A great item of polish that could be added to your game is a particle system. Particle systems can be designed right in SpriteBuilder with the fully featured particle system editor. If you are unsure of what a particle system is, then the best way to learn is by having a look.

In your Flappy Bird project, create a new file. Select Particles and name it FlapParticles.

You should see a new file open up with what looks like a flame in the center of the screen:

This is a particle system. Particle systems allow the spawning of many small sprites that behave in defined ways. You can set many property ranges and each particle that is spawned will be randomly assigned a value in that range. This allows for the creation of some really great effects. The best thing about creating particle systems in SpriteBuilder is that you can see the changes straightaway. This allows for great fine tuning of the effect.

The following are the properties you can edit and their effects: