Hummingbirds are tiny birds weighing just a few grams. The smallest hummingbird, the bee hummingbird, weighs less than 2 g while the largest, the giant hummingbird, can weigh up to 24 g.
They have the highest metabolism of all warm-blooded animals to help regulate their body temperature and to support the rapid beating of their wings as they hover and fly.
You may have heard that hummingbirds need to consume as many calories as humans to maintain their active lifestyle, or about 1,800 calories a day. Some sources put the figure even higher at anything between 6,000 and 25,000 calories a day.
But these figures are wrong, and here’s why.
Let’s look at one species of hummingbird to help us work out the real number. The rufous hummingbird has a basal metabolic rate (BMR) – the rate of energy the body needs at rest to maintain its vital organs and survive – of 1,600 kcal/kg/day.
This means that for every kg it weighs it needs to consume 1,600 calories a day. This may sound a lot, but the rufous hummingbird is tiny and weighs just 3.5 g, or 0.0035 kg.
To find the BMR relative to its mass you need to multiply the number of calories by its weight in kg. And that gives you a figure of just 5.6 calories a day.
Of course, that number assumes the rufous hummingbird does nothing but rest all day and doesn’t move about.
Returning to the previous wild estimates, let’s have a look at how much food a hummingbird would need to eat to consume those many calories.
We know that a gram of nectar, the food that makes up the majority of a hummingbird’s diet, contains about 4 calories.
Even if we take the most conservative guess of 1,800 calories a day that would mean a hummingbird would need to eat 450 g of nectar a day, or about the same amount as in a bag of sugar. And if the oft-quoted 12,000 calories a day was correct this would mean these tiny birds would need to work their way through nearly 7 bags of sugar a day!
Hummingbirds actually only consume about the equivalent of their bodyweight in food each day. Even if the rufous hummingbird was eating pure sugar and no water, insects, spiders, or fruit, the most it could possibly consume is about 14 calories a day.
The extra energy it takes in over that required for its BMR is used for its daily activities like flying, feeding, migration, and breeding.
14 calories may not seem as impressive as 1,800 calories, but if humans had the same metabolism as a hummingbird, then a 12 stone man (76 kg) would need to consume a whopping 121,600 calories a day, or 875 cans of Coke, just to stay alive.