I have just finished reading chapter I of this useful book: The Nature of code. At the end of the chapter there is a challenging exercise: creating a word with different creatures that move around with different patterns.
The sketch I have made this time is a sort of sky where there are two kinds of creatures: flies and birds.
Flies go around with sudden changes in their movement, birds are have more coherent flights but when they get close to flies they try to catch them.
The code is hosted on openprocessing: http://www.openprocessing.org/sketch/84168