According to NPR on Dec. 3, the mice are dead and have been laced with the painkiller acetaminophen. The U.S. Department of Agriculture's Animal and Plant Health Inspection Service has been experimenting with ways to reduce the estimated 2 million brown tree snakes. According to the Washington Times, the snakes made their way onto the island in the '50s and they are now responsible for millions of dollars in damage to electrical equipment at the Andersen Air Force Base.
The reason the mice are parachuted to the snakes is because researchers are trying to reach the snakes, which mostly stay in trees, and also to keep the toxic mice away from other animals. Though, the amount in the mice that is toxic to snakes, shouldn't be enough to harm the other animals.
In a previous report, NPR states that if the snakes traveled similarly about boats to Hawaii, it could lead to increased medical incidences and overall lead to a decrease in tourism.