  We test the reliability of infrared (IR) emission to trace star formation in
individual star-forming sites of M33, and outline a new method for testing the
distribution function of massive stars in newly formed clusters. We select IR
sources from the Spitzer survey of M33 and show that the IR and Halpha
luminosities are not correlated. Complementing the infrared photometry with
GALEX-UV data, we estimate the source bolometric luminosities. For a given
stellar IMF we simulate a theoretical curve for the expected
bolometric-to-Halpha luminosity ratio, along which stellar clusters are born.
We call this the cluster birthline in the Lbol--Lbol/LHal plane. The birthline
is flat for Lbol>3x10^{39}erg/s because all clusters fully sample the IMF and
it increases toward lower luminosities as the upper end of the IMF becomes
incompletely sampled. The observations of M33 show that young isolated clusters
lie close to the theoretical birthline for a wide range of Lbol. The luminosity
is not proportional to Halpha emission for low mass clusters and aging moves
clusters above the birthline. The best fit to the birthline is for a randomly
sampled IMF, in which the mass of most massive star in a cluster is not
strictly limited by the cluster's mass. We also find that the IR luminosity of
young stellar clusters in M33 is not proportional to their bolometric
luminosity. This irregularity could be the result of low and patchy dust
abundance in M33.
