It 是形式主语was calculated是谓语 后面的that从句是主语从句作真主语。其中
when it population reached its highest point是从句中的状语从句 ,they were more than 3 billlion passenger pigeons是从句中的主句—a number是同位语 equal to 24 to 40 percent of the total bird population in the United States是形容词短语作后置定语, making it perhaps the most abundant bird in the world. 是现在分词短语作非限制性定语修饰前面从句中的主句。
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