Question about L5

Chris, please help me.  I didn’t figure out the sentence. In line 54, why was it that birds should sing so beautifully and bring joy to whole families, only to end as a heap of feathers feet up, on the bottom of the cage?   Although I read the key poin you wrote, I still didn’t understand the meaning of the sentence. Thank you.

-->Why should birds that sing beautifully and make whole families happy have to die and end up lying with their feet up in the air, on the bottom of the cage?
為什麼能唱出那麼優美歌曲，且帶給全家人快樂的鳥，結果卻變成兩腳朝上的一團羽毛躺在鳥籠底部呢？
the sentence underlined refers to "it" (同位語)