The Evolving Pigeons are part of the Creatures 2 Life Kit 2. They are critters and eat seeds from a vendor which vends both the pigeons and their food. They were converted to C3 and DS as the Balloon Pigeons.
When the vendor is first pressed, it will release 10 pigeons. It also vends seeds for the pigeons to eat; this can be set from 0 to 10, and the vendor will keep that many seeds in the world at a time. The pigeons will fly around the vendor, trying to eat the seeds. Each pigeon has an energy level which is replenished by eating a seed, and they also have their own specific flying speed, randomly assigned when it is created. Faster pigeons will deplete their energy faster. If a pigeon's energy level gets low, it will become visibly thinner. If a pigeon runs out of energy, or runs into water, it will die. As pigeons die, the vendor will create new ones to keep the population at 10.
The idea behind the 'evolving' aspect is that over time, the population will tend towards a speed that suits the number of seeds being vended at once.