The underlying rhythm of “To a Mouse” is iambic. This means that Burns built the poem using metrical feet called iambs, which are composed of one unstressed syllable and one stressed syllable, as in the word “to-day.” Though the iambic rhythm is consistent throughout, the unique stanza form Burns used for the poem involves lines with different numbers of feet: either four feet (i.e., iambic tetrameter) or two (i.e., iambic dimeter). The so-called Habbie—or “Burns stanza”—begins with three lines of iambic tetrameter, which are then followed by four more lines of alternating iambic dimeter and tetrameter. For a clear instance of this form, consider the poem’s third stanza (lines 13–18):

     I doubt | na, whyles, | but thou | may thieve;
     What then? | poor beas- | tie, thou | maun live!
     A dai- | men-ick- | er in | a thrave
               ’S a sma’ | re-quest:
     I’ll get | a bless- | in wi’ | the lave,
               An’ ne- | ver miss ’t!

Most of the poem follows this model quite strictly. However, Burns does include some minor deviations from strict iambic rhythm. For instance, he occasionally adds extra unstressed syllables at the end of lines. Other times he adds unstressed syllables in the middle of lines. Sometimes he even adds syllables in both places, as in line 4:

     Wi’ bick- | er ing bratt- | le.

Here Burns both ends the line with an unstressed syllable, and he makes the second foot an anapest, which has an extra unstressed syllable at the beginning. These and other minor deviations bring unexpected texture to the poem’s underlying iambic rhythm.

Iambic tetrameter typically has a sing-song cadence, particularly when that rhythm is sustained for long periods. In the case of “To a Mouse,” however, Burns never features sustained passages of iambic tetrameter. Although the first three lines of each stanza are all iambic tetrameter, three lines isn’t quite enough to generate a continuous, song-like rhythm. Indeed, the ear longs to hear one more line of rhymed tetrameter to feel a sense of closure. But the shorter fourth line unexpectedly interrupts the rhythm, and that sense of interruption continues with the following two lines, which repeat the tetrameter–dimeter pattern. This rhythmic variation gives the poem greater metrical complexity than a more straightforward song would have. That is, it demonstrates a degree of poetic sophistication that some readers might be surprised to see in a work of vernacular verse.