How does Joyce Carol Oates use imagery in Four Summers?

Oates uses symbolic imagery to emphasize the squalidness of Sissie's childhood years and her disdain for the kind of life her parents led. She uses this imagery to illustrate Sissie's situation, drawing an implicit connection between Sissie and the blackbird trapped in the lake's polluted waters at the end of the first section, writing, "The bird's wings keep fluttering but it can't get out. If it could get free it would fly and be safe, but the scum holds it down."