Always having to clean these out (once every 3 months, due to having silver birch trees) however once a year take the duck bills out completely and clean them separately (remove wiper arms and scuttle to gain access and prepare to scrape and rub your arms red doing it but best for shifting the gunk)
while there out
I would cut about 2mm off with a pair of scissors. But its up to you.
help to flow better and keeps the slim and sticky residue from trees blocking them up
unfortunately you will still need to clear the leaves