  Gravitational wave interferometers have proved the existence of a new class
of binary black holes (BBHs) weighting tens of solar masses and they have
provided the first reliable measurement of the rate of coalescing black holes
(BHs) in the local universe. On another side, long gamma-ray bursts (GRBs)
detected with gamma-ray satellites are believed to be associated with the birth
of stellar mass BHs, providing a measure of the rate of these events across the
history of the universe, thanks to the measure of their cosmological redshift.
These two types of sources, which are subject to different detection biases and
involve BHs born in different environments with potentially different
characteristics, provide complementary information on the birth rate of stellar
BHs. We compare here the birth rates of BHs found in BBH mergers and in long
GRBs.
  We construct a simple model which makes reasonable assumptions on the history
of GRB formation, and which takes into account some major uncertainties, like
the beaming angle of GRBs or the delay between the formation of BBHs and their
coalescence. We use this model to evaluate the ratio of the number of stellar
mass BHs formed in BBH mergers to those formed in GRBs. We find that in our
reference model the birth rate of stellar BHs in BBH mergers represents from
few percent to 100% of the rate of long GRBs and that comparable birth rates
are favored by models with moderate beaming angles. We briefly discuss this
result in view of our understanding of the progenitors of GRBs and BBH mergers,
and we emphasize that this ratio, which will be better constrained in the
coming years, can be directly compared with the prediction of stellar evolution
models if a single model is used to produce GRBs and of BBH mergers with the
same assumptions.
