Hi Maria.
I remember seeing Jane Fannel (?) covering this on TV and I'm now racking my brains as to how she cured it.
I think it came down to discs......Our dog trainer used it at our dog classes....when a dog was distracted by something rather than doing what you wanted it to do, the small collection of discs, tied together to make a noise, was thrown to land near the dog, the dog totally unsuspecting as to where they came from.....its a distraction from what the dog was focusing on....it was either the discs used or a water pistol.....I think John Fisher uses the same method too.
We never used it with Bonnie our rescue cos she was the nervous type and it made her worse...so I guess it depends on the dog.
Not much use I guess but I found this too...
" Another method to teach a “quiet” command is to wait until your dog is barking, say to a doorbell and while he is barking place a very tasty food treat by his nose. Most dogs will stop barking to sniff the treat. At the same time you must say the word you will use for quiet, such as ‘silent’, ‘hush’ etc. When the dog is quiet (as they will be because dogs cannot sniff and bark at the same time) you can praise him, say ‘good, quiet’ and give the treat. Again, as with all new tasks, numerous repetitions are necessary for lasting learning.
Alternately, distraction can be used to disrupt the barking. One of the most effective means of interrupting barking and ensuring quiet is a remote leash and head halter. A pull on the leash disrupts the dog and closes the mouth, which should also coincide with a verbal command such as ‘quiet’ or ‘hush’. Quiet behavior can then be reinforced first by releasing and then giving a reinforcer such as praise or food if the dog remains quiet. Soon the dog should associate the closed mouth and the word prompt with the absence of noise and begin to stop barking when given the verbal prompt alone. "
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I guess the second section would be useful for the letters problem as you will know roughly when the post will be arriving.