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Mud is important!
Mud is important for building a robin nest. This robin carries mud in her beak. Her breast feathers are muddy too. Using mud lets her make the nest deep and round. She uses her chest to push and shape the nest into a nice curved bowl exactly the right size to hold in her body warmth. She wants her nest snug enough so her body heat doesn't leak out and the eggs, and then the babies, stay warm underneath her.

Image: Wayne Kryduba

Female robin with mud on her breast and a beakful of mud for building her nest.