What is a collector bonus?

Check your pokedex for number of pidgey caught. It is probably the catch bonus for catching a multiple of 100 of a species. It has been renamed a few times. Originally labeled just "Bonus" with no hint as to what it was. Gives 100xp, doubled for the current event.

Every 100th of the same pokemon you catch gives a 100xp bonus. It was called just "bonus" on the catch screen for the longest time, now it's "collector bonus".