Discussion of Challenge Question #3:
"How much time does a female hummer spend incubating her eggs on June 1 in Lansing?"

When you clicked on the photo, you learned that on June 1 in Lansing, Michigan, the sun rises at about 6:02 a.m. EDT and sets about 9:09 p.m. EDT. Daytime in Lansing on June 1 is 15 hours and 7 minutes, and the mother spends about 75% of the daylight hours incubating and she sleeps on the nest at night. Seventy-five percent of daytime is about 11 hours and 20 minutes. Nighttime lasts 8 hours and 53 minutes. That adds up to a total of about 20 hours and 13 minutes incubating on June 1.

Discussion of Challenge Question #4:

"Why do tiny birds like hummingbirds and chickadees lose body heat so much more easily than large birds like ravens and loons?"

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, which means 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.

Discussion of Challenge Question #5:
"Why are lichens, bud scales, thistle and dandelion down, and spider silk perfect building materials for hummer nests?"

Lichens are good hummer nesting materials because they are tiny, strong for their weight, easy for a tiny hummingbird to manipulate, and waterproof. Because they provide good camouflage against branches, lichens are used mostly on the outside of the nest.

Thistle and dandelion down are soft, waterproof, good insulation, and easy for hummers to manipulate. Dandelion down is easy for a hummer to find just about anywhere! This material is too white to use on the outside of the nest, but it's perfect on the inside where it is soft against the baby hummers' fragile bare skin.

Spider silk is a good hummer nesting material because it is easy to find, very strong, sticky enough to hold lichens and down together, waterproof, good camouflage, easy for a hummer to manipulate with its tiny beak, and--best of all--it's the stretchiest material available. The stretchiness allows the nest to grow with the babies.

Discussion of Challenge Question #6:

"Why do hummingbirds almost always lay exactly two eggs?"

One egg is not the best number for hummingbird nests. If anything happens to one baby or the egg doesn't hatch, all the work and energy the mother put into building the nest and incubating will be lost. But more than two eggs take too much energy from the mother. She would have to sacrifice more body heat to the eggs, and so much food to feed three babies, that she might grow too weak before they were ready to be on their own. A hummer nest with three babies could possibly raise all the babies successfully if weather conditions were great and food abundant--but it's a very big risk.

Discussion of Challenge Question #7:
"Why do you think the eggshell looks so thin and papery compared to a chicken egg?"

Hummingbird nestlings don't have an "egg tooth" to help them break out of their shell. Luckily the shell is so easy to open!