The mockingbird's singing at the beginning of Chapter 28 serves to foreshadow the events to come later that night. The mockingbird serves at least two purpose: First, the "mocker," known for its ability to emulate the calls of other birds, lays out a series of warnings to the children. The bird is not singing beautiful music on this night; instead, it gives the "shrill kee, kee of the sunflower bird," followed by the "irascible qua-ack of a bluejay," and then the most ominous of all--
... the sad lament of Poor Will, Poor Will, Poor Will.
Secondly, the mockingbird symbolizes Boo Radley, one of the human mockingbirds of the story, who is looking down upon the children and watching them as they head unescorted to and from the school. The bird keeps watch from one of the trees on the Radley property, just as the hidden Boo must have done.