  We estimate the fraction of AGNs hosted in starburst galaxies (f_bursty) as a
function of the AGN luminosity predicted under the assumption that starburst
events and AGN activity are triggered by galaxy interactions during their
merging histories. The latter are described through Monte Carlo realizations,
and are connected to star formation and BH accretion using a semi-analytic
model of galaxy formation in a cosmological framework. The predicted fraction
f_bursty increases steeply with AGN luminosity from <0.2 at L_X < 10^44 erg/s
to >0.9 at L_X > 10^45 erg/s over a wide redshift interval from z=0 to z=6. We
compare the model predictions with new measurements of f_bursty from a sample
of X-ray selected AGNs in the XMM-COSMOS field at 0.3< z< 2, and from a sample
of QSOs (L_X > 10^45 erg/s) in the redshift range 2< z< 6.5. We find
preliminary indications that under conservative assumptions half of the QSO
hosts are starburst galaxies. This result provide motivation for future
systematic studies of the stellar properties of high luminosity AGN hosts in
order to constrain AGN triggering mechanisms.
