What allows hardy coastal populations of the Yellow-rumped Warbler to feed on winter berries when insects are scarce?

Answer

Specialized high levels of bile salts

Yellow-rumped Warblers in coastal areas can process waxy coatings on winter berries, such as bayberry and juniper, due to specialized high levels of bile salts in their digestive systems.

What allows hardy coastal populations of the Yellow-rumped Warbler to feed on winter berries when insects are scarce?

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