Perhaps it goes without saying that David Singer's things symbolize—yep—David Singer. For a while after his death, his widowed wife keeps all his possessions exactly as he left them. When she finally puts them out onto the curb, she's doing so because she's really trying to coax herself into moving on with her life.
But as Alma stares out her window at the things, the image of his discarded possessions out on the curb is placed in a wider context. The leaves spin by the pile of stuff, suggesting the earth's indifference to this one individual life. And an old man sits momentarily in David's chair, which just makes us sigh yet again at the injustice of Alma's dad having died a relatively young man. Alma fishes out her dad's sweater and wears it for forty-two days straight—literally cloaking herself in his memory, the alpaca wool no substitute for her father's warm embrace.
Later, Bird cries that his mother "should have asked me before she threw away everything that belonged to Dad" (14.5). So, if his mom is trying to escape the crushing weight of grief, and his sister is clinging to the meager connection to her father that remains, Bird's protest symbolizes his having been denied the opportunity to grieve in his own way too, since he hardly knew his father at all.