In Australia there is a problem with magpies diving at cyclists. There are many ideas flying around as to how you can avoid this irritation by putting various adornments on your bike helmet if you wear one.
A group of friends in Canberra did a [not very] scientific study to determine the best solution for stopping the menace, including some of the various ideas of what to put on your helmet.
One might conclude that covering the helmet with something less shiny or even a wig might be a good solution. However, helmets have two primary features that are important. One is absorbing impact and the other is being very slippery. That's why they're designed the way they are, with a slippery, thin outer shell.
If the odds are against you and you go down and the odds are further against you and your head makes contact with the asphalt, you really want that helmet to be slippery so your head slides.
Covering your helmet with fabric or what not will increase the risk of your helmet snagging on the asphalt and this will increase the risk of neck injury or worse.
I'll be doing a post about how it isn't wise to cover helmets with fabric in the near future.