why are there birds in airports? are they there on purpose or do they just wander in? is this a psychology thing? where have they been for my past 16 years of flying?

jack of all trades, master of some
hungarian
i am personally responsible for the following horrible things here:
why are there birds in airports? are they there on purpose or do they just wander in? is this a psychology thing? where have they been for my past 16 years of flying?