Lyric, Tetrameter and Trimeter
"Sympathy" is a lyric poem, since it gives us a glimpse into the speaker's thoughts and emotions. Even though a lot of this poem describes what the caged bird feels, we can understand it as a lyric because the speaker identifies so closely with the caged bird's pain. After all, the poem begins with the words "I know what the caged bird feels." So the speaker gives us an insight into the bird's feelings and pain in order to give us insight into his own feelings and pain.
Form-wise, the meter of this poem is generally written in tetrameter, with varying iambic and anapest stresses (though there are variations). What does that all this jargon mean in practice? Good question, Shmoopers.
For starters, it means that most of the lines of the poem are divided into four feet, or a "tetrameter" ("tetra" means four). However, the last line of each stanza is only three feet long, or a "trimeter." Let's have a look at the first stanza of the poem as an example:
I know what the caged bird feels, alas!
When the sun is bright on the upland slopes;
When the wind stirs soft through the springing grass,
And the river flows like a stream of glass;
When the first bird sings and the first bud opes,
And the faint perfume from its chalice steals—
I know what the caged bird feels! (1-7)
Note that, in the first six lines, there are four stressed syllables per line. These are each accompanied by either one unstressed syllable, or two unstressed syllables. So each of these first six lines is divided into four feet that are either iambs or anapests. The last line of the stanza has only three stressed syllables. It's made up of two iambs and one anapest. So because there are only three feet in this last line, it forms a "trimeter."
In addition to this regular rhythm, another thing to notice, of course, is the rhyme scheme: ABAABCC, where each letter represents that line's end rhyme. The first and third stanzas share the same rhyme scheme, while there's just a slight variation in the second stanza, which features two end rhymes instead of the usual three. The effect is even more confining in terms of the poem's form.
So, what's behind Dunbar's form choices here? We'd say it has a lot to do with imprisonment. The regular form, the consistent meter, the strict pattern of the rhyme scheme—all of these things create a subtle, but persistent, sense of confinement. It's as though we're trapped in the cage right along with that poor bird. Based on Dunbar's goal of articulating sympathy, we'd say he's doing all this on purpose, locking us in and throwing away the key.