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The owl might create a lot of wind drag, plus IMHO a little kitschy. You might consider the wire style spikes or just a loudspeaker with recordings of owls, that would be really cool.
Just get some thin sheet metal, bend them into trays and pop rivet them to the bottom of the crossbars to catch the bird droppings so they don't hit your roof. Paint them black it they will look really cool. Angle them down in the front to create more downforce on your MINI.
Warning. Following above instructions may result in interference with sunroof operation. Use above suggestion at your own risk.