They are relying majorly on radiation. Radiation as a mode of heat transfer is the transfer of heat energy from one body to another without any intervening medium between the source of heat and the body. Without intervening medium means that radiation does not requires material medium for its propagation and it occurs mainly in vacuum.
Example of radiation is the source of heat directly from the sun. Since the kittens are sleeping in a sunny spot, therefore they get their heat through radiation.
