In Chapter 27, when Stargirl (actually, it is "Susan" at this point) is on her way to her oratorical competition with Mr. McShane and Leo, she demands that her teacher stop the car. She hops right out of the car and romps around in the desert for a bit, just because she feels like it. That's where the birds come in.
While she's out there, she hears the call of a mockingbird, which reminds her of something Archie once said: "He said he believes mockingbirds may do more than imitate other living birds. He thinks they may also imitate the sounds of birds that are no longer around."
Mockingbirds are known for mimicking other birdcalls, and it's actually possible that mockingbirds have been passing imitated sounds of long extinct birds down through the generations. That's a pretty cool idea, huh? It's yet another one of Archie's reminders that we are all connected to the past and the natural world.
Hearing the mockingbirds out in the desert prompts Stargirl to ask Mr. McShane about any extinct birds he might know of. It just so turns out that Mr. McShane is a bit of an expert on the Moa, which is a huge bird that once thrived, but has since gone extinct because of humans. Stargirl notes that a man would actually be half the size of a Moa, and a little light bulb goes off in our heads: doesn't the fact that the Moa were killed off by smaller, lesser creatures sound a lot like the fact that Stargirl has allowed her awesome self to be killed off by smaller, lesser creatures like Hillari and the rest of Mica's cruel student body?