After I posted the entry on the water bottles in Burano, I did a bit of searching on the Italian version of Google and found a few discussions where the water bottle approach was discussed and analyzed. Basically it was dismissed by many as an urban legend but those that believed in it gave the following reasons why it worked. Frankly, I find most of them a bit silly but I'm amused by them.
- the cat/dog sees his reflection in the water and is scared-off
- the cat/dog finds the place "occupied" and goes away to find another spot
- the cat flees when he "senses" the humidity in the bottle
- the dog is startled when he sees his urine rolling down the bottle and runs away
- the cat/dog is inhibited by the sight of the water in the bottle
and the funniest and most bizarre of all:
- the cat/dog feels a light electric shock when he sprays the bottle which will send him running away
A possible explanation for the origin of this practice is given here ( in Italian ). It tells of an old Scottish practice of placing half-buried uncapped glass bottles in the crop fields. It seems that the whistling sound made by the wind blowing over the mouth of the bottle scared off foxes and other predators.
Snopes also posted an article on a variation of the use of the water bottle as a cat/dog deterrent. Here's the piece.
So there you have it, the mystery of the water bottles explained. Urban legend or not it makes for an interesting topic.