Why does Second Brother beat Adeline

The children received pet ducks from close family friends, and Adeline took great care with hers.... she treasured it. After digging up a number of worms to feed Precious Little Treasure (the duck's name), she went up to the roof to feed him. In the process of feeding PLT, she shooed the other ducks away and Second Brother became angry with her. His abuse was brought on the fact he believed she should favor his duck ahead of her own.