A woodpecker caused the deep narrow hole in the nut labeled "A". With their sharp, narrow bill, woodpeckers punch a hole through the shell and feed on the kernel inside. Woodpeckers carry off small native pecans and cache them in tree voids for later consumption. Since they can't carry off large, thin-shelled nuts they simply take a quick bite before getting back to work stealing native pecans.
Mice will climb up into trees and start gnawing into pecans as soon as the shucks split open. You can always spot a mouse damaged nut by the neat round hole they chew into the side of the shell (pecan "B"). They usually make the hole just large enough to poke their head into the shell and clean out every morsel of pecan kernel.
A nut with a large, irregularly-cracked hole was attacked by a bluejay or crow (pecan "C"). These birds have large beaks that they use to pound open the shell. Often the nuts are broken in several pieces but they always eat the entire nut kernel inside.
Wildlife can eat a significant proportion of a pecan crop so its always important to harvest as quickly as possible. The list of critters that eat pecans include not only the three pests described above but also includes squirrels, raccoons, deer, coyotes and turkeys.
