  We consider a cosmological scenario where a relativistic particle and a
stable massive particle are simultaneously produced from the decay of a
late-decaying particle after Big-Bang Nucleosynthesis but before
matter-radiation equality. The relativistic and massive particles behave as
dark radiation and warm dark matter, respectively. Due to a common origin, the
warmness and abundances are closely related. We refer to the models that lead
to such a scenario as Common Origin of Warm and Relativistic Decay Products
(COWaRD). We show that COWaRD predicts a correlation between the amount of dark
radiation and suppression of the large scale structure, which can be tested in
future precision cosmology observations. We demonstrate that COWaRD is
realized, as an example, in a class of supersymmetric axion models and that
future observations by the next generation Cosmic Microwave Background, Large
Scale Structure, and 21-cm surveys can reveal the structure of the theory.
