The hobby of a woodpecker to peck a tree is related to his diet. For insect-eating woodpeckers, they peck a tree to split the wood open. From the sidelines of the wood that usually there are insects. The woodpecker then uses his tongue to catch the insect.
For pineapple-eating woodpeckers, they pecked trees to make holes. After that, the pine fruit is inserted into the hole and cleaved for edible seeds. As for oak woodpeckers, they pecked trees to sap the tree can drip. After the sap drips, then they suck the sap.