Because it turns out they only really like red toilets. Specifically, red toilets with four wheels.
A cutting edge survey, which was conducted across five cities in the UK, found 18 per cent of red cars were marked with deposits (i.e. poop), followed by blue (14 per cent), black (11 per cent) and white (seven per cent).
And yes, someone actually paid money for this study to happen.
For the cleanest ride, the best advice, appropriately, is to go green – just one per cent of that colour was smeared.
The report said theories abound on motoring and social networking websites as to why birds are attracted to pooping on some cars more than others.
Some drivers say red signifies danger – clearly to a bowel-loosening degree.
Dropping-stained paintwork costs motorists £57 million (B2.8 billion) a year in repairs in the UK, according to car accessories chain Halfords.
It analysed 1,140 vehicles in Brighton, Glasgow, Leeds, Manchester and Bristol.
A spokesman said: “The problem annoys drivers, causes damaged paintwork and affects the value of vehicles.”