This is just false. While there are more media sources with a liberal bias, conservative sources, including Fox have a stronger bias.
Although this kind of thing is extraordinarily hard to measure, and the studies rely on tracking viewing habits, so really, what they are measuring is confirmation bias.
This study is from 2014, but it is still the most widely cited as far as I know.
http://www.businessinsider.com/what...et-says-about-your-political-ideology-2014-10
I would love to see some good linguistic studies done on media sources. For example, how many adjectives and adverbs does a source use versus nouns? How many informal terms (such as Obamacare) versus formal terms (like the ACA). How many qualifying terms are used like reportedly and speculate...
These things would paint an interesting picture of media bias.