Q1."that live in South American forests"用来修饰birds说明这类鸟栖息在南美森林,with bright red feathers作为定语修饰birds,说这类鸟是“亮红色的羽毛”
Q2.In the past children were content to entertain themselves with kitchen pots and balls of twin; Now they want high-priced electronic toys.
这样两个句子形成平行比较.
In the past vs Now
Children vs They
kitchen pots and balls of twin vs high-priced electronic toys
Q3.In regard to ：about 关于,而 in regards to 不存在；平时记短语时要注意单复数哦!
eg.In regard to your request for information,I regret to inform you that I am unable to help you.
关于你需要情报的事,我遗憾地告诉你我没法帮助你.
Q4.“mountain lions are one of the most formidable wild creatures native to North America”
因为lions作为creatures是可数的,所以用one of 是不妥的.
而"rivers are one of our most important natural source."中rivers“河流”作为natural source“自然资源”强调的是一种资源,资源不可数.
因此,不同的用词,其用法也不一样,尽管子面上看去很相似.