Abstract: We consider the possibility of a non-standard spectrum for the cosmic neutrino background, investigate its effect on various cosmological observables, such as the Cosmic Microwave Background or Big Bang Nucleosynthesis (BBN), and use this to place bounds on these neutrino spectral distortions. We also combine this model with additional dark radiation and investigate whether the two effects can be distinguished. We find that BBN observations can break this degeneracy, thanks to the neutrinos' weak interactions' effect on the proton-to-neutron ratio at freeze-out, and this allows us to place constraints on both phenomena in the scenario where they both occur. By doing this, we test the neutrino nature of the cosmic neutrino background, and can rule out a scenario where more than ~1/2 of its energy density is replaced by dark radiation.