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Spring Battle
Male robins may fight vigorously to establish and defend territories. They can fight with their beaks or wings, but it's safest to use their feet. The wing bones are hollow, so it's dangerous to strike anything with the wings. A robin will attack with its beak if it senses a real threat of physical harm, rather than just a territorial challenge. Using the feet to fight off foes is the safest way to say "stay away!"

Robin: Defending Territory
Andy Wilson