The "mocker" mentioned in Chapter 28 is a mockingbird, pouring out its song in the various voices of other birds. The mockingbird, in addition to being a symbol of innocence throughout the novel, is also a bird capable of great mimicry. It can alter its voice to that of many different songbirds, and is known to also imitate the sounds of insects and even amphibians. On this fateful Halloween night, not realizing that it is sitting in a tree on the dreaded Radley property, the "mocker" imitates the sounds of a sunflower bird, a bluejay, and a whippoorwill. The quote has to do with the bird's dangerous position in the Radley tree, a creepy place to be considering it is a dark, windy Halloween night--and that the mysterious Boo Radley may be lurking nearby. Additionally, the author probably intends the mocker as a foreshadowing element to the noises that the children will hear later on the trail to and from school.
The sunflower bird symbolises the children, the Bluejay symbolises Bob Ewell because he is always full of anger and the whippoorwill symbolises boo radley because they only leave their nest when they are stepped on. Thsi forshadows the attack from Bob Ewell on the children.