Angry Birds performs so well on these so called feature phones that it got people puzzled. I guess many people do not expect much from JavaME, therefore there had been questions whether this is actually a JavaME application or did Nokia open up its C libraries, or some other secret API to Angry Birds.
It is a well developed JavaME application! It does not even need access to APIs that are available to manufacturer/operator signed MIDlets. Below is a list of usual suspect APIs that Angry Birds uses as you can see it is a list of APIs that are available to any JavaME developer that targets Series 40.