the metabolic rates of captive birds drinking nectar at different temperatures.
As forecast by their model, Lotz's team discovered that the metabolic rates of feeding hummingbirds changed dramatically with major changes in nectar temperature. Birds sipping nectar chilled to 4°C burnt up to 27% more energy than those feeding on 39°C nectar. And the costs rocketed when birds supped on low-sugar nectar - individuals fed on dilute 4°C nectar increased their metabolic rates to levels equal to those generated by a 15°C drop in body temperature. Most surprisingly, the costs of food warming were similar whether birds perched or hovered to feed.
By quantifying the influence of nectar temperature and sugar content on food warming costs, Lotz has found that these costs are an important part of hummingbird energy budgets. From this springboard, future research is likely to reveal that the cost of warming a meal is an important component in the energy budgets of other animals that feed on cold, poor-quality food. Perhaps we should all spare a thought for the humble hummingbird next time we pop a pizza in the oven.
