Sure would be great if the various deer whistles out there actually worked, but the evidence and studies all say they don't. Still, there is always the chance that nobody told the deer that (even though they can't hear anything in the most common frequencies used by the whistles). Deer whistles (ultra-sonic) operate at 16 - 20 kHz, but deer can't hear much above 12 kHz, just like us. Even the lower frequency ones don't seem to elicit any "flight" response by the deer, and in some cases rutting bucks actually attacked the whistle. Lots of studies on the web, easy to look up. So discretion remains the better part of valor - slow down, pay all your attention to the road, cover the brakes, and avoid riding as the sun is going down or coming up if you can. Maybe not for everyone, but as I get older, by the end of the day I just want to be at the hotel or home with some good bourbon or single malt. Ride safe.