How does the Mockingbird and moa symbolize Stargirl/Susan?

In thesechapters 27/28 we see Susan clearly becoming Stargirl again. Her behavior is becoming spontaneous again. She also shows her reverence for the earth in two ways: First when she practices her speech in the desert to the cacti. Second when she calls her speech "I Might Have Heard a Moa." The Moa is a large extinct, flightless bird, resembling the emu. She is asking the audience to listen to the past, respect it, but she is also saying that if we try hard enough we can hear things that have been lost. Mockingbirds also represent the past. They imotate calls of birds long extinct: when a mockingbird calls out they are "pitching fossils into the air."