what is the subject of the sentence...Two lost species are the passenger pigeon and the carolina parakeet.

Answer 1

There are three nouns and no pronouns in this sentence. Which noun do you think is the subject?

species
pigeon
parakeet

(If you think that passenger and carolina are nouns, too, you're right in a way, but they are being used as adjectives in this particular sentence.)

Let us know what you think.

Answer 2

http://grammar.ccc.commnet.edu/grammar/subjects.htm

http://www.uottawa.ca/academic/arts/writcent/hypergrammar/rvsubj.html

http://www.ucalgary.ca/UofC/eduweb/grammar/index.htm

=)