This is one of our resident blackbirds a bit earlier this year. It looks like he’s having a bad hair day. He didn’t seem to mind being on camera though!
You might have seen birds look like this yourself and it is nothing to worry about; they’re moulting.
When, and for how long, birds moult can vary but it is a normal process they go through each year.
You can learn more about this on the RSPB’s website, HERE