  We study the shape and evolution of the star formation main sequence in three
independently developed semi-analytic models of galaxy formation. We focus, in
particular, on the characterization of the model galaxies that are
significantly above the main sequence, and that can be identified with galaxies
classified as `starburst' in recent observational work. We find that, in all
three models considered, star formation triggered by merger events (both minor
and major) contribute to only a very small fraction of the cosmic density of
star formation. While mergers are associated to bursts of star formation in all
models, galaxies that experienced recent merger events are not necessarily
significantly above the main sequence. On the other hand, `starburst galaxies'
are not necessarily associated with merger episodes, especially at the low-mass
end. Galaxies that experienced recent mergers can have relatively low levels of
star formation when/if the merger is gas-poor, and galaxies with no recent
merger can experience episodes of starbursts due to a combination of large
amount of cold gas available from cooling/accretion events and/or small disk
radii which increases the cold gas surface density.
