MATH PUZZLE: Can you replace...

Deep within a forest a little turtle began to climb a tree. After hours of effort he reached the top, jumped into the air waving his front legs and crashed to the ground. After recovering, he slowly climbed the tree again, jumped, and fell to the ground.
The turtle tried again and again while a couple of birds sitting on a branch watched his sad efforts. Finally, the female bird turned to her mate.
“Dear,” she chirped, “I think it's time to tell him he's adopted.”