According to the report, 27% of smartphone traffic arrived direct; 16% arrived from email; and 14% arrived from referrals.
The Index also revealed that Apple devices accounted for 59% of all smartphone visits in the quarter. Visits from users running iOS 8 grew 30%, while iOS 7 dropped 42% from the prior quarter. Google is scheduled to change its search ranking algorithm to weight mobile-optimized sites more heavily.
"Search is the leading source of traffic to retailers' smartphone sites," said Chris Mason, Branding Brand CEO and co-founder. "Retailers need to focus on providing mobile friendly experiences that not only convert but can also get found in the first place."
The 100 mobile-optimized sites in the study received a total of 280 million visits from January 1 to March 31, 2015 and produced $233 million in revenue. During this time, 28% of all online revenue was generated by mobile devices.