  Digital news outlets rely on a variety of outside contributors, from
freelance journalists, to political commentators, to executives and
politicians. These external dependencies create a network among news outlets,
traced along the contributors they share. Using connections between outlets, we
demonstrate how contributors' publishing trajectories tend to align with outlet
political leanings. We also show how polarized clustering of outlets translates
to differences in the topics of news covered and the style and tone of articles
published. In addition, we demonstrate how contributors who cross partisan
divides tend to focus on less explicitly political topics. This work addresses
an important gap in the media polarization literature, by highlighting how
structural factors on the production side of news media create an ecosystem
shaped by political leanings, independent of the priorities of any one person
or organization.
