When discussing a character's fear or hesitation, focus on both the internal feelings (lack of confidence, self-doubt) and the external factors (the vast sea, the high ledge) that contribute to their state of mind. This provides a more complete answer.
The young seagull was afraid of flying because he lacked confidence in his wings. He believed that his wings would not support him over the vast, intimidating sea below. Despite his siblings flying fearlessly with shorter wings, he was overcome by fear and couldn't muster the courage to take the plunge from the ledge.