If you or I were to cling to a branch we would tighten our fingers from their normally relaxed state. Birds do that in reverse. These photos show an eastern bluebird and a black-capped chickadee with relaxed, dangling feet. That would be the default position for us, but not for them. As soon as the birds landed on something they could grip -- a branch, a perch -- their toes automatically would tighten around the perch to provide grip. That grip is the default position. 

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A loud fight ensued

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Why do crows harass owls?