Free mobile apps using third party services to display ads are draining users' batteries, researchers from Purdue University and Microsoft say.
The team monitored the power use of Android and Windows Phone devices using a tool called Eprof, and found that up to 70 percent of the power used by free apps was caused by downloading advertisements and tracking users.
In the case of Angry Birds, just a fifth of the power use was caused by actually playing the game, with almost half accounted for by location services used to target advertisements.
In addition, the team found that the game left 3G connections open for up to ten seconds after downloading information, accounting for over a quarter of the app's total energy consumption.
But other apps were almost as bad, with the team finding energy wastage on all 21 apps examined, including browsers, newspaper apps and Twitter.
There is a solution, say the researchers.
"Most of the energy in smartphone apps is spent in I/O, and I/O events are clustered, often due to a few routines," they say in their report.
"This motivates us to propose bundles, a new accounting presentation of app I/O energy, which helps the developer to quickly understand and optimize the energy drain of her app. Using the bundle presentation, we reduced the energy consumption of four apps by 20 percent to 65 percent."