May 11, 2001: Happy Birthday YET AGAIN!

All four eggs have hatched! Compare Julie's robin babies to some baby hummingbirds Why do you think robins lay at least twice as many eggs as hummingbirds? Think about their weights:




Number of babies



Average weight of each baby when hatched

5.5 grams

0.62 grams

Average weight of mother

75 grams

3.5 grams

Q. What percentage of a mother robin's body weight is her brood of newly-hatched babies? What percentage of a mother hummingbird's body weight is her brood? Do you think this may be related to the reason that robins lay four or five eggs while hummers lay two?

Photo by Julie Brophy

Discussion of Previous Questions

Q. What is a reason why most birds don't grow feathers over their whole bodies but only in tracts?

Except for a very few birds like penguins, who swim in VERY cold water, most birds don't need to grow feathers over their whole bodies. One individual feather is a lot wider than one hair, and can cover a bigger space on a bird's body than one individual hair can cover on a mammal's body. But feathers take a lot of energy to produce. To save energy, birds produce just enough outer feathers to completely cover their bodies when the feathers are groomed properly. These robins will each have about 2200 to 2600 feathers when full grown, and add more in the fall for winter insulation.

See the next Robin Nest Photo Lesson and discussion of today's questions.