In the questions Why not kill animals to hasten their rebirth? and Do the same rules of Karma apply to animals? and others, it in part discusses how animals have a very difficult time to be reborn as humans, and stuck endlessly in a cycle as animals. As I understand it, because they are not capable of Meritorious Deeds on there own.
However, it seems to be possible for humans to transfer some good Karma to them, which may improve their chances of a higher rebirth.
If it is possible, how is this accomplished? Take this case:
Let's say there is a housefly in my house. Of course I could just grab a fly swatter and kill it. However, I choose to take 15 minutes to wait for it to fly to the screen door and then let it out.
If this provides some good Karma for me, can it be transferred to the fly which may be helpful upon its rebirth? Or am I the only beneficiary of saving it (or at least for not killing it)?