Zygodactyl Toe Arrangement
Most birds have four toes, three pointing forward and one back. A quick look in the snow beneath your bird feeder will confirm this. Exceptions to this rule include woodpeckers, owls and ospreys, among others, whose toe arrangement is 2 and 2: two of their toes (the middle two) point forward, and the outer toes point backwards. The term for this arrangement is zygodactyl.
Different species have evolved this toe arrangement to meet different needs. It enhances the ability of woodpeckers to hold onto limbs and climb up vertical tree trunks, while owls and ospreys can get a better grasp of slippery or wiggly prey. (Woodpeckers can pivot one of their back toes to the side, and owls can pivot one of their back toes forward, as well.)
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