Just the News reports that, according to MRC's tracking and research data, these four applications sit on nearly every smartphone in America and serve as a dominant news window for an estimated 58 percent of Americans.

According to the MRC's special report, released earlier this month, all four aggregators exhibit a heavy left-wing bias by overwhelmingly surfacing mainstream and left-leaning publications while filtering out conservative outlets.

Out of 487 top stories last February on Google News, the MRC reports that only 11, or 2.2 percent, were right-leaning; 314, or 64.5 percent, were left-leaning and 162, or 33.3 percent, were in the center.

David Bozell, who leads the Media Research Center, reported that after analyzing one month of stories on Apple News, his group found that of 620 stories that Apple had promoted on its top feeds, "not a single one of those stories was from a right-wing news source. Not one. Zero."

According to the MRC, out of the 620 top stories on Apple News last January, there were zero right-leaning ones and 440 left-leaning.

Meanwhile, articles on popular websites such as Fox News and the New York Post were never promoted.

A more in-depth "Big Four" analysis in April showed the apps promoted 92 percent left-leaning outlets and only 17 percent of right-leaning ones, according to Just the News.

"NPR was linked by Apple 155 times in six months," Bozell told Full Measure. "It was linked by Google 121 times in six months. ... How is that? NPR, National Public Radio, [is] the 20th most-popular news website in America today, whereas the New York Post is in the top 10 and Breitbart is in the top 35." Right-leaning news "gets no love from these guys."

According to Just the News, Bozell says the audience is left with little choice because, for example, Apple News is preloaded on iPhones while Google News is preloaded on Androids.

Bozell called for news consumers speak up and demand a more diverse mix of information from the Big Four news apps.