The vast majority of mobile video viewing now takes place over Wi-Fi networks, according to the latest quarterly report from mobile video ad network Rhythm NewMedia.

The firm said 70% of its videos were viewed across Wi-Fi connections in the second quarter, up from 51% a year ago. That could be a result of various factors including the proliferation of Wi-Fi networks, wider adoption of tablets (used mostly at home), and people shifting video-watching from wireless networks to Wi-Fi to avoid incurring heavy data charges.

Whatever the case, the trend likely correlates with growing video consumption on devices. The Rhythm report pointed to a Cisco forecast projecting that video will account for 58% of all mobile traffic this year, up from 52% in 2011. Tablets are playing a role in that upward trajectory.

In its first-quarter report, Rhythm noted that people prefer tablets to smartphones for video, watching 50% to 175% more videos on the former than the latter. In the new study, it referred to the tablet audience as “heavy video viewers,” more than half of who watch programs or clips on a tablet more than once a week.

As prior research has noted, tablets are also more likely to be shared devices than smartphones. More than half (56%) of tablet users surveyed by Rhythm in the third quarter had 2 or more users. Multiple users within a family likely add up to more video viewing overall than a single user of a tablet or smartphone.

When it comes to advertising, virtually all campaigns (92%) running with Rhythm include in-stream video ads, which it defines as a commercial break in full episodes or before video clips. In-stream ads are 80% more likely to be noticed than those served in other contexts such as the launch of an app or between screen changes.

 In-Stream ads on Rhythm’s network made up more than 200 apps and sites owned by publishers such as NBC Universal, ABC, IAC, and Warner Bros, had an average completion rate of 89% compared to 68% online. The majority of campaigns (58%) included some type of custom button, such as for getting more product information or social sharing, in the ad creative.

Rhythm said an even larger share of campaigns (72%) now feature full-page ads on smartphones and or tablets, and that 90% include some type of display advertising. Combining video and display leads to higher engagement rates, according to the company’s research.

On the media side, entertainment-related content continues to prove especially sticky. Of users visiting a mobile entertainment news destination today, 45%-60% would have visited yesterday. 

The Rhythm findings are based on ads served in the U.S. across iPhone, iPod Touch, iPad, Android and other devices in the first quarter. More than 200 brands in 2012 so far have run campaigns with Rhythm including Unilever, McDonald’s, Disney, Paramount, General Motors, Ford, Verizon and Marriott.