|I have found only four of these|
So what next? (I have downloaded the Angry Birds "Seasons" set of level, but am waiting for the holiday to play that.) The game itself suggests I could try for three stars on every level. Now, I admit I did a few levels more than once as I went through, when it seemed I just lucked out, or when I wanted to explore an alternative solution. But, I prefer to move on once I have achieved what's needed, which here is to "clear the level" of pigs.
It's important not to spend too much time perfecting what you have done, in place of doing new things. Do something as well as it needs to be done, then move on. If you have moved on, don't go back and redo, knowing you could do it better now. (Of course you can do it better now: you've moved on! Only go back if the earlier imperfect version is proving a shaky foundation.) Polishing is much easier than progression, but is often only a displacement activity.
So, I will maybe try to get better scores on a few more levels (maybe try for no one star levels, anyway) in my "spare time", but it isn't anything like the same as marching on to new levels.