"They were beckoning to him, calling shrilly." Why did the seagull's father and mother threaten him and cajole him to fly?
The young seagull's parents threatened and cajoled him because he was extremely afraid to take his first flight. He felt certain that his wings would never support him, so he refused to leave the safety of his ledge.
They wanted him to overcome his cowardice and learn the art of flight, just like his brothers and sister had. By threatening to let him starve on his ledge unless he flew away, they tried to push him to take the plunge. It was crucial for his survival to conquer his fear and fly.
Explanation
The parents used a mix of threats and encouragement to push the young seagull out of his comfort zone. The text explicitly states they were 'upbraiding him, threatening to let him starve on his ledge unless he flew away' and 'taunting him with his cowardice'. Their ultimate goal was to make him overcome his irrational fear and take his first flight, which was essential for his survival.