Should

A farm boy from Colorado loved to hike and rock climb. One day while climbing in the mountains, he found an eagle's nest with an egg in it. He took the egg from the nest, and when he got home, he put it under a hen along with her other eggs. Since he hatched among chicks, the eagle thought he was a chicken. He learned chicken behavior from his "mother" and scratched in the chicken yard along with his "siblings." And when he sometimes felt strange stirrings within him, he didn't know what to do with them, so he ignored them or suppressed them. After all, if he was a chicken, he should behave like a chicken.

I wonder if the meaning of "should" in the "1" is obligation or expectation or anything else.
1. After all, if he was a chicken, he should behave like a chicken.