Why do birds clean out gutters?
Birds clean out gutters because they want to put gutter cleaning professionals out of a job...
...In all seriousness, birds and other wild animals find gutters an ideal source of shelter and sustenance. Guttering can provide adequate protection from predators, while also harbouring moss, which provides insects for birds to eat. Unfortunately, moss also absorbs rainwater and slows run-off - making it unwelcome in your system.
Remember, in no circumstance can you remove an occupied bird’s nest from your guttering system - it could result in a six-month prison sentence!