This is probably more than some want to know, but what the hell. About those deer and whistles etc.
There is plenty of testimonials that they work, but no hard data from sound research that they do! The fact is sometimes deer will react to a whistle and sometimes not. Just as they will sometimes reaat to flashing your headlights and sometimes not and sometimes react to a horn and sometimes not.
I live in the middle of a woods and have been watching, studying, (and hunting & eating!) deer for many years. Here's what we know. Deer can't see very well. They are also color blind. (That's why hunters can wear blaze orange and deer still run up to them!) They can detect movement well, but not discern details. That's why if they see you in the woods and you are not moving they will often stomp their foot to try and make you move so they can see you better. So during the day they can see some movement sometimes on a road, (wave your arm, etc) but at night not. That's also why they freeze in the headlights. They see a light but cannot pick up the movement.
They also have a very keen sense of smell..no help in our riding situation. And they can hear very well...here comes the deer whistles. They can hear the whistles, but so what. They do not know what it is and do not automatically react because they don't. It's just a noise to them and, as often as not, they will just freeze and try to see what's up. (Just you want from a deer in the middle of the road!) An old hunting trick is to talk or grunt at a running deer. Often they will stop dead in their tracks just to figure out what is making the noise! (Makes an easier shot!)
The whole thing is even more complicated because deer react differently at diferent times of the year and different sexes react differently. For example, during the rutting season, the buck has only one thing on his mind and no dam whistle is going to deter him. I was riding in a Jeep during the rut and a buck was standing in the middle of the road ahead. We came to a full stop, honked the horn, flashed the lights and the buck took a look at us and proceeded to ram the jeep!! Put his antler through the radiator!
In early summer the does have their fawns and will "kick out" the yearlings from the nest so to speak. They then a roam around and are dumber than a post. They just don't know what to do and act and react really unpredicatable.
So if you want to use the whisles, go ahead. They probably don't hurt. (They also "keep away the elephants"!) But don't relax and think you are safe. You can't a trust a deer to react in any particular way. (Until they are in the freezer!)