Journaling Question: Digesting the Information Question: How do you think longer intestines help Yellow-rumped Warblers and Tree Swallows? Discussion: Plants have cell walls, which animals don't have. The cell walls are made of cellulose, and protect the nutritious food within the cells. Mammals that eat plants have sturdy teeth, sometimes complicated stomachs, and long intestines to break down those strong cell walls. Birds lack teeth, so no birds eat grass and only one species (the Hoatzin of South America) eats leaves. Most warblers and swallows eat only insects, which lack cell walls so are easy to digest. Yellow-rumped Warblers and Tree Swallows DO eat berries and some seeds when very hungry, and need longer intestines to help them break down the cell walls of this food. Copyright 2004 Journey North. All Rights Reserved. Please send all questions, comments, and suggestions to our feedback form