Birds are true gas guzzlers - and most require high octane fuel. Animals take in "fuel" in the form of food. The metabolic rate is the rate at which an animal burns this fuel.

The amount of energy an animal requires depends on its body size -larger animals generally have slower metabolisms and eat less food for their size than smaller animals. Because most birds are small and have active lifestyles (flapping takes a lot of energy), they need a lot of fuel. If you "eat like a bird" you really have a healthy appetite. Rather than eating a vast amount of low-energy food that would make them heavy for flight, most birds eat smaller quantities of high-energy foods. Carbohydrates, fats and proteins provide energy.

Birds can use as much as 90% of the energy found in these foods:

Seeds are rich in carbohydrates and fats.	Most fruit is not high in protein or calories, so fruit eating birds need to spend long periods of their day feeding to get enough food.
Insects are great food for birds. They're high in protein, fats, and calories, but they are not always available. Billions of insects hatch out in the spring and summer, then die off in the fall and winter. When this happens, many insect-eaters switch to other foods or migrate to warmer climates.	Nectar- One-fifth of all bird species probe flowers for nectar. Nectar is mostly sugar and provides a fast-burning, high energy fuel. Nectar is useful for quick energy, but it is short on protein.

Other foods provide much less energy and must be eaten in large quantities. Leaves and grasses are a staple for many different mammals and reptiles, but only a handful of bird species eat leaves. Birds can digest only 30% of the energy in leaves. They can't afford to carry a lot of heavy, slowly digesting leaves in their gut, nor the weight of extra intestines needed to digest the leaves. The Hoatzin is an unusual bird because its diet consists mainly of leaves.

Classroom Activity: What's for dinner?