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Minus 3: Baby Cradle This nest is just big enough to hold two tiny eggs and the mother hummer. The eggs are snuggled in by the nest's soft sides and bottom — and also by the mother's tummy. She must warm her eggs with body heat. Q: Why do tiny birds like hummingbirds and chickadees lose body heat so much more easily than large birds? (Answer below photo.) Photo: Dorothy Edgington A: The amount of heat a warm-blooded animal produces is related to its body's volume. The heat is produced by the animal's muscles and blood and organs. The bigger the volume of the body, the more of these heat-producing tissues fit inside. How much heat a body loses is related to its surface area: how much skin it has where heat leaks out. Fur or feathers help hold heat inside a body, but some heat always escapes. (When you touch your face, you can feel the heat your body produced leaking out into the air, or onto your hand.) This is why tiny birds need to shiver so much more than larger birds. Shivering makes their muscles do extra work to produce more heat than larger birds need. And it's why hummers build such a tiny, tight nest. A tiny nest holds in their body heat so the babies inside their eggs can warm up and develop without wasting the mother's heat and energy.