Final answer:
The boy in 'The Sand Castle' does not know what eagles are due to his limited knowledge and experience with birds.
Step-by-step explanation:
The most likely reason the boy in 'The Sand Castle' does not know what eagles are is because he has limited knowledge and experience with birds. Throughout the story, it is mentioned that the boy is familiar with local birds like the cat-birds, herons, and white herons. However, he is unaware of eagles, which suggests that eagles are not commonly found or discussed in his environment. This lack of exposure and knowledge about eagles is the most likely reason for his unfamiliarity with them.