  We study the possibility of detecting dark radiation (DR) produced by a
combination of interactions with the thermal bath and ultra-light primordial
black hole (PBH) evaporation in the early universe. We show that the detection
prospects via cosmic microwave background (CMB) measurements of the effective
relativistic degrees of freedom ${\rm \Delta N_{eff}}$ get enhanced in some
part of the parameter space compared to the purely non-thermal case where DR is
produced solely from PBH. On the other hand, for certain part of the parameter
space, DR which initially decouples from the bath followed by its production
from PBH evaporation, can re-enter the thermal bath leading to much tighter
constraints on the PBH parameter space. We also discuss the complementary
detection prospects via observation of stochastic gravitational wave (GW)
sourced by PBH density perturbations. The complementary probes offered by CMB
and GW observations keep the detection prospects of such light degrees of
freedom very promising in spite of limited discovery prospects at particle
physics experiments.
